Incandescent Light Bulbs Based on a Refractory Metasurface

Hirofumi Toyoda; Kazunari Kimino; Akihiro Kawano; Junichi Takahara

doi:10.3390/photonics6040105

Incandescent Light Bulbs Based on a Refractory Metasurface

Toyoda, Hirofumi;Kimino, Kazunari;Kawano, Akihiro;Takahara, Junichi 2019-10-12 00:00:00 hv photonics Article Incandescent Light Bulbs Based on a Refractory Metasurface 1 1 1 1 , 2 , Hirofumi Toyoda , Kazunari Kimino , Akihiro Kawano and Junichi Takahara * Graduate School of Engineering, Osaka University, Osaka 565-0871, Japan; toyoda@ap.eng.osaka-u.ac.jp (H.T.); kimino@ap.eng.osaka-u.ac.jp (K.K.); a.kawano@ap.eng.osaka-u.ac.jp (A.K.) Photonics Center, Graduate School of Engineering, Osaka University, Osaka 565-0871, Japan * Correspondence: takahara@ap.eng.osaka-u.ac.jp; Tel.: +81-6-6879-8503 y Invited paper. Received: 5 September 2019; Accepted: 9 October 2019; Published: 12 October 2019 Abstract: A thermal radiation light source, such as an incandescent light bulb, is considered a legacy light source with low luminous ecacy. However, it is an ideal energy source converting light with high eciency from electric power to radiative power. In this work, we evaluate a thermal radiation light source and propose a new type of ﬁlament using a refractory metasurface to fabricate an ecient light bulb. We demonstrate visible-light spectral control using a refractory metasurface made of tantalum with an optical microcavity inserted into an incandescent light bulb. We use a nanoimprint method to fabricate the ﬁlament that is suitable for mass production. A 1.8 times enhancement of thermal radiation intensity is observed from the microcavity ﬁlament compared to the ﬂat ﬁlament. Then, we demonstrate the thermal radiation control of the metasurface using a refractory plasmonic cavity made of hafnium nitride. A single narrow resonant peak is observed at the designed wavelength as well as the suppression of thermal radiation in wide mid-IR range under the condition of constant surface temperature. Keywords: incandescent light bulb; thermal radiation; refractory metal; microcavity; metamaterial; metasurface; surface plasmon; infrared emitter; nanoimprint 1. Introduction An incandescent ﬁlament shining in a transparent glass bulb is the origin of the beauty of lighting. The presence of incandescent light can stimulate a brilliant human mind and exert a calming eect. In addition, incandescent light displays a continuous spectrum of thermal radiation that is attractive from the point of view of architecture, lighting design, and print color-matching. However, a thermal radiation light source like an incandescent light bulb is a legacy light source, and in recent years, its applications have decreased. This is because the luminous ecacy of an incandescent light bulb is only 15 lmW , which is much lower than the emerging solid-state light sources, viz., the light-emitting diode (LED) light bulbs. Most of the radiative power emitted from an incandescent light bulb is in infrared (IR) light. Hence the luminous ecacy of an incandescent light bulb is low compared to an LED light bulb because the luminous ecacy is limited to the narrow absorption band of the human eye. Additionally, the lifetime of LED light bulbs is 40,000–50,000 h which is much longer than that of incandescent light bulbs (1000–2000 h). In contradiction to the trend favoring the use of LEDs over incandescent light bulbs, we propose that a thermal radiation light source is an ideal, high-eciency converter of electric input power to a radiative output power source. The energy conversion eciency of incandescent light bulbs is higher than 90%, which is a result of the Joule heating of a ﬁlament [1,2]. Hence thermal radiation light Photonics 2019, 6, 105; doi:10.3390/photonics6040105 www.mdpi.com/journal/photonics Photonics 2019, 6, 105 2 of 20 Photonics 2019, 6, x FOR PEER REVIEW 2 of 20 sources have great potential for use as ecient light sources. Although the thermal radiation spectrum obeys Planck’s law that depends on physical constants and temperature, its luminous ecacy can be spectrum obeys Planck’s law that depends on physical constants and temperature, its luminous improved beyond that suggested by Planck’s law, if the thermal radiation spectra from a ﬁlament can efficacy can be improved beyond that suggested by Planck’s law, if the thermal radiation spectra be controlled artiﬁcially and optimized to the absorption band of human eyes. from a filament can be controlled artificially and optimized to the absorption band of human eyes. The basic concept for improving the eciency of an incandescent light bulb by thermal radiation The basic concept for improving the efficiency of an incandescent light bulb by thermal radiation control of a nanostructured ﬁlament was proposed by Waymouth in 1989 [3,4]. He reasoned that control of a nanostructured filament was proposed by Waymouth in 1989 [3,4]. He reasoned that IR rad IR i radiation ation (lon(longer ger waveleng wavelengths) ths) can be can sube ppressed suppressed by the cut-o by theff e cut-o ffect of ea ect microca of a micr vity while v ocavity while isible visible light (shorter wavelengths) is radiated into free-space, which is analogous to the operation of light (shorter wavelengths) is radiated into free-space, which is analogous to the operation of a microwave w a microwave a waveguide. veguide. ThThis is idea idea is kno is known wn as t as he W the a W ymouth hyp aymouth hypothesis, othesis, and t and he lamp is called the lamp is called a a “microcavity lamp.” Figure 1 shows the concept of the microcavity lamp, where a cuboidal hole “microcavity lamp.” Figure 1 shows the concept of the microcavity lamp, where a cuboidal hole array is form array is ed on t formed he sur on the face of surface a tungst of aen tungsten (W) fila(W) ment ﬁlament set into a bulb. Fo set into a bulb. llowing t Following he viewpoint the viewpoint of modern of modern photonics, such an artiﬁcial surface structure is called a “metasurface,” i.e., a two-dimensional photonics, such an artificial surface structure is called a “metasurface,” i.e., a two-dimensional (2D) meta (2D) ma metamaterial. terial. Figure 1. The concept of a microcavity lamp: an incandescent light bulb with a microcavity array Figure 1. The concept of a microcavity lamp: an incandescent light bulb with a microcavity array ﬁlament acting as a refractory metasurface. filament acting as a refractory metasurface. Independent of the Waymouth hypothesis, the modiﬁcation of a thermal radiation spectrum Independent of the Waymouth hypothesis, the modification of a thermal radiation spectrum (deviation from Planck’s law) using a microstructured surface was demonstrated ﬁrst in 1986 by a deep (deviation from Planck’s law) using a microstructured surface was demonstrated first in 1986 by a silicon grating [5]. This was studied further using various types of micro and nanostructures such deep silicon grating [5]. This was studied further using various types of micro and nanostructures as an open-end metallic microcavity [6,7], photonic crystal [8,9], plasmonic cavity [10,11], and Mie such as an open-end metallic microcavity [6,7], photonic crystal [8,9], plasmonic cavity [10,11], and resonators [12]. Additionally, narrow-band thermal radiation has been reported using surface waves Mie resonators [12]. Additionally, narrow-band thermal radiation has been reported using surface such as surface phonon polaritons [13], surface plasmon polaritons [14,15], spoof surface plasmons [16], waves such as surface phonon polaritons [13], surface plasmon polaritons [14,15], spoof surface and Tamm plasmons [17]. In 2008, perfect absorbers based on metamaterials were proposed [18,19]. plasmons [16], and Tamm plasmons [17]. In 2008, perfect absorbers based on metamaterials were In addition, thermal radiation emitters based on metasurfaces using metal-dielectric-metal (MDM) proposed [18,19]. In addition, thermal radiation emitters based on metasurfaces using metal- structures were demonstrated in the infrared (IR) range [20,21]. In the last decade, much research dielectric-metal (MDM) structures were demonstrated in the infrared (IR) range [20,21]. In the last was done on perfect absorbers based on MDM metasurfaces, and their applications such as thermal decade, much research was done on perfect absorbers based on MDM metasurfaces, and their radiation emitters or gas sensing in the IR range [22–29]. There are many papers about thermal applications such as thermal radiation emitters or gas sensing in the IR range [22–29]. There are many radiation control based on metasurfaces in the IR range. A comprehensive list of these eorts can be papers about thermal radiation control based on metasurfaces in the IR range. A comprehensive list found in the literature [30,31]. of these efforts can be found in the literature [30,31]. In contrast to the IR range, there are a few studies about thermal radiation control in the visible In contrast to the IR range, there are a few studies about thermal radiation control in the visible spectrum. This occurs because the melting points of typical plasmonic materials such as gold (Au) spectrum. This occurs because the melting points of typical plasmonic materials such as gold (Au) and silver (Ag) are below 1500 K. Additionally, damage during the nanofabrication process induces and silver (Ag) are below 1500 K. Additionally, damage during the nanofabrication process induces a decrease in the melting points of refractory metals due to defects. This reduces the durability of a decrease in the melting points of refractory metals due to defects. This reduces the durability of nanostructures compared with bulk materials. It was reported that Tungsten microcavity structures nanostructures compared with bulk materials. It was reported that Tungsten microcavity structures degrade the melting point to just above 1500 K, which was then held at 1400 K, less than one-half its degrade the melting point to just above 1500 K, which was then held at 1400 K, less than one-half its melting point [32]. In 2015, we fabricated a microcavity lamp utilizing nanoimprint technology and melting point [32]. In 2015, we fabricated a microcavity lamp utilizing nanoimprint technology and demonstrated the enhancement of visible light using a microcavity filament [33,34]. In 2016, Ilic et al. reported an efficient thermal light source that radiated only visible light (cut IR light emission from Photonics 2019, 6, 105 3 of 20 demonstrated the enhancement of visible light using a microcavity ﬁlament [33,34]. In 2016, Ilic et al. Photonics 2019, 6, x FOR PEER REVIEW 3 of 20 reported an ecient thermal light source that radiated only visible light (cut IR light emission from the ﬁlament) by directly sandwiching a dielectric multilayer ﬁlter [35]. However, it is still challenging to the filament) by directly sandwiching a dielectric multilayer filter [35]. However, it is still challenging construct an ecient incandescent light bulb with a ﬁlament controlled by a refractory metasurface. to construct an efficient incandescent light bulb with a filament controlled by a refractory metasurface. In this paper, we review our recent work on thermal radiation control for incandescent light bulbs In this paper, we review our recent work on thermal radiation control for incandescent light based-on refractory metasurfaces. First, we demonstrate the spectral control of visible light using an bulbs based-on refractory metasurfaces. First, we demonstrate the spectral control of visible light optical microcavity array fabricated on a ﬁlament inserted in a light bulb. Here, a nanoimprint method using an optical microcavity array fabricated on a filament inserted in a light bulb. Here, a is used to mass-produce the ﬁlaments. Then, to overcome the drawback of a microcavity, we introduce nanoimprint method is used to mass-produce the filaments. Then, to overcome the drawback of a a refractory metasurface based on a plasmonic cavity made from hafnium nitride (HfN), which is a microcavity, we introduce a refractory metasurface based on a plasmonic cavity made from hafnium new kind of refractory plasmonic material. We demonstrate spectral control in the mid IR range using nitride (HfN), which is a new kind of refractory plasmonic material. We demonstrate spectral control this new refractory metasurface. in the mid IR range using this new refractory metasurface. 2. The Eciency of Thermal Radiation Light Sources 2. The Efficiency of Thermal Radiation Light Sources Figure 2 shows the power ﬂow for typical commercial light sources: an incandescent light bulb Figure 2 shows the power flow for typical commercial light sources: an incandescent light bulb with and without inert gases (inert gases were not used in old incandescent lamps), a ﬂuorescent lamp, with and without inert gases (inert gases were not used in old incandescent lamps), a fluorescent and an LED light bulb [1,2]. In Figure 2, we show the ratio (percentage) of output power to input lamp, and an LED light bulb [1,2]. In Figure 2, we show the ratio (percentage) of output power to electric power. Here, all power ﬂow data are taken from [36–40]. input electric power. Here, all power flow data are taken from [36–40]. Figure 2. The power flow ratio (percentage) of typical commercial light sources: (a) an incandescent Figure 2. The power ﬂow ratio (percentage) of typical commercial light sources: (a) an incandescent light bulb with inert gases (100 W) [36,37], (b) an incandescent light bulb without inert gases (10W) light bulb with inert gases (100 W) [36,37], (b) an incandescent light bulb without inert gases (10W) [38], [38], (c) a fluorescent lamp (40 W) [39], and (d) a light-emitting diode (LED) light bulb (blue LED + (c) a ﬂuorescent lamp (40 W) [39], and (d) a light-emitting diode (LED) light bulb (blue LED + yellow yellow phosphor) [40]. phosphor) [40]. For an LED light bulb, the conversion ratio from input power to visible light is 30–50% while it is only 10% for an incandescent light bulb with inert gas. The LED light bulb is more efficient than the incandescent light bulb. The energy loss of an LED light bulb is caused by various physical processes such as wavelength-conversion losses, inner absorption, or non-radiative phonon Photonics 2019, 6, 105 4 of 20 For an LED light bulb, the conversion ratio from input power to visible light is 30–50% while it is only 10% for an incandescent light bulb with inert gas. The LED light bulb is more ecient than the incandescent light bulb. The energy loss of an LED light bulb is caused by various physical processes such as wavelength-conversion losses, inner absorption, or non-radiative phonon excitation, resulting in the dissipation of energy to the environment around the bulb. However, considering the conversion ratio from input power to total electromagnetic radiation, it is higher than 80% for the incandescent light bulb. Additionally, it exceeds 90% (~94%) for an incandescent light bulb without inert gas. This latter result is obtained because the loss of heat conduction from the ﬁlament to inert gas is negligible. Thus, we can conclude that a thermal radiation light source is an ideal high-eciency energy converter from input electric power to output radiative power. If we suppress the IR light and convert it to visible light, incandescent light bulbs can be recreated as an ecient light source. 3. The Basic Principle of Thermal Radiation Control by a Refractory Metasurface There are two ways to suppress IR light from incandescent lamps: (i) use of an optical ﬁlter coated on a bulb and (ii) thermal radiation control of a ﬁlament. For optical ﬁlters, dielectric multilayers are used for short-pass (IR rejection) optical ﬁlters. Although short-pass optical ﬁlters are used for commercially available halogen light bulbs, the shapes of blubs are limited to elliptical, and the transparency of the bulb is reduced due to coloring of the dielectric multilayers, resulting in a reduction of the beauty of incandescent light bulbs. In contrast, using thermal radiation control, we can modify the thermal radiation spectrum for a ﬁlament directly by forming nanostructures on it. In this method, IR light is suppressed, and visible light is enhanced from a ﬁlament directly, resulting in a signiﬁcant improvement in the luminous ecacy. 2 1 1 Spectral radiant intensity I (,T) [Wm m sr ] of blackbody radiation per area and per solid bb angle at temperature T and wavelength is given by Equation (1): 2hc 1 I (, T) = , (1) bb 5 hc/k T e 1 where c is the speed of light, h is the Planck constant, and k is the Boltzmann constant. The thermal radiation spectrum from a real surface can be calculated by the product of I (,T) and spectral bb emissivity "(). Hence, we can control the radiation spectrum artiﬁcially by specifying "(). This is the basic principle of thermal radiation control. In a microcavity lamp, "() can be controlled by a microcavity array formed on the surface of a refractory metal ﬁlament. Figure 3a shows a schematic view of a cuboidal hole microcavity array. Such a cuboidal hole behaves as an open-end cavity for optical electromagnetic (EM) ﬁelds, and they are conﬁned inside the hole. Contrary to the Waymouth hypothesis, previous experimental studies in the IR range demonstrated that a microcavity enhances thermal radiation at speciﬁc wavelengths by resonance instead of suppression by the cut-o eect [6,7]. The resonant wavelength of the microcavity (Figure 3a) is given by Equation (2): = q , (2) 2 2 2 n y n x z + + a a 2d where n , n = 0, 1, 2, 3 ::: are mode numbers of the x- or y- (horizontal) direction, respectively, and x y n = 0, 1, 3, 5 ::: is the mode number of the z- (vertical) direction, a and d are the width in the x-y direction and the depth of the cuboidal cavity, respectively [5,6]. Photonics 2019, 6, 105 5 of 20 Photonics 2019, 6, x FOR PEER REVIEW 5 of 20 Figure 3. Principle of thermal radiation control by a metasurface: (a) an array of a microcavity on Figure 3. Principle of thermal radiation control by a metasurface: (a) an array of a microcavity on a a refractory metal ﬁlament, (b) resonant modes for n = 1, 3, and 5 inside a microcavity with perfect refractory metal filament, (b) resonant modes for n = 1, 3, and 5 inside a microcavity with perfect conductor walls, and (c) the thermal radiation spectrum can be controlled by the product of spectral conductor walls, and (c) the thermal radiation spectrum can be controlled by the product of spectral emissivity of the metasurface and Planck’s law. emissivity of the metasurface and Planck’s law. Figure 3b shows typical resonant modes in the cavity. In principle, such resonant modes Figure 3b shows typical resonant modes in the cavity. In principle, such resonant modes can can enhance absorption at the resonant wavelengths. According to Kirchho’s law, such resonant enhance absorption at the resonant wavelengths. According to Kirchhoff’s law, such resonant absorption increases the emissivity of opaque materials in thermal radiation. On a metallic surface, absorption increases the emissivity of opaque materials in thermal radiation. On a metallic surface, emissivity is very low at non-resonant wavelengths, as shown in Figure 3c, resulting in a steep resonant emissivity is very low at non-resonant wavelengths, as shown in Figure 3c, resulting in a steep enhancement of the spectral emissivity "(). Hence, the total radiation spectrum can be controlled resonant enhancement of the spectral emissivity ε(λ). Hence, the total radiation spectrum can be by "(). controlled by ε(λ). 4. Thermal Radiation Control by a Microcavity Array 4. Thermal Radiation Control by a Microcavity Array 4.1. Fabrication by Nanoimprint 4.1. Fabrication by Nanoimprint To build a microcavity lamp, we fabricated microcavity array structures on a refractory metal To build a microcavity lamp, we fabricated microcavity array structures on a refractory metal substrate, cut it into ﬁlament strips then inserted them into incandescent light bulbs. In the substrate, cut it into filament strips then inserted them into incandescent light bulbs. In the nanofabrication process, we used a nanoimprint method to fabricate microcavity array patterns nanofabrication process, we used a nanoimprint method to fabricate microcavity array patterns looking forward to a mass-production process. The details of the fabrication process are shown in looking forward to a mass-production process. The details of the fabrication process are shown in Appendix A. Appendix A. Figure 4a shows a photograph of a 20 20 mm polished tantalum (Ta) substrate with a thickness Figure 4a shows a photograph of a 20 × 20 mm polished tantalum (Ta) substrate with a thickness of 100 m, on which microcavity structures are formed. The structures were fabricated on a single side of 100 µm, on which microcavity structures are formed. The structures were fabricated on a single or both sides of the substrate. Figure 4b shows a scanning ion microscope (SIM) image of the structures. side or both sides of the substrate. Figure 4b shows a scanning ion microscope (SIM) image of the The pattern sizes of the mold are width a = 300 nm, depth d = ~200 nm, and period P = 600 nm. From structures. The pattern sizes of the mold are width a = 300 nm, depth d = ~200 nm, and period P = 600 Figure 4b it is conﬁrmed that a 350-nm-squared cuboidal microcavity with P = 600 nm formed on the nm. From Figure 4b it is confirmed that a 350-nm-squared cuboidal microcavity with P = 600 nm substrate. The depth of the cavity is estimated to be ~280 nm by the slanted angle of the SIM image at formed on the substrate. The depth of the cavity is estimated to be ~280 nm by the slanted angle of 30 . The measured depth is shallower than the designed depth of 500 nm. This is because the depth the SIM image at 30°. The measured depth is shallower than the designed depth of 500 nm. This is is limited by the dierences in the dry etching rate between the Cr mask and the Ta substrate. After because the depth is limited by the differences in the dry etching rate between the Cr mask and the fabricating the pattern, the substrate was cut into strips (length: 20 mm, width: 500 m) using a dicing Ta substrate. After fabricating the pattern, the substrate was cut into strips (length: 20 mm, width: saw. A single strip was placed into two holding stems to form a ﬁlament by welding into a bulb made 500 µm) using a dicing saw. A single strip was placed into two holding stems to form a filament by of Pyrex glass (borosilicate glass). Inert gases (75% Ar and 25% N ) were put into the bulb. As a result, welding into a bulb made of Pyrex glass (borosilicate glass). Inert gases (75% Ar and 25% N2) were put into the bulb. As a result, we prepared two types of light bulbs with a structure on both sides and a single side. We also prepared a light bulb with a flat filament for reference. Photonics 2019, 6, 105 6 of 20 we prepared two types of light bulbs with a structure on both sides and a single side. We also prepared Photonics 2019, 6, x FOR PEER REVIEW 6 of 20 Photonics 2019, 6, x FOR PEER REVIEW 6 of 20 a light bulb with a ﬂat ﬁlament for reference. Figure 4. Microcavity ﬁlament: (a) 20-mm-squared Ta substrate and its split into strips using a Figure 4. Microcavity filament: (a) 20-mm-squared Ta substrate and its split into strips using a dicing- Figure 4. Microcavity filament: (a) 20-mm-squared Ta substrate and its split into strips using a dicing- dicing-saw process and (b) scanning ion microscope (SIM) image of the microcavity. The horizontal saw process and (b) scanning ion microscope (SIM) image of the microcavity. The horizontal scale bar saw process and (b) scanning ion microscope (SIM) image of the microcavity. The horizontal scale bar scale bar is 350 nm. is 350 nm. is 350 nm. Figure 5a shows a photograph of a microcavity lamp prototype. The fact that rainbow colors are Figure 5a shows a photograph of a microcavity lamp prototype. The fact that rainbow colors are Figure 5a shows a photograph of a microcavity lamp prototype. The fact that rainbow colors are seen on the ﬁlament shows that periodic structures are formed successfully on the ﬁlament. As shown seen on the filament shows that periodic structures are formed successfully on the filament. As shown seen on the filament shows that periodic structures are formed successfully on the filament. As shown in Figure 5b, the light bulb was set into an E26 socket, and it emitted visible light from the ﬁlament by in Figure 5b, the light bulb was set into an E26 socket, and it emitted visible light from the filament in Figure 5b, the light bulb was set into an E26 socket, and it emitted visible light from the filament connecting an electric power supply. by connecting an electric power supply. by connecting an electric power supply. Figure 5. A prototype of the microcavity lamp: (a) turning off and (b) on. Figure 5. A prototype of the microcavity lamp: (a) turning o and (b) on. Figure 5. A prototype of the microcavity lamp: (a) turning off and (b) on. 4.2. Measurements 4.2. Measurements 4.2. Measurements Measurements of thermal radiation spectra were performed using an integrating sphere for Measurements of thermal radiation spectra were performed using an integrating sphere for Measurements of thermal radiation spectra were performed using an integrating sphere for collecting the total luminous ﬂux. A light bulb with a microcavity ﬁlament was set into the integrating collecting the total luminous flux. A light bulb with a microcavity filament was set into the integrating collecting the total luminous flux. A light bulb with a microcavity filament was set into the integrating sphere (LMS-200, Labsphere, Inc., North Sutton, NH, USA) with a diameter of 25 cm. The radiation sphere (LMS-200, Labsphere, Inc., North Sutton, NH, USA) with a diameter of 25 cm. The radiation sphere (LMS-200, Labsphere, Inc., North Sutton, NH, USA) with a diameter of 25 cm. The radiation spectra were measured using a ﬁber multichannel spectrometer (QE65Pro, Ocean Optics, Inc., Largo, spectra were measured using a fiber multichannel spectrometer (QE65Pro, Ocean Optics, Inc., Largo, spectra were measured using a fiber multichannel spectrometer (QE65Pro, Ocean Optics, Inc., Largo, FL, USA) over the wavelength range of 500–1100 nm. A voltage source was used to heat the ﬁlament FL, USA) over the wavelength range of 500–1100 nm. A voltage source was used to heat the filament FL, USA) over the wavelength range of 500–1100 nm. A voltage source was used to heat the filament under a constant DC voltage of 1.5 V, where the two-terminal resistance of the light bulb was ~0.1 W at under a constant DC voltage of 1.5 V, where the two-terminal resistance of the light bulb was ~0.1 Ω under a constant DC voltage of 1.5 V, where the two-terminal resistance of the light bulb was ~0.1 Ω room temperature. Next, the light bulb with a ﬂat ﬁlament (without a microcavity) was measured at room temperature. Next, the light bulb with a flat filament (without a microcavity) was measured at room temperature. Next, the light bulb with a flat filament (without a microcavity) was measured under the same conditions as the reference. All measurements were done under a constant electric under the same conditions as the reference. All measurements were done under a constant electric under the same conditions as the reference. All measurements were done under a constant electric power of 7.9 W. power of 7.9 W. power of 7.9 W. We note that the conditions for measuring thermal radiation spectra should be identical for all We note that the conditions for measuring thermal radiation spectra should be identical for all samples. Two measurement conditions are standard: (i) constant temperature mode and (ii) constant samples. Two measurement conditions are standard: (i) constant temperature mode and (ii) constant power mode. In constant temperature mode, the radiation spectra are measured, maintaining the power mode. In constant temperature mode, the radiation spectra are measured, maintaining the same filament temperature for all samples. In constant power mode, radiation spectra are measured same filament temperature for all samples. In constant power mode, radiation spectra are measured Photonics 2019, 6, 105 7 of 20 We note that the conditions for measuring thermal radiation spectra should be identical for all samples. Two measurement conditions are standard: (i) constant temperature mode and (ii) constant Photonics 2019, 6, x FOR PEER REVIEW 7 of 20 power mode. In constant temperature mode, the radiation spectra are measured, maintaining the same maintﬁlamen aining const t temperatur ant elect erfor ic power t all samples. o heat In a constant filamentpower . Since mode, it is dif radiation ficult to d spectra irectly me are measur asure ted he maintaining temperature of constant a filament insi electric de power a light to bulb, we used constant po heat a ﬁlament. Since it is wer mode here. dicult to directly measure the temperature of a ﬁlament inside a light bulb, we used constant power mode here. 4.3. Results and Discussion 4.3. Results and Discussion Figure 6 shows the results of the thermal radiation spectra of the total flux from two light bulbs: Figure 6 shows the results of the thermal radiation spectra of the total ﬂux from two light bulbs: a light bulb with a two-sided microcavity filament (Φc(λ)) and one with a flat filament (ΦF(λ)) [33]. a light bulb with a two-sided microcavity ﬁlament ( ()) and one with a ﬂat ﬁlament ( ()) [33]. We can clearly see that the total flux of the microcavity c filament is higher than the flat filame F nt. This We can clearly see that the total ﬂux of the microcavity ﬁlament is higher than the ﬂat ﬁlament. This suggests that the emissivity of the microcavity filament increases compared with the flat filament suggests that the emissivity of the microcavity ﬁlament increases compared with the ﬂat ﬁlament because the temperature of both filaments was almost identical due to the use of the same input because the temperature of both ﬁlaments was almost identical due to the use of the same input power. power. Figure 6. Thermal radiation spectra of the total ﬂux from a microcavity surface (solid line) and Figure 6. Thermal radiation spectra of the total flux from a microcavity surface (solid line) and flat ﬂat surface (dotted line). The ratio of total ﬂux (solid red line) is also plotted, representing the surface (dotted line). The ratio of total flux (solid red line) is also plotted, representing the enhancement factor. enhancement factor. To analyze the enhancement mechanism, we plotted the enhancement factor deﬁned as ()/ (), To analyze the enhancement mechanism, we plotted the enhancement factor defined as as shown in Figure 6. In the enhancement factor plot, we observe a single broad peak at ~700 nm. The Φc(λ)/Φf(λ), as shown in Figure 6. In the enhancement factor plot, we observe a single broad peak at enhancement factor physically means relative spectral emissivity, which is deﬁned as the ratio of the ~700 nm. The enhancement factor physically means relative spectral emissivity, which is defined as spectral emissivity of a microcavity surface to that of a ﬂat surface: i.e., " ()/" (), where " () and c f c the ratio of the spectral emissivity of a microcavity surface to that of a flat surface: i.e., εc(λ)/εf(λ), " () are spectral emissivities at of the microcavity and the ﬂat surface, respectively. where εc(λ) and εf(λ) are spectral emissivities at λ of the microcavity and the flat surface, respectively. 4.4. Simulated Results 4.4. Simulated Results To analyze the enhancement eect quantitatively, we performed numerical calculations on the To analyze the enhancement effect quantitatively, we performed numerical calculations on the spectral absorptivity for the microcavity ﬁlament versus the depth of the cavity using a commercially spectral absorptivity for the microcavity filament versus the depth of the cavity using a commercially available numerical simulator and the rigorous coupled-wave analysis (RCWA) method (Diract Mod, available numerical simulator and the rigorous coupled-wave analysis (RCWA) method (Diffract RSoft Inc.). Mod, RSoft Inc.). Figure 7a shows the spectral map, (,d), of the calculated absorptivity versus the depth, d, of the Figure 7a shows the spectral map, α(λ,d), of the calculated absorptivity versus the depth, d, of cavity. We see that (,d) has a peak at ~600–900 nm with an value of 0.9, which then decreases to the cavity. We see that α(λ,d) has a peak at ~600–900 nm with an α value of 0.9, which then decreases to ~0.1 at λ > 1.0 µm. These peaks in absorptivity are attributed to the resonant modes inside a single microcavity, and the rapid decrease is due to the cut-off effect in the cavity. However, no peak structure is observed in the measured spectrum, Φc(λ), as shown in Figure 6. Photonics 2019, 6, 105 8 of 20 ~0.1 at > 1.0 m. These peaks in absorptivity are attributed to the resonant modes inside a single Photonics 2019, 6, x FOR PEER REVIEW 8 of 20 microcavity, and the rapid decrease is due to the cut-o eect in the cavity. However, no peak structure is observed in the measured spectrum, (), as shown in Figure 6. Figure 7. Simulated spectral maps: (a) spectral absorptivity/emissivity of a Ta microcavity metasurface Figure 7. Simulated spectral maps: (a) spectral absorptivity/emissivity of a Ta microcavity to the depth of a microcavity with w = 350 nm and P = 600 nm and (b) relative spectral metasurface to the depth of a microcavity with w = 350 nm and P = 600 nm and (b) relative spectral absorptivity/emissivity for the ﬂat surface of Ta. absorptivity/emissivity for the flat surface of Ta. To compare the simulation to the experimental results, we calculated the relative spectral absorptivity, which is equal to the relative spectral emissivity, " ()/" (), from Kirchho’s law. By taking To compare the simulation to the experimental results, we calculated the relative spectral c f the absorpti ratio of vity, absorptivity which is equa to the l to the ﬂat surface, relative spectra we obtainlthe emi relative ssivity, spectral εc(λ)/εf(absorptivity λ), from Kirchho /emissivity ff’s law. map By (,d)/(,0) shown in Figure 7b. Note that even a ﬂat Ta surface has moderate broad absorption taking the ratio of absorptivity to the flat surface, we obtain the relative spectral of absorpti = ~0.5 vity/ at emi <ssi 0.6 vi ty m m. W ap eα see (λ,that d)/α(absorption λ,0) shown enhancement in Figure 7b. occurs Note th at at~800 evennm, a flat and Tathe surrfelative ace has absorptivity moderate bro incr ad ab eases sorptio as the n of caviα = ty~0 depth .5 at λ incr < 0eases, .6 µm.as We see t shownhin at ab Figur sorp e t7 ion en b. Athancem a sample entdepth occurs of at d~= 800 280 nm, nmain nd the Figur rela e 4ti b, ve absorpti the peak position vity increa for ses a the sr elative the cavi emissivity ty depth inis crea at ses, ~700–900 as shown i nm (Figur n Figure 7 e 7b). b. This At a samp is consistent le depth with of the d = 28 peak 0 nm i position n Figure 4 of the b, the peak posi experiment in tion f Figuro er the rela 6. Thus, ti the ve br emi oad ssi peak vity iobserved s at ~700– for 900 nm the relative (Figure 7b emissivity ). This is is consist attributed ent wi to the th th micr e pe ocavity ak position o eect.f the experiment in Figure 6. Thus, the broad pe If weak can ob fabricate served for t a su h e relative ciently deep emissivi micr ty ocavity is attrib with uted to the mi d > 500 nm, croca we vexpect ity effect. that the thermal radiation If we spectr can fab umricate will have a suffic a narr iently ower deep resonant microcav peak ity with at ~850 d > nm 500 and nm, we expect tha its relative absorptivity t the therma will l incr rad ease iation to spe ﬁve, ctrum will h as seen in Figur ave a n e 7b. arrowe However r resonant , increasing peakthe at ~ cavity 850 nm depth and further its relais tive di ab cult sorp due tivito ty w the ill limit incre of ase to f the dry ive, as etching seen prin ocess Figu using re 7b. Ho refractory wever, in metals. creasing the c As a Ta substrate avity depth furth is a hard e material r is diffic compar ult due to ed with the limit o Si, the fetching the dry etch contrast ing proce ratio between ss using re thefr resist actory mask metal and s. As thea Ta substr Ta substrate ate is inot s a har su dcient mate for rial deeper compared w etching ith Si, the etc (see Appendix hing contr A). Besides, ast ratio it is betwee challenging n the res to contr ist mask ol the and absorptivity the Ta sub insthe trate is visible not range sufficbecause ient for typical deeper etchin refractory g (see Appen metals such dix A as T)a, . B Mo, esidand es, it is W ar challe e “dielectric” nging to cont fromrol the the negative absorp value tivity of inthe the visi dielectric, ble rar nge because typi esulting in absorption cal refr in actory meta the visiblels spectr such um as T (see a, Mo Appendix , and W B ar )e “ [41 d ]. iel Actually ectric” f , reven om th ae ﬂat negative v Ta surface alue of the die (d = 0) haslec antric, result absorption ing of in abso = ~0.5 rptiat on in the visible = 0.4–0.7 m, spectrum (see as shown in Appendix B Figure 7a. )This [41]. means Actualthat ly, eve these n a fl metals at Ta s ar u erfac fare ( from d = 0) h perfect as an ab conductors sorption of in theα visible = ~0.5 at range. λ = 0.Hence, 4–0.7 µm a , new as sho kind wn in of metasurface Figure 7a. Thi is needed s means t beyond hat ththe ese m performance etals are far f ofra om micr pe ocavity rfect conduct array to ors in the v enhance further isible ran the gemissivity e. Hence, a in new kind the visible of metasur spectrum. face is needed Plasmonic bey materials ond the perf and its or metasurfaces mance of a microcav are needed ity array to enh beyond conventional ance further the emissivity in the visible spectrum. Plasmonic materials and its metasurfaces are needed beyond refractory metals to control the thermal radiation spectra in the visible range. conventional refractory metals to control the thermal radiation spectra in the visible range. 5. Thermal Radiation Control by a Refractory Plasmonic Metasurface 5. Thermal Radiation Control by a Refractory Plasmonic Metasurface 5.1. Thermal Radiation Control by Plasmonic Cavities 5.1. Thermal Radiation Control by Plasmonic Cavities As described in Section 4, we achieved thermal radiation control using a microcavity ﬁlament in the visible range. As a next step, we propose a new kind of ﬁlament using a plasmonic metasurface, As described in Section 4, we achieved thermal radiation control using a microcavity filament in as illustrated in Figure 8. Figure 8 shows the concept of a plasmonic metasurface where thick the visible range. As a next step, we propose a new kind of filament using a plasmonic metasurface, microcavities on the refractory metal are replaced by very thin MDM plasmonic cavities. A plasmonic as illustrated in Figure 8. Figure 8 shows the concept of a plasmonic metasurface where thick microcavities on the refractory metal are replaced by very thin MDM plasmonic cavities. A plasmonic resonator is very thin (<<λ) compared with the wavelength while a microcavity needs a deep trench structure on the order of the controlled optical wavelength (~λ). Since the thickness of the metasurface Photonics 2019, 6, 105 9 of 20 resonator is very thin (<<) compared with the wavelength while a microcavity needs a deep trench Photonics 2019, 6, x FOR PEER REVIEW 9 of 20 structure on the order of the controlled optical wavelength (~). Since the thickness of the metasurface is much smaller than the wavelength, the heat capacity is small and is compatible with a planar is much smaller than the wavelength, the heat capacity is small and is compatible with a planar fabrication process. However, the melting point of conventional plasmonic metals such as Ag and Au fabrication process. However, the melting point of conventional plasmonic metals such as Ag and are not high enough for thermal radiation control in the visible spectrum. Au are not high enough for thermal radiation control in the visible spectrum. Figure 8. Refractory metasurface (a) from a microcavity array to (b) a plasmonic cavity array. Figure 8. Refractory metasurface (a) from a microcavity array to (b) a plasmonic cavity array. In recent years, nitride ceramics such as titanium nitride (TiN) have been proposed and studied as In recent years, nitride ceramics such as titanium nitride (TiN) have been proposed and studied new plasmonic materials operating at higher temperatures (T > 1500 K [42–45]). Melting points of as new plasmonic materials operating at higher temperatures (T > 1500 K [42–45]). Melting points of typical plasmonic materials and nitride ceramics are summarized in Table 1. The melting points of typical plasmonic materials and nitride ceramics are summarized in Table 1. The melting points of these materials are similar to conventional refractory metals, and the permittivity of these materials is these materials are similar to conventional refractory metals, and the permittivity of these materials negative in the visible range. Hence, those are called “refractory plasmonic materials.” is negative in the visible range. Hence, those are called “refractory plasmonic materials.” Table 1. Refractory metals and refractory plasmonic materials in order of its melting point. Table 1. Refractory metals and refractory plasmonic materials in order of its melting point. Material Melting Point (K) Permittivity in Visible Range Material Melting Point (K) Permittivity in Visible Range Ag 1235 ND Ag 1235 ND Au 1337 ND Au 1337 ND SiO 1983 D SiO2 1983 D Mo 2896 D Mo 2896 D HfO 3031 D 2 HfO2 3031 D TiN 3203 ND TiN 3203 ND Ta 3290 D/ND Ta 3290 D/ND HfN HfN 3607 3607 ND ND W W 3695 3695 D D ND: Negati ND: Negative veDielectric; Dielectric; D D: Dielectric. : Dielectric. In this study, we used hafnium nitride (HfN) since the melting point of HfN is higher than that In this study, we used hafnium nitride (HfN) since the melting point of HfN is higher than that of of TiN and it is the same order as W. The most crucial property of nitride ceramics is that its TiN and it is the same order as W. The most crucial property of nitride ceramics is that its permittivity permittivity is negative in the visible range like the noble metals. Such a feature is useful for is negative in the visible range like the noble metals. Such a feature is useful for plasmonic materials. plasmonic materials. The spectral permittivity of conventional and plasmonic refractory metals are The spectral permittivity of conventional and plasmonic refractory metals are shown in Appendix C. shown in Appendix C. If we realize plasmonic metasurfaces using plasmonic refractory materials If we realize plasmonic metasurfaces using plasmonic refractory materials instead of noble metals, we instead of noble metals, we can control the thermal radiation spectra more precisely and obtain higher can control the thermal radiation spectra more precisely and obtain higher Q-value of the plasmonic Q-value of the plasmonic cavity than that of the microcavity. cavity than that of the microcavity. Figure 9 shows a schematic of a cross-sectional view of a refractory MDM metasurface, where Figure 9 shows a schematic of a cross-sectional view of a refractory MDM metasurface, where the Fabry–Pérot (FP) plasmonic resonator disk type based on HfN are arranged in a periodic array. the Fabry–Pérot (FP) plasmonic resonator disk type based on HfN are arranged in a periodic array. The diameter d, the period P of the resonator, the gap thickness in the dielectric layer, Tg, and the top The diameter d, the period P of the resonator, the gap thickness in the dielectric layer, T , and the top metal layer (HfN) Td are shown in Figure 9. We note that the dielectric layer should be selected in metal layer (HfN) T are shown in Figure 9. We note that the dielectric layer should be selected in accordance with the operating temperature T as such that HfO2 for T > 2000 K or SiO2 for T < 2000 K. accordance with the operating temperature T as such that HfO for T > 2000 K or SiO for T < 2000 K. 2 2 Photonics 2019, 6, x FOR PEER REVIEW 10 of 20 Photonics 2019, 6, 105 10 of 20 Photonics 2019, 6, x FOR PEER REVIEW 10 of 20 Figure 9. A schematic and cross-sectional view of a metal-dielectric-metal (MDM) metasurface based on hafnium nitride (HfN). Figure 9. A schematic and cross-sectional view of a metal-dielectric-metal (MDM) metasurface based Figure 9. A schematic and cross-sectional view of a metal-dielectric-metal (MDM) metasurface based on hafnium nitride (HfN). on hafnium nitride (HfN). To confirm the efficiency of thermal radiation control by this refractory plasmonic metasurface, we calculated the theoretical radiation spectrum of the MDM metasurface and compared it to a To conﬁrm the eciency of thermal radiation control by this refractory plasmonic metasurface, we To confirm the efficiency of thermal radiation control by this refractory plasmonic metasurface, blackbody surface under the condition that both radiation powers are identical, i.e., a constant power calculated the theoretical radiation spectrum of the MDM metasurface and compared it to a blackbody we calculated the theoretical radiation spectrum of the MDM metasurface and compared it to a mode as described in Section 4.2. Figure 10 shows the simulated results for the thermal radiation surface under the condition that both radiation powers are identical, i.e., a constant power mode as blackbody surface under the condition that both radiation powers are identical, i.e., a constant power spectrum obtained from the metasurface composed of HfN and HfO2 at T = 2500 K (red line) with d described in Section 4.2. Figure 10 shows the simulated results for the thermal radiation spectrum mode as described in Section 4.2. Figure 10 shows the simulated results for the thermal radiation = 40 nm, P = 80 nm, Tg = 60 nm, and Td = 20 nm. Here, we observe that the highest power radiated obtained from the metasurface composed of HfN and HfO at T = 2500 K (red line) with d = 40 nm, spectrum obtained from the metasurface composed of HfN and HfO2 at T = 2500 K (red line) with d from the metasurface is focused on the resonant peak at ~700 nm with a full-width half-maximum P = 80 nm, T = 60 nm, and T = 20 nm. Here, we observe that the highest power radiated from the g d = 40 nm, P = 80 nm, Tg = 60 nm, and Td = 20 nm. Here, we observe that the highest power radiated (FWHM) value of 571 nm due to the plasmonic resonance in an FP resonator disk. From Figure 10, metasurface is focused on the resonant peak at ~700 nm with a full-width half-maximum (FWHM) from the metasurface is focused on the resonant peak at ~700 nm with a full-width half-maximum the equivalent power from the metasurface at T = 2500 K corresponds to the power from a blackbody value of 571 nm due to the plasmonic resonance in an FP resonator disk. From Figure 10, the equivalent (FWHM) value of 571 nm due to the plasmonic resonance in an FP resonator disk. From Figure 10, at T = 1777 K. This means that radiated power from the metasurface at T = 2500 K equals that from power from the metasurface at T = 2500 K corresponds to the power from a blackbody at T = 1777 K. the equivalent power from the metasurface at T = 2500 K corresponds to the power from a blackbody the blackbody at only T = 1777 K. According to the Stefan–Boltzmann law, the efficiency is improved This means that radiated power from the metasurface at T = 2500 K equals that from the blackbody at T = 1777 K. This means that radiated power from the metasurface at T = 2500 K equals that from by a factor of (2500/1700) = 3.9; i.e., the metasurface is 3.9 times more efficient than the blackbody at only T = 1777 K. According to the Stefan–Boltzmann law, the eciency is improved by a factor of the blackbody at only T = 1777 K. According to the Stefan–Boltzmann law, the efficiency is improved from the viewpoint of power consumption. Additionally, from Figure 10, the radiation intensity at (2500/1700) = 3.9; i.e., the metasurface is 3.9 times more ecient than the blackbody from the viewpoint by a factor of (2500/1700) = 3.9; i.e., the metasurface is 3.9 times more efficient than the blackbody the plasmonic resonant wavelength is more than 10 times greater than that of the blackbody at T = of power consumption. Additionally, from Figure 10, the radiation intensity at the plasmonic resonant from the viewpoint of power consumption. Additionally, from Figure 10, the radiation intensity at 1777 K. wavelength is more than 10 times greater than that of the blackbody at T = 1777 K. the plasmonic resonant wavelength is more than 10 times greater than that of the blackbody at T = 1777 K. Figure 10. Simulated thermal radiation spectra in constant power mode: radiation spectra from the Figure 10. Simulated thermal radiation spectra in constant power mode: radiation spectra from the MDM metasurface (red line) composed of HfN and HfO at T = 2500 K with d = 40 nm, P = 80 nm, MDM metasurface (red line) composed of HfN and HfO2 at T = 2500 K with d = 40 nm, P = 80 nm, Tg T Figure 10. = 60 nm, T Simu = 20 lated thermal radiation nm, and the referencespec blackbody tra in consta (blue nt line) power m at T = 1777 ode: r K. adiation spectra from the = 60 nm, Td = 20 nm, and the reference blackbody (blue line) at T = 1777 K. MDM metasurface (red line) composed of HfN and HfO2 at T = 2500 K with d = 40 nm, P = 80 nm, Tg 5.2. Fabrication = 60 nm, Td = 20 nm, and the reference blackbody (blue line) at T = 1777 K. 5.2. Fabrication To demonstrate the thermal radiation control by a refractory plasmonic metasurface, we fabricated To demonstrate the thermal radiation control by a refractory plasmonic metasurface, we 5.2. Fabrication MDM metasurfaces based on HfN. In this study, we designed the metasurface to be a perfect absorber fabricated MDM metasurfaces based on HfN. In this study, we designed the metasurface to be a in the mid-IR range (~4 m) instead of in the visible as the ﬁrst step towards the fabrication of a To demonstrate the thermal radiation control by a refractory plasmonic metasurface, we perfect absorber in the mid-IR range (~4 µm) instead of in the visible as the first step towards the fabricated MDM metasurfaces based on HfN. In this study, we designed the metasurface to be a perfect absorber in the mid-IR range (~4 µm) instead of in the visible as the first step towards the Photonics 2019, 6, x FOR PEER REVIEW 11 of 20 Photonics 2019, 6, 105 11 of 20 fabrication of a “plasmonic” thermal radiation light source. To design and optimize the size “plasmonic” thermal radiation light source. To design and optimize the size parameters for a perfect parameters for a perfect absorber operating at ~4 µm, we performed numerical simulations using the absorber operating at ~4 m, we performed numerical simulations using the commercially available commercially available finite-difference time-domain method (FDTD) software (Lumerical Inc., ﬁnite-dierence time-domain method (FDTD) software (Lumerical Inc., Vancouver, BC, Canada) for Vancouver, BC, Canada) for the metasurface composed of HfN and SiO2 as shown in Appendix F. the metasurface composed of HfN and SiO as shown in Appendix F. From Figure A6, the designed From Figure A6, the designed value of diameter d = 1.2 µm was determined for achieving the value of diameter d = 1.2 m was determined for achieving the absorption peak of 4 m. absorption peak of 4 µm. Figure 11 shows the MDM metasurface sample with d = 1.14 m, P = 2.0 m, T = 130 nm, and Figure 11 shows the MDM metasurface sample with d = 1.14 µm, P = 2.0 µm, Tg g = 130 nm, and Td T= 20 = 0 nm 200 nm. . ThThe e me metasurface tasurface wa was s fafabricated bricated on ona a115 5 × 115 5 m mm m s squar quare e q quartz uartz substrate substrate us using ing R RFF sputtering and electron beam (EB) lithography. The details of the fabrication process are described sputtering and electron beam (EB) lithography. The details of the fabrication process are described in in Appendix D Appendix .D The SE . The SEM M ima image ge ofof met meta-atoms a-atoms isis show shown n in in FFigur igure e111 1c.c. Add Additionally itionally, we , we fabr fabricated icated a a blackbody reference sample by spraying a blackbody spray (TA410KS, Ichinen TASCO Co., Ltd., blackbody reference sample by spraying a blackbody spray (TA410KS, Ichinen TASCO Co., Ltd., Osaka, Osaka, J Japan) apan) t too t the he 1 155 × 15 15 m mm msquar squar eequartz quartzsubstrate, substrate, as as shown shown in F in Figur igue re11 11 a.a. Thi Thiss b blackbody lackbody reference has an average absorptivity of ~0.989 at = 3–10 m. reference has an average absorptivity of α ~0.989 at λ = 3–10 µm. Figure 11. Refractory MDM metasurface and blackbody reference: (a) a photograph of the blackbody Figure 11. Refractory MDM metasurface and blackbody reference: (a) a photograph of the blackbody reference sample, (b) the MDM metasurface sample composed of HfN and SiO2 on a quartz substrate reference sample, (b) the MDM metasurface sample composed of HfN and SiO on a quartz substrate with d = 1.14 µm, P = 2.0 µm, Tg = 130 nm, and Td = 200 nm. The patterned area is 10 × 10 mm, and (c) with d = 1.14 m, P = 2.0 m, T = 130 nm, and T = 200 nm. The patterned area is 10 10 mm, and g d SEM image of plasmonic resonators. (c) SEM image of plasmonic resonators. 5.3. Measurements 5.3. Measurements Before we measured the thermal radiation, we measured the spectral reﬂectivity R() of the Before we measured the thermal radiation, we measured the spectral reflectivity R(λ) of the metasurface at = 3–12 m using a confocal infrared microscope (HYPERION2000, Bruker Inc., metasurface at λ = 3–12 µm using a confocal infrared microscope (HYPERION2000, Bruker Inc., Billerica, MA, USA) and a Fourier transform infrared (FTIR) spectrometer (VERTEX 70v, Bruker Inc., Billerica, MA, USA) and a Fourier transform infrared (FTIR) spectrometer (VERTEX 70v, Bruker Inc., Billerica, MA, USA) at room temperature. IR light partially shielded by slits was focused on a sample Billerica, MA, USA) at room temperature. IR light partially shielded by slits was focused on a sample through an 15 (NA: 0.4) Schwarzschild objective lens. The reﬂected light from the sample was through an ×15 (NA: 0.4) Schwarzschild objective lens. The reflected light from the sample was corrected through the objective using a detector and converted through a Fourier transform to calculate corrected through the objective using a detector and converted through a Fourier transform to reﬂectivity. Spectral absorptivity, A(), can be calculated by A() = 1 R() if the sample is opaque. calculate reflectivity. Spectral absorptivity, A(λ), can be calculated by A(λ) = 1 − R(λ) if the sample is The thermal radiation spectra were measured using an FTIR spectrometer (FT/IR 6000, JASCO opaque. Co., Tokyo, Japan) at = 3–12 m. The setup of the thermal radiation measurement is shown in The thermal radiation spectra were measured using an FTIR spectrometer (FT/IR 6000, JASCO Appendix E. To avoid oxidation of the surface, the sample was set in a vacuum chamber connected to Co., Tokyo, Japan) at λ = 3–12 µm. The setup of the thermal radiation measurement is shown in the FTIR spectrometer and heated on a ceramic heater. A DC power supply was used to control the Appendix E. To avoid oxidation of the surface, the sample was set in a vacuum chamber connected temperature. The temperature was measured by a thermocouple placed on the surface of the sample. to the FTIR spectrometer and heated on a ceramic heater. A DC power supply was used to control The radiation spectra were measured for both the metasurface and the blackbody sample at 573 K. the temperature. The temperature was measured by a thermocouple placed on the surface of the Hence, all measurements were done under the constant temperature of 573 K. sample. The radiation spectra were measured for both the metasurface and the blackbody sample at 573 K. Hence, all measurements were done under the constant temperature of 573 K. 5.4. Results and Discussion 5.4. Results and Discussion The measured absorptivity spectrum of the MDM metasurface at room temperature is shown in Figure 12a. We observed a single resonant peak at 4.11 m. To identify the physical origin of the The measured absorptivity spectrum of the MDM metasurface at room temperature is shown in peak, we calculated the spectral absorptivity (see the dashed line in Figure 12a) and ﬁeld distribution Figure 12a. We observed a single resonant peak at 4.11 µm. To identify the physical origin of the peak, by the FDTD method. The measured spectrum is in good agreement with the simulated spectrum. we calculated the spectral absorptivity (see the dashed line in Figure 12a) and field distribution by Figure 12b shows a cross-sectional view of the spatial distribution of the electric ﬁeld normal to the the FDTD method. The measured spectrum is in good agreement with the simulated spectrum. Figure 12b shows a cross-sectional view of the spatial distribution of the electric field normal to the Photonics 2019, 6, 105 12 of 20 Photonics 2019, 6, x FOR PEER REVIEW 12 of 20 incident electric ﬁeld at 4.11 m around a meta-atom (plasmonic cavity). Here, we can conﬁrm that incident electric field at 4.11 µm around a meta-atom (plasmonic cavity). Here, we can confirm that the gap plasmon is excited to an FP resonant mode between two metal layers. Hence, the peak in the gap plasmon is excited to an FP resonant mode between two metal layers. Hence, the peak in absorptivity around 4 m is attributed to the plasmonic resonance inside a single plasmonic cavity. absorptivity around 4 µm is attributed to the plasmonic resonance inside a single plasmonic cavity. Note that the resonant peak position is robust against incident angle for both p- and s-polarizations Note that the resonant peak position is robust against incident angle for both p- and s-polarizations as shown in Appendix G. The measured FWHM of the peak (~2 m) is higher than the simulated as shown in Appendix G. The measured FWHM of the peak (~2 µm) is higher than the simulated value (~1.5 m) while the peak position and the peak value are red-shifted slightly and decreased, value (~1.5 µm) while the peak position and the peak value are red-shifted slightly and decreased, respectively. The dierence in the FWHM is attributed to the unexpected loss increase in the real respectively. The difference in the FWHM is attributed to the unexpected loss increase in the real materials. The dierence in the peak value is probably due to the o-axial arrangement of the incident materials. The difference in the peak value is probably due to the off-axial arrangement of the incident light through the Schwarzschild objective lens of the infrared microscope. From this measurement, light through the Schwarzschild objective lens of the infrared microscope. From this measurement, we were able to conﬁrm that the sample was correctly fabricated and operating as designed for a we were able to confirm that the sample was correctly fabricated and operating as designed for a perfect absorber. perfect absorber. Figure 12. Absorptivity spectra and electric ﬁeld distribution of the MDM metasurface composed of Figure 12. Absorptivity spectra and electric field distribution of the MDM metasurface composed of HfN and SiO with P = 2.0 m, d = 1.14 m, T = 130 nm, and T = 200 nm: (a) measured (solid line) HfN and SiO2 2with P = 2.0 µm, d = 1.14 µm, Tg = 130 g nm, and Td = 200 d nm: (a) measured (solid line) and and simulated (dashed line) absorptivity spectra at room temperature, and (b) normalized electric ﬁeld simulated (dashed line) absorptivity spectra at room temperature, and (b) normalized electric field distribution around the meta-atom for the resonance at 4.11 m. distribution around the meta-atom for the resonance at 4.11 µm. Next, we performed a thermal radiation experiment. Figure 13a shows the thermal radiation Next, we performed a thermal radiation experiment. Figure 13a shows the thermal radiation spectra at 573 K for the MDM metasurface and reference blackbody sample. We observe that the spectra at 573 K for the MDM metasurface and reference blackbody sample. We observe that the radiation intensity is suppressed at > 5 m compared with the blackbody level. Such suppression radiation intensity is suppressed at λ > 5 µm compared with the blackbody level. Such suppression is caused by the lower absorptivity (emissivity) at > 5 m, as seen in Figure 12a. Additionally, we is caused by the lower absorptivity (emissivity) at λ > 5 µm, as seen in Figure 12a. Additionally, we can derive the spectral emissivity, " (), of the metasurface from Figure 13a. From Kirchho’s law, can derive the spectral emissivity, ε (λ), of the metasurface from Figure 13a. From Kirchhoff’s law, this must be equal to () shown in Figure 12a if the temperature of a sample is identical. Figure 13b this must be equal to α (λ) shown in Figure 12a if the temperature of a sample is identical. Figure 13b shows the measured spectral emissivity at 573 K of the MDM metasurface. The resonant peak value shows the measured spectral emissivity at 573 K of the MDM metasurface. The resonant peak value of " = ~1 at 4.1 m with FWHM of ~3 m is obtained from Figure 13a. This is consistent with the of ε = ~1 at 4.1 µm with FWHM of ~3 µm is obtained from Figure 13a. This is consistent with the simulated results for absorptivity in Figure 12a (see also the solid line in Figure 13b). These results simulated results for absorptivity in Figure 12a (see also the solid line in Figure 13b). These results suggest that perfect absorption/emission occurred as designed, and the cavity loss was increased due suggest that perfect absorption/emission occurred as designed, and the cavity loss was increased due to the temperature increase. The FWHM of the measured peak actually is broader than the calculated to the temperature increase. The FWHM of the measured peak actually is broader than the calculated result of ~1.5 m. This is evidence of the loss increase caused by the thermal eect. result of ~1.5 µm. This is evidence of the loss increase caused by the thermal effect. Finally, we note that the measured radiation spectrum in Figure 13a is not an intrinsic radiation spectrum, but it includes the transmission function of the optical system in the spectrometer (see Appendix E). Hence, it is necessary to separate it out so we can estimate the intrinsic radiation spectrum from the sample. Since we obtained " (), as shown in Figure 13b, we can determine the intrinsic radiation spectrum of the sample by calculating the product of " () and Planck’s law (Equation (1)). Figure 14 shows the presumed spectrum of the intrinsic radiation as well as the blackbody radiation at 573 K. It is clearly observed that thermal radiation from the metasurface is signiﬁcantly suppressed at longer wavelength region at > 5 m while the surface temperature of the sample is 573 K. Here, we Photonics 2019, 6, x FOR PEER REVIEW 13 of 20 Photonics 2019, 6, 105 13 of 20 point out a crucial fact that the area under the spectral curve of the metasurface is much smaller than that of the blackbody. This indicates that the radiative power from the metasurface is signiﬁcantly suppressed compared with the blackbody resulting in the achievement of an ecient IR emitter, i.e., we are able to heat a sample quite eciently by a small amount of power. Such behavior is typical for constant-temperature-mode measurements, which is dierent from the constant power mode. Photonics 2019, 6, x FOR PEER REVIEW 13 of 20 Figure 13. Experimental thermal radiation spectra for the MDM metasurface: (a) thermal radiation spectra for the MDM metasurface (red line) and blackbody reference sample (solid line) at 573 K. (b) Experimental spectral emissivity at 573 K (red line) derived from (a) and simulated absorptivity at room temperature (solid line). Finally, we note that the measured radiation spectrum in Figure 13a is not an intrinsic radiation spectrum, but it includes the transmission function of the optical system in the spectrometer (see Appendix E). Hence, it is necessary to separate it out so we can estimate the intrinsic radiation spectrum from the sample. Since we obtained ε (λ), as shown in Figure 13b, we can determine the intrinsic radiation spectrum of the sample by calculating the product of ε (λ) and Planck’s law (Equation (1)). Figure 14 shows the presumed spectrum of the intrinsic radiation as well as the blackbody radiation at 573 K. It is clearly observed that thermal radiation from the metasurface is significantly suppressed at longer wavelength region at λ > 5 µm while the surface temperature of the sample is 573 K. Here, we point out a crucial fact that the area under the spectral curve of the metasurface is much smaller than that of the blackbody. This indicates that the radiative power from the metasurface is significantly suppressed compared with the blackbody resulting in the Figure 13. Experimental thermal radiation spectra for the MDM metasurface: (a) thermal radiation Figure 13. Experimental thermal radiation spectra for the MDM metasurface: (a) thermal radiation achievement of an efficient IR emitter, i.e., we are able to heat a sample quite efficiently by a small spectra for the MDM metasurface (red line) and blackbody reference sample (solid line) at 573 K. spectra for the MDM metasurface (red line) and blackbody reference sample (solid line) at 573 K. (b) amount of power. Such behavior is typical for constant-temperature-mode measurements, which is (b) Experimental spectral emissivity at 573 K (red line) derived from (a) and simulated absorptivity at Experimental spectral emissivity at 573 K (red line) derived from (a) and simulated absorptivity at different from the constant power mode. room temperature (solid line). room temperature (solid line). Finally, we note that the measured radiation spectrum in Figure 13a is not an intrinsic radiation spectrum, but it includes the transmission function of the optical system in the spectrometer (see Appendix E). Hence, it is necessary to separate it out so we can estimate the intrinsic radiation spectrum from the sample. Since we obtained ε (λ), as shown in Figure 13b, we can determine the intrinsic radiation spectrum of the sample by calculating the product of ε (λ) and Planck’s law (Equation (1)). Figure 14 shows the presumed spectrum of the intrinsic radiation as well as the blackbody radiation at 573 K. It is clearly observed that thermal radiation from the metasurface is significantly suppressed at longer wavelength region at λ > 5 µm while the surface temperature of the sample is 573 K. Here, we point out a crucial fact that the area under the spectral curve of the metasurface is much smaller than that of the blackbody. This indicates that the radiative power from the metasurface is significantly suppressed compared with the blackbody resulting in the achievement of an efficient IR emitter, i.e., we are able to heat a sample quite efficiently by a small amount of power. Such behavior is typical for constant-temperature-mode measurements, which is different from the constant power mode. Figure 14. Calculated thermal radiation spectra at 573 K: The radiation spectrum from the MDM Figure 14. Calculated thermal radiation spectra at 573 K: The radiation spectrum from the MDM metasurface (red line) is calculated from the measured emissivity shown in Figure 13b. The theoretical metasurface (red line) is calculated from the measured emissivity shown in Figure 13b. The theoretical blackbody radiation spectrum (Equation (1)) at 573 K (solid line) is plotted for reference. blackbody radiation spectrum (Equation (1)) at 573 K (solid line) is plotted for reference. 6. Conclusions We fabricated a prototype of microcavity lamp by a nanoimprint method that is suitable for mass production and demonstrated to control visible-light spectrum using a refractory metasurface made of Ta with an optical microcavity implemented into an incandescent light bulb. It was conﬁrmed that thermal radiation intensity from the microcavity ﬁlament was increased 1.8 times compared to the ﬂat ﬁlament under the constant power input. Then, we fabricated and demonstrated the thermal radiation control in mid-IR range by using an MDM plasmonic metasurface composed of a refractory plasmonic cavity made of HfN. A single narrow resonant peak was observed at designed wavelength Figure 14. Calculated thermal radiation spectra at 573 K: The radiation spectrum from the MDM metasurface (red line) is calculated from the measured emissivity shown in Figure 13b. The theoretical blackbody radiation spectrum (Equation (1)) at 573 K (solid line) is plotted for reference. Photonics 2019, 6, 105 14 of 20 as well as the suppression of thermal radiation in wide mid-IR range under the condition of constant surface temperature. We revaluated a thermal radiation light source as an ecient light source from the perspective of energy conversion. For a future energy-saving society, it is vital to reconsider thermal radiation sources as energy-saving technology. Author Contributions: J.T. conceived the idea of incandescent light bulbs based on refractory metasurface. H.T. and A.K. performed the numerical simulations. H.T. and K.K. performed the experiments. J.T. analyzed the experimental data and wrote the initial draft of the manuscript. J.T. supervised the project. All the authors discussed the results and contributed to the writing of the manuscript. Funding: This research was funded in part by the Photonics Advanced Research Center (PARC) from the Ministry of Education, Culture, Sports, Science and Technology, Japan (MEXT) and the JSPS Core-to-Core Program, and A. Advanced Research Networks (Advanced Nanophotonics in the Emerging Fields of Nano-imaging, Spectroscopy, Nonlinear Optics, Plasmonics/Metamaterials, and Devices). Acknowledgments: We would like to thank Yosuke Ueba and Yusuke Nagasaki for useful discussions. A part of this work was supported by the “Nanotechnology Platform Project (Nanotechnology Open Facilities in Osaka University)” of the Ministry of Education, Culture, Sports, Science, and Technology, Japan [No.: F-17-OS-0011, S-17-OS-0011]. Conﬂicts of Interest: The authors declare no conﬂicts of interest. The funders had no role in the design of the study, in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results. Appendix A The microcavity ﬁlaments were fabricated by a nanoimprint process suitable for mass production as described below. Figure A1 shows the fabrication process of a microcavity array onto a Ta substrate. At ﬁrst, a 3-inch Si wafer (thickness 380m) was prepared, and the electron beam (EB) resist (ZEP520A-7) spin-coated to a thickness of 300 nm using a spinner for 60 s at 4000 rpm. It was then baked for 5 min at 180 C. After that, a 20 20 mm square microcavity array pattern was drawn onto the EB resist using a 50-kV electron beam (EB) lithography system (F5112+VD01; ADVANTEST Co., Tokyo, Japan). After developing the resist, the Si wafer was dry-etched by an inductively coupled plasma (ICP) etching machine (EIS-700, ELIONIX Inc., Tokyo, Japan) to transfer the pattern to the substrate, resulting in a Si master mold with 300 300 nm square holes, 300 nm wall width, and approximately 200 nm depth. Then, the master mold was duplicated onto a photocrosslinkable resin ﬁlm under heat and high-pressure conditions, resulting in an intermediate resin membrane (IRM). These IRMs are used for mass production in the future so the master mold can be preserved. Next, we prepared polished Ta substrates (20 20 mm squares with a thickness of 100 m) for making ﬁlaments. A 30-nm-thickness Cr layer was deposited on the Ta substrate, and a photoresist (MUR-XR2-150, Maruzen Petrochemical Co., Ltd., Tokyo, Japan) was spin-coated on the substrate to a thickness of 215 nm by using a spinner at 3000 rpm. Then, the sample was placed in a vacuum chamber, and the IRM placed on a quartz cylinder was pressed onto the resist under UV light irradiation, resulting in stamping the pattern onto the resist residing on the substrate with the Cr layer. Since the IRM and the cylinder are transparent, it can be used as a template to transfer a pattern to a photoresist under UV irradiation. After removing the IRM, the Ta substrate was dry-etched through the resist and the Cr mask ﬁlm using an electron cyclotron resonance (ECR) ion shower machine (EIS-200ER, ELIONIX Inc., Tokyo, Japan) and ICP etching machine (EIS-700, ELIONIX Inc., Tokyo, Japan). The etching depths are 3.2 nm for the Cr layer and 18 nm for the Ta substrate. Finally, the resist pattern was transferred to the Ta substrate, as shown in Figure 4b. The resulting cavity size was wider than the mold (350 350 nm square holes, 250-nm wall width, and ~280-nm depth). Note that such a widening eect was caused by the dry-etching process and was calibrated by designing the EB lithography process. We fabricated three kinds of Ta substrate: (i) the microcavity on a single side, (ii) the microcavity on both sides, and (iii) a plane without patterning for reference. In the case of (i), we performed the Photonics 2019, 6, x FOR PEER REVIEW 15 of 20 After removing the IRM, the Ta substrate was dry-etched through the resist and the Cr mask film using an electron cyclotron resonance (ECR) ion shower machine (EIS-200ER, ELIONIX Inc., Tokyo, Japan) and ICP etching machine (EIS-700, ELIONIX Inc., Tokyo, Japan). The etching depths are 3.2 nm for the Cr layer and 18 nm for the Ta substrate. Finally, the resist pattern was transferred to the Ta substrate, as shown in Figure 4b. The resulting cavity size was wider than the mold (350 × 350 nm square holes, 250-nm wall width, and ~280-nm depth). Note that such a widening effect was Photonics 2019, 6, 105 15 of 20 caused by the dry-etching process and was calibrated by designing the EB lithography process. We fabricated three kinds of Ta substrate: (i) the microcavity on a single side, (ii) the microcavity on both sides, and (iii) a plane without patterning for reference. In the case of (i), we performed the process once. For (ii), we repeated the process twice. Finally, the Ta substrates were cut into narrow process once. For (ii), we repeated the process twice. Finally, the Ta substrates were cut into narrow 500-m strips as shown in Figure 4a. 500-µm strips as shown in Figure 4a. Figure A1. Nanoimprint process for fabricating microcavity ﬁlaments. Figure A1. Nanoimprint process for fabricating microcavity filaments. Appendix B Appendix B Figure A2 shows the relative permittivity spectra of conventional refractory metals (W, Ta, and Figure A2 shows the relative permittivity spectra of conventional refractory metals (W, Ta, and Mo) [41]. The real part of the permittivity for W and Mo are both positive in the visible range (λ < 0.8 Mo) [41]. The real part of the permittivity for W and Mo are both positive in the visible range µm). The real part of the permittivity of Ta switches from negative to positive below 0.6 µm. The ( < 0.8 m). The real part of the permittivity of Ta switches from negative to positive below 0.6 m. imaginary part of the permittivity of Ta is ~1/2 that of W and Mo. The imaginary part of the permittivity of Ta is ~1/2 that of W and Mo. Photonics 2019, 6, x FOR PEER REVIEW 16 of 20 Figure A2. Spectral relative permittivities of conventional refractory metals (W, Ta, and Mo): (a) real Figure A2. Spectral relative permittivities of conventional refractory metals (W, Ta, and Mo): (a) real and (b) imaginary part of the relative permittivity [41]. and (b) imaginary part of the relative permittivity [41]. Appendix C Figure A3 shows the relative permittivity spectra of conventional plasmonic metals (Au, Ag, and Al) [41], and refractory plasmonic materials (TiN and HfN) [43]. The real part of the permittivity of HfN is negative within the entire visible range and is very close to that of Au at λ < 0.6 µm. The imaginary part of the permittivity of HfN is ~1/2 that of TiN in the visible range. Figure A3. Spectral relative permittivity of conventional plasmonic metals (Au, Ag, and Al) [41] and refractory plasmonic materials (TiN and HfN) [43]: (a) real and (b) imaginary part of the relative permittivities. Appendix D Figure A4 shows the fabrication process for the refractory MDM metasurface. First, a HfN layer and a 130-nm-thick SiO2 layer were deposited on a quartz substrate (15 × 15 mm) using an RF sputtering system (SVC-700LRF, SANYU Electron, Tokyo, Japan). In fabricating the HfN layer, we used an HfN target (Toshima Manufacturing Co., Ltd., Saitama, Japan) under an Ar gas flow rate of −4 25 sccm at 2 × 10 Pa. Next, hexamethyldisilazane (HMDS) was spin-coated using a spinner for 90 s at 5000 rpm. Then, a photoresist (TSMR-8900) was spin-coated to a thickness of 700 nm using a spinner for 90 s at 5000 rpm. The metasurface patterns were exposed using a mask-less UV lithography system (DL-1000, Nanosystem Solutions Inc., Tokyo, Japan) then developed. Next, a 200- Photonics 2019, 6, x FOR PEER REVIEW 16 of 20 Photonics 2019, 6, 105 16 of 20 Figure A2. Spectral relative permittivities of conventional refractory metals (W, Ta, and Mo): (a) real and (b) imaginary part of the relative permittivity [41]. Appendix C Appendix C Figure A3 shows the relative permittivity spectra of conventional plasmonic metals (Au, Ag, and Figure A3 shows the relative permittivity spectra of conventional plasmonic metals (Au, Ag, and Al) [41], and refractory plasmonic materials (TiN and HfN) [43]. The real part of the permittivity of HfN Al) [41], and refractory plasmonic materials (TiN and HfN) [43]. The real part of the permittivity of is negative within the entire visible range and is very close to that of Au at < 0.6 m. The imaginary HfN is negative within the entire visible range and is very close to that of Au at λ < 0.6 µm. The part of the permittivity of HfN is ~1/2 that of TiN in the visible range. imaginary part of the permittivity of HfN is ~1/2 that of TiN in the visible range. Figure A3. Spectral relative permittivity of conventional plasmonic metals (Au, Ag, and Al) [41] and Figure A3. Spectral relative permittivity of conventional plasmonic metals (Au, Ag, and Al) [41] refractory plasmonic materials (TiN and HfN) [43]: (a) real and (b) imaginary part of the relative and refractory plasmonic materials (TiN and HfN) [43]: (a) real and (b) imaginary part of the permittivities. relative permittivities. Appendix D Appendix D Figure A4 shows the fabrication process for the refractory MDM metasurface. First, a HfN layer Figure A4 shows the fabrication process for the refractory MDM metasurface. First, a HfN and a 130-nm-thick SiO2 layer were deposited on a quartz substrate (15 × 15 mm) using an RF layer and a 130-nm-thick SiO layer were deposited on a quartz substrate (15 15 mm) using an RF sputtering system (SVC-700LRF, SANYU Electron, Tokyo, Japan). In fabricating the HfN layer, we sputtering system (SVC-700LRF, SANYU Electron, Tokyo, Japan). In fabricating the HfN layer, we used an HfN target (Toshima Manufacturing Co., Ltd., Saitama, Japan) under an Ar gas flow rate of used an HfN target (Toshima Manufacturing Co., Ltd., Saitama, Japan) under an Ar gas ﬂow rate −4 25 sccm at 2 × 10 Pa. Next, hexamethyldisilazane (HMDS) was spin-coated using a spinner for 90 s of 25 sccm at 2 10 Pa. Next, hexamethyldisilazane (HMDS) was spin-coated using a spinner at 5000 rpm. Then, a photoresist (TSMR-8900) was spin-coated to a thickness of 700 nm using a for 90sp sinner fo at 5000r rpm. 90 s at Then, 5000 rp a photor m. The m esistet(TSMR-8900) asurface patterns wer was spin-coated e exposed u tosing a thickness a mask-less of 700 UV nm lithography system (DL-1000, Nanosystem Solutions Inc., Tokyo, Japan) then developed. Next, a 200- using a spinner for 90 s at 5000 rpm. The metasurface patterns were exposed using a mask-less UV lithography system (DL-1000, Nanosystem Solutions Inc., Tokyo, Japan) then developed. Next, a Photonics 2019, 6, x FOR PEER REVIEW 17 of 20 200-nm-thick HfN layer was deposited by RF sputtering. Finally, the HfN layer was lifted o using nm-thick HfN layer was deposited by RF sputtering. Finally, the HfN layer was lifted off using N- N-methyl-2-pyrrolidone (NMP). methyl-2-pyrrolidone (NMP). Figure A4. Fabricating the refractory MDM metasurface. Figure A4. Fabricating the refractory MDM metasurface. Appendix E Figure A5 shows the experimental setup for measuring the thermal radiation spectrum. The optical system was placed in a vacuum chamber connected to an FTIR spectrometer (FT/IR 6000, JASCO Co., Tokyo, Japan) through a tunnel tube. The vacuum chamber and the FTIR were pumped 2 2 −1 to 2.0 × 10 Pa and 1.4 × 10 Pa, respectively. The spectral resolution was set to 4 cm , and a DLATGS detector was used for the measurement. The device is shown in Figure 12 and was placed on a micro- ceramic heater (MS-1000, Sakaguchi E.H VOC Corp., Tokyo, Japan). The temperature of the sample was measured by a K-type sheath thermocouple (T350251H, Sakaguchi E.H VOC Corp., Tokyo, Japan) placed on the surface of the sample. The measurements were performed at 573 K. Figure A5. Experimental setup for measuring the thermal radiation spectrum. The sample is set on a ceramic heater in a vacuum chamber that is connected to the FTIR spectrometer through a tunnel tube. Appendix F Figure A6 shows simulated spectral absorptivity to the diameter d of an MDM metasurface composed of HfN and SiO2 (see Figure 9) with P = 2.0 µm, Tg = 130 nm, and Td = 200 nm. The peak Photonics 2019, 6, x FOR PEER REVIEW 17 of 20 nm-thick HfN layer was deposited by RF sputtering. Finally, the HfN layer was lifted off using N- methyl-2-pyrrolidone (NMP). Photonics 2019, 6, 105 17 of 20 Figure A4. Fabricating the refractory MDM metasurface. Appendix E Figur Appendix E e A5 shows the experimental setup for measuring the thermal radiation spectrum. The optical system was placed in a vacuum chamber connected to an FTIR spectrometer (FT/IR 6000, Figure A5 shows the experimental setup for measuring the thermal radiation spectrum. The JASCO Co., Tokyo, Japan) through a tunnel tube. The vacuum chamber and the FTIR were pumped to optical system was placed in a vacuum chamber connected to an FTIR spectrometer (FT/IR 6000, 2 2 1 2.0 10 Pa and 1.4 10 Pa, respectively. The spectral resolution was set to 4 cm , and a DLATGS JASCO Co., Tokyo, Japan) through a tunnel tube. The vacuum chamber and the FTIR were pumped 2 2 −1 detector to 2.was 0 × 10used Pa and for 1.the 4 × 10 measur Pa, respecti ement. vely The . The spectra device is l resol shown ution in wa Figur s set to e 4 12 cm and , an was d a DL placed ATGSon a detector was used for the measurement. The device is shown in Figure 12 and was placed on a micro- micro-ceramic heater (MS-1000, Sakaguchi E.H VOC Corp., Tokyo, Japan). The temperature of the ceramic heater (MS-1000, Sakaguchi E.H VOC Corp., Tokyo, Japan). The temperature of the sample sample was measured by a K-type sheath thermocouple (T350251H, Sakaguchi E.H VOC Corp., Tokyo, was measured by a K-type sheath thermocouple (T350251H, Sakaguchi E.H VOC Corp., Tokyo, Japan) placed on the surface of the sample. The measurements were performed at 573 K. Japan) placed on the surface of the sample. The measurements were performed at 573 K. Figure A5. Experimental setup for measuring the thermal radiation spectrum. The sample is set on a Figure A5. Experimental setup for measuring the thermal radiation spectrum. The sample is set on a ceramic heater in a vacuum chamber that is connected to the FTIR spectrometer through a tunnel tube. ceramic heater in a vacuum chamber that is connected to the FTIR spectrometer through a tunnel tube. Appendix F Appendix F Figure A6 shows simulated spectral absorptivity to the diameter d of an MDM metasurface Photonics 2019, 6, x FOR PEER REVIEW 18 of 20 Figure A6 shows simulated spectral absorptivity to the diameter d of an MDM metasurface composed of HfN and SiO (see Figure 9) with P = 2.0 m, T = 130 nm, and T = 200 nm. The peak 2 g d composed of HfN and SiO2 (see Figure 9) with P = 2.0 µm, Tg = 130 nm, and Td = 200 nm. The peak position of absorption caused by gap plasmon mode in the circular cavity can be changed from 3.0 to position of absorption caused by gap plasmon mode in the circular cavity can be changed from 3.0 to 7.0 µm by changing d. 7.0 m by changing d. Figure A6. Simulated spectral absorptivity/emissivity map to the diameter of an MDM metasurface Figure A6. Simulated spectral absorptivity/emissivity map to the diameter of an MDM metasurface composed of HfN and SiO with P = 2.0 m, T = 130 nm, and T = 200 nm. 2 g d composed of HfN and SiO2 with P = 2.0 µm, Tg = 130 nm, and Td = 200 nm. Appendix G Figure A7 shows simulated spectral absorptivity to the incident angle to an MDM metasurface composed of HfN and SiO2 (see Figure 9). The single absorption peak caused by gap plasmon mode in the circular cavity is observed at ~4.5 µm for both polarizations, which is not strongly dependent on incident angle. The strong angle-dependent steep absorption is caused by diffraction at 2.5–4.0 µm only for p-polarization as shown in (a). Note that the designed value of d = 1.2 µm is slightly greater than that of the experimental value (d = 1.14 µm). Figure A7. Simulated spectral absorptivity/emissivity maps to the incident angle to an MDM metasurface composed of HfN and SiO2 with P = 2.0 µm, d = 1.2 µm, Tg = 130 nm, and Td = 200 nm: (a) p-polarization and (b) s-polarization. References 1. Takahara, J.; Ueba, Y.; Nagatsuma, T. Thermal radiation control by microcavity and ecological incandescent lamps. Jpn. J. Opt. 2010, 39, 482–488. Photonics 2019, 6, x FOR PEER REVIEW 18 of 20 position of absorption caused by gap plasmon mode in the circular cavity can be changed from 3.0 to 7.0 µm by changing d. Figure A6. Simulated spectral absorptivity/emissivity map to the diameter of an MDM metasurface Photonics 2019, 6, 105 18 of 20 composed of HfN and SiO2 with P = 2.0 µm, Tg = 130 nm, and Td = 200 nm. Appendix G Appendix G Figure A7 shows simulated spectral absorptivity to the incident angle to an MDM metasurface Figure A7 shows simulated spectral absorptivity to the incident angle to an MDM metasurface composed of HfN and SiO (see Figure 9). The single absorption peak caused by gap plasmon mode in composed of HfN and SiO 22 (see Figure 9). The single absorption peak caused by gap plasmon mode the circular cavity is observed at ~4.5 m for both polarizations, which is not strongly dependent on in the circular cavity is observed at ~4.5 µm for both polarizations, which is not strongly dependent incident angle. The strong angle-dependent steep absorption is caused by diraction at 2.5–4.0 m on incident angle. The strong angle-dependent steep absorption is caused by diffraction at 2.5–4.0 only for p-polarization as shown in (a). Note that the designed value of d = 1.2 m is slightly greater µm only for p-polarization as shown in (a). Note that the designed value of d = 1.2 µm is slightly than that of the experimental value (d = 1.14 m). greater than that of the experimental value (d = 1.14 µm). Figure A7. Simulated spectral absorptivity/emissivity maps to the incident angle to an MDM Figure A7. 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DOI: 10.3390/photonics6040105
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Abstract

hv photonics Article Incandescent Light Bulbs Based on a Refractory Metasurface 1 1 1 1 , 2 , Hirofumi Toyoda , Kazunari Kimino , Akihiro Kawano and Junichi Takahara * Graduate School of Engineering, Osaka University, Osaka 565-0871, Japan; toyoda@ap.eng.osaka-u.ac.jp (H.T.); kimino@ap.eng.osaka-u.ac.jp (K.K.); a.kawano@ap.eng.osaka-u.ac.jp (A.K.) Photonics Center, Graduate School of Engineering, Osaka University, Osaka 565-0871, Japan * Correspondence: takahara@ap.eng.osaka-u.ac.jp; Tel.: +81-6-6879-8503 y Invited paper. Received: 5 September 2019; Accepted: 9 October 2019; Published: 12 October 2019 Abstract: A thermal radiation light source, such as an incandescent light bulb, is considered a legacy light source with low luminous ecacy. However, it is an ideal energy source converting light with high eciency from electric power to radiative power. In this work, we evaluate a thermal radiation light source and propose a new type of ﬁlament using a refractory metasurface to fabricate an ecient light bulb. We demonstrate visible-light spectral control using a refractory metasurface made of tantalum with an optical microcavity inserted into an incandescent light bulb. We use a nanoimprint method to fabricate the ﬁlament that is suitable for mass production. A 1.8 times enhancement of thermal radiation intensity is observed from the microcavity ﬁlament compared to the ﬂat ﬁlament. Then, we demonstrate the thermal radiation control of the metasurface using a refractory plasmonic cavity made of hafnium nitride. A single narrow resonant peak is observed at the designed wavelength as well as the suppression of thermal radiation in wide mid-IR range under the condition of constant surface temperature. Keywords: incandescent light bulb; thermal radiation; refractory metal; microcavity; metamaterial; metasurface; surface plasmon; infrared emitter; nanoimprint 1. Introduction An incandescent ﬁlament shining in a transparent glass bulb is the origin of the beauty of lighting. The presence of incandescent light can stimulate a brilliant human mind and exert a calming eect. In addition, incandescent light displays a continuous spectrum of thermal radiation that is attractive from the point of view of architecture, lighting design, and print color-matching. However, a thermal radiation light source like an incandescent light bulb is a legacy light source, and in recent years, its applications have decreased. This is because the luminous ecacy of an incandescent light bulb is only 15 lmW , which is much lower than the emerging solid-state light sources, viz., the light-emitting diode (LED) light bulbs. Most of the radiative power emitted from an incandescent light bulb is in infrared (IR) light. Hence the luminous ecacy of an incandescent light bulb is low compared to an LED light bulb because the luminous ecacy is limited to the narrow absorption band of the human eye. Additionally, the lifetime of LED light bulbs is 40,000–50,000 h which is much longer than that of incandescent light bulbs (1000–2000 h). In contradiction to the trend favoring the use of LEDs over incandescent light bulbs, we propose that a thermal radiation light source is an ideal, high-eciency converter of electric input power to a radiative output power source. The energy conversion eciency of incandescent light bulbs is higher than 90%, which is a result of the Joule heating of a ﬁlament [1,2]. Hence thermal radiation light Photonics 2019, 6, 105; doi:10.3390/photonics6040105 www.mdpi.com/journal/photonics Photonics 2019, 6, 105 2 of 20 Photonics 2019, 6, x FOR PEER REVIEW 2 of 20 sources have great potential for use as ecient light sources. Although the thermal radiation spectrum obeys Planck’s law that depends on physical constants and temperature, its luminous ecacy can be spectrum obeys Planck’s law that depends on physical constants and temperature, its luminous improved beyond that suggested by Planck’s law, if the thermal radiation spectra from a ﬁlament can efficacy can be improved beyond that suggested by Planck’s law, if the thermal radiation spectra be controlled artiﬁcially and optimized to the absorption band of human eyes. from a filament can be controlled artificially and optimized to the absorption band of human eyes. The basic concept for improving the eciency of an incandescent light bulb by thermal radiation The basic concept for improving the efficiency of an incandescent light bulb by thermal radiation control of a nanostructured ﬁlament was proposed by Waymouth in 1989 [3,4]. He reasoned that control of a nanostructured filament was proposed by Waymouth in 1989 [3,4]. He reasoned that IR rad IR i radiation ation (lon(longer ger waveleng wavelengths) ths) can be can sube ppressed suppressed by the cut-o by theff e cut-o ffect of ea ect microca of a micr vity while v ocavity while isible visible light (shorter wavelengths) is radiated into free-space, which is analogous to the operation of light (shorter wavelengths) is radiated into free-space, which is analogous to the operation of a microwave w a microwave a waveguide. veguide. ThThis is idea idea is kno is known wn as t as he W the a W ymouth hyp aymouth hypothesis, othesis, and t and he lamp is called the lamp is called a a “microcavity lamp.” Figure 1 shows the concept of the microcavity lamp, where a cuboidal hole “microcavity lamp.” Figure 1 shows the concept of the microcavity lamp, where a cuboidal hole array is form array is ed on t formed he sur on the face of surface a tungst of aen tungsten (W) fila(W) ment ﬁlament set into a bulb. Fo set into a bulb. llowing t Following he viewpoint the viewpoint of modern of modern photonics, such an artiﬁcial surface structure is called a “metasurface,” i.e., a two-dimensional photonics, such an artificial surface structure is called a “metasurface,” i.e., a two-dimensional (2D) meta (2D) ma metamaterial. terial. Figure 1. The concept of a microcavity lamp: an incandescent light bulb with a microcavity array Figure 1. The concept of a microcavity lamp: an incandescent light bulb with a microcavity array ﬁlament acting as a refractory metasurface. filament acting as a refractory metasurface. Independent of the Waymouth hypothesis, the modiﬁcation of a thermal radiation spectrum Independent of the Waymouth hypothesis, the modification of a thermal radiation spectrum (deviation from Planck’s law) using a microstructured surface was demonstrated ﬁrst in 1986 by a deep (deviation from Planck’s law) using a microstructured surface was demonstrated first in 1986 by a silicon grating [5]. This was studied further using various types of micro and nanostructures such deep silicon grating [5]. This was studied further using various types of micro and nanostructures as an open-end metallic microcavity [6,7], photonic crystal [8,9], plasmonic cavity [10,11], and Mie such as an open-end metallic microcavity [6,7], photonic crystal [8,9], plasmonic cavity [10,11], and resonators [12]. Additionally, narrow-band thermal radiation has been reported using surface waves Mie resonators [12]. Additionally, narrow-band thermal radiation has been reported using surface such as surface phonon polaritons [13], surface plasmon polaritons [14,15], spoof surface plasmons [16], waves such as surface phonon polaritons [13], surface plasmon polaritons [14,15], spoof surface and Tamm plasmons [17]. In 2008, perfect absorbers based on metamaterials were proposed [18,19]. plasmons [16], and Tamm plasmons [17]. In 2008, perfect absorbers based on metamaterials were In addition, thermal radiation emitters based on metasurfaces using metal-dielectric-metal (MDM) proposed [18,19]. In addition, thermal radiation emitters based on metasurfaces using metal- structures were demonstrated in the infrared (IR) range [20,21]. In the last decade, much research dielectric-metal (MDM) structures were demonstrated in the infrared (IR) range [20,21]. In the last was done on perfect absorbers based on MDM metasurfaces, and their applications such as thermal decade, much research was done on perfect absorbers based on MDM metasurfaces, and their radiation emitters or gas sensing in the IR range [22–29]. There are many papers about thermal applications such as thermal radiation emitters or gas sensing in the IR range [22–29]. There are many radiation control based on metasurfaces in the IR range. A comprehensive list of these eorts can be papers about thermal radiation control based on metasurfaces in the IR range. A comprehensive list found in the literature [30,31]. of these efforts can be found in the literature [30,31]. In contrast to the IR range, there are a few studies about thermal radiation control in the visible In contrast to the IR range, there are a few studies about thermal radiation control in the visible spectrum. This occurs because the melting points of typical plasmonic materials such as gold (Au) spectrum. This occurs because the melting points of typical plasmonic materials such as gold (Au) and silver (Ag) are below 1500 K. Additionally, damage during the nanofabrication process induces and silver (Ag) are below 1500 K. Additionally, damage during the nanofabrication process induces a decrease in the melting points of refractory metals due to defects. This reduces the durability of a decrease in the melting points of refractory metals due to defects. This reduces the durability of nanostructures compared with bulk materials. It was reported that Tungsten microcavity structures nanostructures compared with bulk materials. It was reported that Tungsten microcavity structures degrade the melting point to just above 1500 K, which was then held at 1400 K, less than one-half its degrade the melting point to just above 1500 K, which was then held at 1400 K, less than one-half its melting point [32]. In 2015, we fabricated a microcavity lamp utilizing nanoimprint technology and melting point [32]. In 2015, we fabricated a microcavity lamp utilizing nanoimprint technology and demonstrated the enhancement of visible light using a microcavity filament [33,34]. In 2016, Ilic et al. reported an efficient thermal light source that radiated only visible light (cut IR light emission from Photonics 2019, 6, 105 3 of 20 demonstrated the enhancement of visible light using a microcavity ﬁlament [33,34]. In 2016, Ilic et al. Photonics 2019, 6, x FOR PEER REVIEW 3 of 20 reported an ecient thermal light source that radiated only visible light (cut IR light emission from the ﬁlament) by directly sandwiching a dielectric multilayer ﬁlter [35]. However, it is still challenging to the filament) by directly sandwiching a dielectric multilayer filter [35]. However, it is still challenging construct an ecient incandescent light bulb with a ﬁlament controlled by a refractory metasurface. to construct an efficient incandescent light bulb with a filament controlled by a refractory metasurface. In this paper, we review our recent work on thermal radiation control for incandescent light bulbs In this paper, we review our recent work on thermal radiation control for incandescent light based-on refractory metasurfaces. First, we demonstrate the spectral control of visible light using an bulbs based-on refractory metasurfaces. First, we demonstrate the spectral control of visible light optical microcavity array fabricated on a ﬁlament inserted in a light bulb. Here, a nanoimprint method using an optical microcavity array fabricated on a filament inserted in a light bulb. Here, a is used to mass-produce the ﬁlaments. Then, to overcome the drawback of a microcavity, we introduce nanoimprint method is used to mass-produce the filaments. Then, to overcome the drawback of a a refractory metasurface based on a plasmonic cavity made from hafnium nitride (HfN), which is a microcavity, we introduce a refractory metasurface based on a plasmonic cavity made from hafnium new kind of refractory plasmonic material. We demonstrate spectral control in the mid IR range using nitride (HfN), which is a new kind of refractory plasmonic material. We demonstrate spectral control this new refractory metasurface. in the mid IR range using this new refractory metasurface. 2. The Eciency of Thermal Radiation Light Sources 2. The Efficiency of Thermal Radiation Light Sources Figure 2 shows the power ﬂow for typical commercial light sources: an incandescent light bulb Figure 2 shows the power flow for typical commercial light sources: an incandescent light bulb with and without inert gases (inert gases were not used in old incandescent lamps), a ﬂuorescent lamp, with and without inert gases (inert gases were not used in old incandescent lamps), a fluorescent and an LED light bulb [1,2]. In Figure 2, we show the ratio (percentage) of output power to input lamp, and an LED light bulb [1,2]. In Figure 2, we show the ratio (percentage) of output power to electric power. Here, all power ﬂow data are taken from [36–40]. input electric power. Here, all power flow data are taken from [36–40]. Figure 2. The power flow ratio (percentage) of typical commercial light sources: (a) an incandescent Figure 2. The power ﬂow ratio (percentage) of typical commercial light sources: (a) an incandescent light bulb with inert gases (100 W) [36,37], (b) an incandescent light bulb without inert gases (10W) light bulb with inert gases (100 W) [36,37], (b) an incandescent light bulb without inert gases (10W) [38], [38], (c) a fluorescent lamp (40 W) [39], and (d) a light-emitting diode (LED) light bulb (blue LED + (c) a ﬂuorescent lamp (40 W) [39], and (d) a light-emitting diode (LED) light bulb (blue LED + yellow yellow phosphor) [40]. phosphor) [40]. For an LED light bulb, the conversion ratio from input power to visible light is 30–50% while it is only 10% for an incandescent light bulb with inert gas. The LED light bulb is more efficient than the incandescent light bulb. The energy loss of an LED light bulb is caused by various physical processes such as wavelength-conversion losses, inner absorption, or non-radiative phonon Photonics 2019, 6, 105 4 of 20 For an LED light bulb, the conversion ratio from input power to visible light is 30–50% while it is only 10% for an incandescent light bulb with inert gas. The LED light bulb is more ecient than the incandescent light bulb. The energy loss of an LED light bulb is caused by various physical processes such as wavelength-conversion losses, inner absorption, or non-radiative phonon excitation, resulting in the dissipation of energy to the environment around the bulb. However, considering the conversion ratio from input power to total electromagnetic radiation, it is higher than 80% for the incandescent light bulb. Additionally, it exceeds 90% (~94%) for an incandescent light bulb without inert gas. This latter result is obtained because the loss of heat conduction from the ﬁlament to inert gas is negligible. Thus, we can conclude that a thermal radiation light source is an ideal high-eciency energy converter from input electric power to output radiative power. If we suppress the IR light and convert it to visible light, incandescent light bulbs can be recreated as an ecient light source. 3. The Basic Principle of Thermal Radiation Control by a Refractory Metasurface There are two ways to suppress IR light from incandescent lamps: (i) use of an optical ﬁlter coated on a bulb and (ii) thermal radiation control of a ﬁlament. For optical ﬁlters, dielectric multilayers are used for short-pass (IR rejection) optical ﬁlters. Although short-pass optical ﬁlters are used for commercially available halogen light bulbs, the shapes of blubs are limited to elliptical, and the transparency of the bulb is reduced due to coloring of the dielectric multilayers, resulting in a reduction of the beauty of incandescent light bulbs. In contrast, using thermal radiation control, we can modify the thermal radiation spectrum for a ﬁlament directly by forming nanostructures on it. In this method, IR light is suppressed, and visible light is enhanced from a ﬁlament directly, resulting in a signiﬁcant improvement in the luminous ecacy. 2 1 1 Spectral radiant intensity I (,T) [Wm m sr ] of blackbody radiation per area and per solid bb angle at temperature T and wavelength is given by Equation (1): 2hc 1 I (, T) = , (1) bb 5 hc/k T e 1 where c is the speed of light, h is the Planck constant, and k is the Boltzmann constant. The thermal radiation spectrum from a real surface can be calculated by the product of I (,T) and spectral bb emissivity "(). Hence, we can control the radiation spectrum artiﬁcially by specifying "(). This is the basic principle of thermal radiation control. In a microcavity lamp, "() can be controlled by a microcavity array formed on the surface of a refractory metal ﬁlament. Figure 3a shows a schematic view of a cuboidal hole microcavity array. Such a cuboidal hole behaves as an open-end cavity for optical electromagnetic (EM) ﬁelds, and they are conﬁned inside the hole. Contrary to the Waymouth hypothesis, previous experimental studies in the IR range demonstrated that a microcavity enhances thermal radiation at speciﬁc wavelengths by resonance instead of suppression by the cut-o eect [6,7]. The resonant wavelength of the microcavity (Figure 3a) is given by Equation (2): = q , (2) 2 2 2 n y n x z + + a a 2d where n , n = 0, 1, 2, 3 ::: are mode numbers of the x- or y- (horizontal) direction, respectively, and x y n = 0, 1, 3, 5 ::: is the mode number of the z- (vertical) direction, a and d are the width in the x-y direction and the depth of the cuboidal cavity, respectively [5,6]. Photonics 2019, 6, 105 5 of 20 Photonics 2019, 6, x FOR PEER REVIEW 5 of 20 Figure 3. Principle of thermal radiation control by a metasurface: (a) an array of a microcavity on Figure 3. Principle of thermal radiation control by a metasurface: (a) an array of a microcavity on a a refractory metal ﬁlament, (b) resonant modes for n = 1, 3, and 5 inside a microcavity with perfect refractory metal filament, (b) resonant modes for n = 1, 3, and 5 inside a microcavity with perfect conductor walls, and (c) the thermal radiation spectrum can be controlled by the product of spectral conductor walls, and (c) the thermal radiation spectrum can be controlled by the product of spectral emissivity of the metasurface and Planck’s law. emissivity of the metasurface and Planck’s law. Figure 3b shows typical resonant modes in the cavity. In principle, such resonant modes Figure 3b shows typical resonant modes in the cavity. In principle, such resonant modes can can enhance absorption at the resonant wavelengths. According to Kirchho’s law, such resonant enhance absorption at the resonant wavelengths. According to Kirchhoff’s law, such resonant absorption increases the emissivity of opaque materials in thermal radiation. On a metallic surface, absorption increases the emissivity of opaque materials in thermal radiation. On a metallic surface, emissivity is very low at non-resonant wavelengths, as shown in Figure 3c, resulting in a steep resonant emissivity is very low at non-resonant wavelengths, as shown in Figure 3c, resulting in a steep enhancement of the spectral emissivity "(). Hence, the total radiation spectrum can be controlled resonant enhancement of the spectral emissivity ε(λ). Hence, the total radiation spectrum can be by "(). controlled by ε(λ). 4. Thermal Radiation Control by a Microcavity Array 4. Thermal Radiation Control by a Microcavity Array 4.1. Fabrication by Nanoimprint 4.1. Fabrication by Nanoimprint To build a microcavity lamp, we fabricated microcavity array structures on a refractory metal To build a microcavity lamp, we fabricated microcavity array structures on a refractory metal substrate, cut it into ﬁlament strips then inserted them into incandescent light bulbs. In the substrate, cut it into filament strips then inserted them into incandescent light bulbs. In the nanofabrication process, we used a nanoimprint method to fabricate microcavity array patterns nanofabrication process, we used a nanoimprint method to fabricate microcavity array patterns looking forward to a mass-production process. The details of the fabrication process are shown in looking forward to a mass-production process. The details of the fabrication process are shown in Appendix A. Appendix A. Figure 4a shows a photograph of a 20 20 mm polished tantalum (Ta) substrate with a thickness Figure 4a shows a photograph of a 20 × 20 mm polished tantalum (Ta) substrate with a thickness of 100 m, on which microcavity structures are formed. The structures were fabricated on a single side of 100 µm, on which microcavity structures are formed. The structures were fabricated on a single or both sides of the substrate. Figure 4b shows a scanning ion microscope (SIM) image of the structures. side or both sides of the substrate. Figure 4b shows a scanning ion microscope (SIM) image of the The pattern sizes of the mold are width a = 300 nm, depth d = ~200 nm, and period P = 600 nm. From structures. The pattern sizes of the mold are width a = 300 nm, depth d = ~200 nm, and period P = 600 Figure 4b it is conﬁrmed that a 350-nm-squared cuboidal microcavity with P = 600 nm formed on the nm. From Figure 4b it is confirmed that a 350-nm-squared cuboidal microcavity with P = 600 nm substrate. The depth of the cavity is estimated to be ~280 nm by the slanted angle of the SIM image at formed on the substrate. The depth of the cavity is estimated to be ~280 nm by the slanted angle of 30 . The measured depth is shallower than the designed depth of 500 nm. This is because the depth the SIM image at 30°. The measured depth is shallower than the designed depth of 500 nm. This is is limited by the dierences in the dry etching rate between the Cr mask and the Ta substrate. After because the depth is limited by the differences in the dry etching rate between the Cr mask and the fabricating the pattern, the substrate was cut into strips (length: 20 mm, width: 500 m) using a dicing Ta substrate. After fabricating the pattern, the substrate was cut into strips (length: 20 mm, width: saw. A single strip was placed into two holding stems to form a ﬁlament by welding into a bulb made 500 µm) using a dicing saw. A single strip was placed into two holding stems to form a filament by of Pyrex glass (borosilicate glass). Inert gases (75% Ar and 25% N ) were put into the bulb. As a result, welding into a bulb made of Pyrex glass (borosilicate glass). Inert gases (75% Ar and 25% N2) were put into the bulb. As a result, we prepared two types of light bulbs with a structure on both sides and a single side. We also prepared a light bulb with a flat filament for reference. Photonics 2019, 6, 105 6 of 20 we prepared two types of light bulbs with a structure on both sides and a single side. We also prepared Photonics 2019, 6, x FOR PEER REVIEW 6 of 20 Photonics 2019, 6, x FOR PEER REVIEW 6 of 20 a light bulb with a ﬂat ﬁlament for reference. Figure 4. Microcavity ﬁlament: (a) 20-mm-squared Ta substrate and its split into strips using a Figure 4. Microcavity filament: (a) 20-mm-squared Ta substrate and its split into strips using a dicing- Figure 4. Microcavity filament: (a) 20-mm-squared Ta substrate and its split into strips using a dicing- dicing-saw process and (b) scanning ion microscope (SIM) image of the microcavity. The horizontal saw process and (b) scanning ion microscope (SIM) image of the microcavity. The horizontal scale bar saw process and (b) scanning ion microscope (SIM) image of the microcavity. The horizontal scale bar scale bar is 350 nm. is 350 nm. is 350 nm. Figure 5a shows a photograph of a microcavity lamp prototype. The fact that rainbow colors are Figure 5a shows a photograph of a microcavity lamp prototype. The fact that rainbow colors are Figure 5a shows a photograph of a microcavity lamp prototype. The fact that rainbow colors are seen on the ﬁlament shows that periodic structures are formed successfully on the ﬁlament. As shown seen on the filament shows that periodic structures are formed successfully on the filament. As shown seen on the filament shows that periodic structures are formed successfully on the filament. As shown in Figure 5b, the light bulb was set into an E26 socket, and it emitted visible light from the ﬁlament by in Figure 5b, the light bulb was set into an E26 socket, and it emitted visible light from the filament in Figure 5b, the light bulb was set into an E26 socket, and it emitted visible light from the filament connecting an electric power supply. by connecting an electric power supply. by connecting an electric power supply. Figure 5. A prototype of the microcavity lamp: (a) turning off and (b) on. Figure 5. A prototype of the microcavity lamp: (a) turning o and (b) on. Figure 5. A prototype of the microcavity lamp: (a) turning off and (b) on. 4.2. Measurements 4.2. Measurements 4.2. Measurements Measurements of thermal radiation spectra were performed using an integrating sphere for Measurements of thermal radiation spectra were performed using an integrating sphere for Measurements of thermal radiation spectra were performed using an integrating sphere for collecting the total luminous ﬂux. A light bulb with a microcavity ﬁlament was set into the integrating collecting the total luminous flux. A light bulb with a microcavity filament was set into the integrating collecting the total luminous flux. A light bulb with a microcavity filament was set into the integrating sphere (LMS-200, Labsphere, Inc., North Sutton, NH, USA) with a diameter of 25 cm. The radiation sphere (LMS-200, Labsphere, Inc., North Sutton, NH, USA) with a diameter of 25 cm. The radiation sphere (LMS-200, Labsphere, Inc., North Sutton, NH, USA) with a diameter of 25 cm. The radiation spectra were measured using a ﬁber multichannel spectrometer (QE65Pro, Ocean Optics, Inc., Largo, spectra were measured using a fiber multichannel spectrometer (QE65Pro, Ocean Optics, Inc., Largo, spectra were measured using a fiber multichannel spectrometer (QE65Pro, Ocean Optics, Inc., Largo, FL, USA) over the wavelength range of 500–1100 nm. A voltage source was used to heat the ﬁlament FL, USA) over the wavelength range of 500–1100 nm. A voltage source was used to heat the filament FL, USA) over the wavelength range of 500–1100 nm. A voltage source was used to heat the filament under a constant DC voltage of 1.5 V, where the two-terminal resistance of the light bulb was ~0.1 W at under a constant DC voltage of 1.5 V, where the two-terminal resistance of the light bulb was ~0.1 Ω under a constant DC voltage of 1.5 V, where the two-terminal resistance of the light bulb was ~0.1 Ω room temperature. Next, the light bulb with a ﬂat ﬁlament (without a microcavity) was measured at room temperature. Next, the light bulb with a flat filament (without a microcavity) was measured at room temperature. Next, the light bulb with a flat filament (without a microcavity) was measured under the same conditions as the reference. All measurements were done under a constant electric under the same conditions as the reference. All measurements were done under a constant electric under the same conditions as the reference. All measurements were done under a constant electric power of 7.9 W. power of 7.9 W. power of 7.9 W. We note that the conditions for measuring thermal radiation spectra should be identical for all We note that the conditions for measuring thermal radiation spectra should be identical for all samples. Two measurement conditions are standard: (i) constant temperature mode and (ii) constant samples. Two measurement conditions are standard: (i) constant temperature mode and (ii) constant power mode. In constant temperature mode, the radiation spectra are measured, maintaining the power mode. In constant temperature mode, the radiation spectra are measured, maintaining the same filament temperature for all samples. In constant power mode, radiation spectra are measured same filament temperature for all samples. In constant power mode, radiation spectra are measured Photonics 2019, 6, 105 7 of 20 We note that the conditions for measuring thermal radiation spectra should be identical for all samples. Two measurement conditions are standard: (i) constant temperature mode and (ii) constant Photonics 2019, 6, x FOR PEER REVIEW 7 of 20 power mode. In constant temperature mode, the radiation spectra are measured, maintaining the same maintﬁlamen aining const t temperatur ant elect erfor ic power t all samples. o heat In a constant filamentpower . Since mode, it is dif radiation ficult to d spectra irectly me are measur asure ted he maintaining temperature of constant a filament insi electric de power a light to bulb, we used constant po heat a ﬁlament. Since it is wer mode here. dicult to directly measure the temperature of a ﬁlament inside a light bulb, we used constant power mode here. 4.3. Results and Discussion 4.3. Results and Discussion Figure 6 shows the results of the thermal radiation spectra of the total flux from two light bulbs: Figure 6 shows the results of the thermal radiation spectra of the total ﬂux from two light bulbs: a light bulb with a two-sided microcavity filament (Φc(λ)) and one with a flat filament (ΦF(λ)) [33]. a light bulb with a two-sided microcavity ﬁlament ( ()) and one with a ﬂat ﬁlament ( ()) [33]. We can clearly see that the total flux of the microcavity c filament is higher than the flat filame F nt. This We can clearly see that the total ﬂux of the microcavity ﬁlament is higher than the ﬂat ﬁlament. This suggests that the emissivity of the microcavity filament increases compared with the flat filament suggests that the emissivity of the microcavity ﬁlament increases compared with the ﬂat ﬁlament because the temperature of both filaments was almost identical due to the use of the same input because the temperature of both ﬁlaments was almost identical due to the use of the same input power. power. Figure 6. Thermal radiation spectra of the total ﬂux from a microcavity surface (solid line) and Figure 6. Thermal radiation spectra of the total flux from a microcavity surface (solid line) and flat ﬂat surface (dotted line). The ratio of total ﬂux (solid red line) is also plotted, representing the surface (dotted line). The ratio of total flux (solid red line) is also plotted, representing the enhancement factor. enhancement factor. To analyze the enhancement mechanism, we plotted the enhancement factor deﬁned as ()/ (), To analyze the enhancement mechanism, we plotted the enhancement factor defined as as shown in Figure 6. In the enhancement factor plot, we observe a single broad peak at ~700 nm. The Φc(λ)/Φf(λ), as shown in Figure 6. In the enhancement factor plot, we observe a single broad peak at enhancement factor physically means relative spectral emissivity, which is deﬁned as the ratio of the ~700 nm. The enhancement factor physically means relative spectral emissivity, which is defined as spectral emissivity of a microcavity surface to that of a ﬂat surface: i.e., " ()/" (), where " () and c f c the ratio of the spectral emissivity of a microcavity surface to that of a flat surface: i.e., εc(λ)/εf(λ), " () are spectral emissivities at of the microcavity and the ﬂat surface, respectively. where εc(λ) and εf(λ) are spectral emissivities at λ of the microcavity and the flat surface, respectively. 4.4. Simulated Results 4.4. Simulated Results To analyze the enhancement eect quantitatively, we performed numerical calculations on the To analyze the enhancement effect quantitatively, we performed numerical calculations on the spectral absorptivity for the microcavity ﬁlament versus the depth of the cavity using a commercially spectral absorptivity for the microcavity filament versus the depth of the cavity using a commercially available numerical simulator and the rigorous coupled-wave analysis (RCWA) method (Diract Mod, available numerical simulator and the rigorous coupled-wave analysis (RCWA) method (Diffract RSoft Inc.). Mod, RSoft Inc.). Figure 7a shows the spectral map, (,d), of the calculated absorptivity versus the depth, d, of the Figure 7a shows the spectral map, α(λ,d), of the calculated absorptivity versus the depth, d, of cavity. We see that (,d) has a peak at ~600–900 nm with an value of 0.9, which then decreases to the cavity. We see that α(λ,d) has a peak at ~600–900 nm with an α value of 0.9, which then decreases to ~0.1 at λ > 1.0 µm. These peaks in absorptivity are attributed to the resonant modes inside a single microcavity, and the rapid decrease is due to the cut-off effect in the cavity. However, no peak structure is observed in the measured spectrum, Φc(λ), as shown in Figure 6. Photonics 2019, 6, 105 8 of 20 ~0.1 at > 1.0 m. These peaks in absorptivity are attributed to the resonant modes inside a single Photonics 2019, 6, x FOR PEER REVIEW 8 of 20 microcavity, and the rapid decrease is due to the cut-o eect in the cavity. However, no peak structure is observed in the measured spectrum, (), as shown in Figure 6. Figure 7. Simulated spectral maps: (a) spectral absorptivity/emissivity of a Ta microcavity metasurface Figure 7. Simulated spectral maps: (a) spectral absorptivity/emissivity of a Ta microcavity to the depth of a microcavity with w = 350 nm and P = 600 nm and (b) relative spectral metasurface to the depth of a microcavity with w = 350 nm and P = 600 nm and (b) relative spectral absorptivity/emissivity for the ﬂat surface of Ta. absorptivity/emissivity for the flat surface of Ta. To compare the simulation to the experimental results, we calculated the relative spectral absorptivity, which is equal to the relative spectral emissivity, " ()/" (), from Kirchho’s law. By taking To compare the simulation to the experimental results, we calculated the relative spectral c f the absorpti ratio of vity, absorptivity which is equa to the l to the ﬂat surface, relative spectra we obtainlthe emi relative ssivity, spectral εc(λ)/εf(absorptivity λ), from Kirchho /emissivity ff’s law. map By (,d)/(,0) shown in Figure 7b. Note that even a ﬂat Ta surface has moderate broad absorption taking the ratio of absorptivity to the flat surface, we obtain the relative spectral of absorpti = ~0.5 vity/ at emi <ssi 0.6 vi ty m m. W ap eα see (λ,that d)/α(absorption λ,0) shown enhancement in Figure 7b. occurs Note th at at~800 evennm, a flat and Tathe surrfelative ace has absorptivity moderate bro incr ad ab eases sorptio as the n of caviα = ty~0 depth .5 at λ incr < 0eases, .6 µm.as We see t shownhin at ab Figur sorp e t7 ion en b. Athancem a sample entdepth occurs of at d~= 800 280 nm, nmain nd the Figur rela e 4ti b, ve absorpti the peak position vity increa for ses a the sr elative the cavi emissivity ty depth inis crea at ses, ~700–900 as shown i nm (Figur n Figure 7 e 7b). b. This At a samp is consistent le depth with of the d = 28 peak 0 nm i position n Figure 4 of the b, the peak posi experiment in tion f Figuro er the rela 6. Thus, ti the ve br emi oad ssi peak vity iobserved s at ~700– for 900 nm the relative (Figure 7b emissivity ). This is is consist attributed ent wi to the th th micr e pe ocavity ak position o eect.f the experiment in Figure 6. Thus, the broad pe If weak can ob fabricate served for t a su h e relative ciently deep emissivi micr ty ocavity is attrib with uted to the mi d > 500 nm, croca we vexpect ity effect. that the thermal radiation If we spectr can fab umricate will have a suffic a narr iently ower deep resonant microcav peak ity with at ~850 d > nm 500 and nm, we expect tha its relative absorptivity t the therma will l incr rad ease iation to spe ﬁve, ctrum will h as seen in Figur ave a n e 7b. arrowe However r resonant , increasing peakthe at ~ cavity 850 nm depth and further its relais tive di ab cult sorp due tivito ty w the ill limit incre of ase to f the dry ive, as etching seen prin ocess Figu using re 7b. Ho refractory wever, in metals. creasing the c As a Ta substrate avity depth furth is a hard e material r is diffic compar ult due to ed with the limit o Si, the fetching the dry etch contrast ing proce ratio between ss using re thefr resist actory mask metal and s. As thea Ta substr Ta substrate ate is inot s a har su dcient mate for rial deeper compared w etching ith Si, the etc (see Appendix hing contr A). Besides, ast ratio it is betwee challenging n the res to contr ist mask ol the and absorptivity the Ta sub insthe trate is visible not range sufficbecause ient for typical deeper etchin refractory g (see Appen metals such dix A as T)a, . B Mo, esidand es, it is W ar challe e “dielectric” nging to cont fromrol the the negative absorp value tivity of inthe the visi dielectric, ble rar nge because typi esulting in absorption cal refr in actory meta the visiblels spectr such um as T (see a, Mo Appendix , and W B ar )e “ [41 d ]. iel Actually ectric” f , reven om th ae ﬂat negative v Ta surface alue of the die (d = 0) haslec antric, result absorption ing of in abso = ~0.5 rptiat on in the visible = 0.4–0.7 m, spectrum (see as shown in Appendix B Figure 7a. )This [41]. means Actualthat ly, eve these n a fl metals at Ta s ar u erfac fare ( from d = 0) h perfect as an ab conductors sorption of in theα visible = ~0.5 at range. λ = 0.Hence, 4–0.7 µm a , new as sho kind wn in of metasurface Figure 7a. Thi is needed s means t beyond hat ththe ese m performance etals are far f ofra om micr pe ocavity rfect conduct array to ors in the v enhance further isible ran the gemissivity e. Hence, a in new kind the visible of metasur spectrum. face is needed Plasmonic bey materials ond the perf and its or metasurfaces mance of a microcav are needed ity array to enh beyond conventional ance further the emissivity in the visible spectrum. Plasmonic materials and its metasurfaces are needed beyond refractory metals to control the thermal radiation spectra in the visible range. conventional refractory metals to control the thermal radiation spectra in the visible range. 5. Thermal Radiation Control by a Refractory Plasmonic Metasurface 5. Thermal Radiation Control by a Refractory Plasmonic Metasurface 5.1. Thermal Radiation Control by Plasmonic Cavities 5.1. Thermal Radiation Control by Plasmonic Cavities As described in Section 4, we achieved thermal radiation control using a microcavity ﬁlament in the visible range. As a next step, we propose a new kind of ﬁlament using a plasmonic metasurface, As described in Section 4, we achieved thermal radiation control using a microcavity filament in as illustrated in Figure 8. Figure 8 shows the concept of a plasmonic metasurface where thick the visible range. As a next step, we propose a new kind of filament using a plasmonic metasurface, microcavities on the refractory metal are replaced by very thin MDM plasmonic cavities. A plasmonic as illustrated in Figure 8. Figure 8 shows the concept of a plasmonic metasurface where thick microcavities on the refractory metal are replaced by very thin MDM plasmonic cavities. A plasmonic resonator is very thin (<<λ) compared with the wavelength while a microcavity needs a deep trench structure on the order of the controlled optical wavelength (~λ). Since the thickness of the metasurface Photonics 2019, 6, 105 9 of 20 resonator is very thin (<<) compared with the wavelength while a microcavity needs a deep trench Photonics 2019, 6, x FOR PEER REVIEW 9 of 20 structure on the order of the controlled optical wavelength (~). Since the thickness of the metasurface is much smaller than the wavelength, the heat capacity is small and is compatible with a planar is much smaller than the wavelength, the heat capacity is small and is compatible with a planar fabrication process. However, the melting point of conventional plasmonic metals such as Ag and Au fabrication process. However, the melting point of conventional plasmonic metals such as Ag and are not high enough for thermal radiation control in the visible spectrum. Au are not high enough for thermal radiation control in the visible spectrum. Figure 8. Refractory metasurface (a) from a microcavity array to (b) a plasmonic cavity array. Figure 8. Refractory metasurface (a) from a microcavity array to (b) a plasmonic cavity array. In recent years, nitride ceramics such as titanium nitride (TiN) have been proposed and studied as In recent years, nitride ceramics such as titanium nitride (TiN) have been proposed and studied new plasmonic materials operating at higher temperatures (T > 1500 K [42–45]). Melting points of as new plasmonic materials operating at higher temperatures (T > 1500 K [42–45]). Melting points of typical plasmonic materials and nitride ceramics are summarized in Table 1. The melting points of typical plasmonic materials and nitride ceramics are summarized in Table 1. The melting points of these materials are similar to conventional refractory metals, and the permittivity of these materials is these materials are similar to conventional refractory metals, and the permittivity of these materials negative in the visible range. Hence, those are called “refractory plasmonic materials.” is negative in the visible range. Hence, those are called “refractory plasmonic materials.” Table 1. Refractory metals and refractory plasmonic materials in order of its melting point. Table 1. Refractory metals and refractory plasmonic materials in order of its melting point. Material Melting Point (K) Permittivity in Visible Range Material Melting Point (K) Permittivity in Visible Range Ag 1235 ND Ag 1235 ND Au 1337 ND Au 1337 ND SiO 1983 D SiO2 1983 D Mo 2896 D Mo 2896 D HfO 3031 D 2 HfO2 3031 D TiN 3203 ND TiN 3203 ND Ta 3290 D/ND Ta 3290 D/ND HfN HfN 3607 3607 ND ND W W 3695 3695 D D ND: Negati ND: Negative veDielectric; Dielectric; D D: Dielectric. : Dielectric. In this study, we used hafnium nitride (HfN) since the melting point of HfN is higher than that In this study, we used hafnium nitride (HfN) since the melting point of HfN is higher than that of of TiN and it is the same order as W. The most crucial property of nitride ceramics is that its TiN and it is the same order as W. The most crucial property of nitride ceramics is that its permittivity permittivity is negative in the visible range like the noble metals. Such a feature is useful for is negative in the visible range like the noble metals. Such a feature is useful for plasmonic materials. plasmonic materials. The spectral permittivity of conventional and plasmonic refractory metals are The spectral permittivity of conventional and plasmonic refractory metals are shown in Appendix C. shown in Appendix C. If we realize plasmonic metasurfaces using plasmonic refractory materials If we realize plasmonic metasurfaces using plasmonic refractory materials instead of noble metals, we instead of noble metals, we can control the thermal radiation spectra more precisely and obtain higher can control the thermal radiation spectra more precisely and obtain higher Q-value of the plasmonic Q-value of the plasmonic cavity than that of the microcavity. cavity than that of the microcavity. Figure 9 shows a schematic of a cross-sectional view of a refractory MDM metasurface, where Figure 9 shows a schematic of a cross-sectional view of a refractory MDM metasurface, where the Fabry–Pérot (FP) plasmonic resonator disk type based on HfN are arranged in a periodic array. the Fabry–Pérot (FP) plasmonic resonator disk type based on HfN are arranged in a periodic array. The diameter d, the period P of the resonator, the gap thickness in the dielectric layer, Tg, and the top The diameter d, the period P of the resonator, the gap thickness in the dielectric layer, T , and the top metal layer (HfN) Td are shown in Figure 9. We note that the dielectric layer should be selected in metal layer (HfN) T are shown in Figure 9. We note that the dielectric layer should be selected in accordance with the operating temperature T as such that HfO2 for T > 2000 K or SiO2 for T < 2000 K. accordance with the operating temperature T as such that HfO for T > 2000 K or SiO for T < 2000 K. 2 2 Photonics 2019, 6, x FOR PEER REVIEW 10 of 20 Photonics 2019, 6, 105 10 of 20 Photonics 2019, 6, x FOR PEER REVIEW 10 of 20 Figure 9. A schematic and cross-sectional view of a metal-dielectric-metal (MDM) metasurface based on hafnium nitride (HfN). Figure 9. A schematic and cross-sectional view of a metal-dielectric-metal (MDM) metasurface based Figure 9. A schematic and cross-sectional view of a metal-dielectric-metal (MDM) metasurface based on hafnium nitride (HfN). on hafnium nitride (HfN). To confirm the efficiency of thermal radiation control by this refractory plasmonic metasurface, we calculated the theoretical radiation spectrum of the MDM metasurface and compared it to a To conﬁrm the eciency of thermal radiation control by this refractory plasmonic metasurface, we To confirm the efficiency of thermal radiation control by this refractory plasmonic metasurface, blackbody surface under the condition that both radiation powers are identical, i.e., a constant power calculated the theoretical radiation spectrum of the MDM metasurface and compared it to a blackbody we calculated the theoretical radiation spectrum of the MDM metasurface and compared it to a mode as described in Section 4.2. Figure 10 shows the simulated results for the thermal radiation surface under the condition that both radiation powers are identical, i.e., a constant power mode as blackbody surface under the condition that both radiation powers are identical, i.e., a constant power spectrum obtained from the metasurface composed of HfN and HfO2 at T = 2500 K (red line) with d described in Section 4.2. Figure 10 shows the simulated results for the thermal radiation spectrum mode as described in Section 4.2. Figure 10 shows the simulated results for the thermal radiation = 40 nm, P = 80 nm, Tg = 60 nm, and Td = 20 nm. Here, we observe that the highest power radiated obtained from the metasurface composed of HfN and HfO at T = 2500 K (red line) with d = 40 nm, spectrum obtained from the metasurface composed of HfN and HfO2 at T = 2500 K (red line) with d from the metasurface is focused on the resonant peak at ~700 nm with a full-width half-maximum P = 80 nm, T = 60 nm, and T = 20 nm. Here, we observe that the highest power radiated from the g d = 40 nm, P = 80 nm, Tg = 60 nm, and Td = 20 nm. Here, we observe that the highest power radiated (FWHM) value of 571 nm due to the plasmonic resonance in an FP resonator disk. From Figure 10, metasurface is focused on the resonant peak at ~700 nm with a full-width half-maximum (FWHM) from the metasurface is focused on the resonant peak at ~700 nm with a full-width half-maximum the equivalent power from the metasurface at T = 2500 K corresponds to the power from a blackbody value of 571 nm due to the plasmonic resonance in an FP resonator disk. From Figure 10, the equivalent (FWHM) value of 571 nm due to the plasmonic resonance in an FP resonator disk. From Figure 10, at T = 1777 K. This means that radiated power from the metasurface at T = 2500 K equals that from power from the metasurface at T = 2500 K corresponds to the power from a blackbody at T = 1777 K. the equivalent power from the metasurface at T = 2500 K corresponds to the power from a blackbody the blackbody at only T = 1777 K. According to the Stefan–Boltzmann law, the efficiency is improved This means that radiated power from the metasurface at T = 2500 K equals that from the blackbody at T = 1777 K. This means that radiated power from the metasurface at T = 2500 K equals that from by a factor of (2500/1700) = 3.9; i.e., the metasurface is 3.9 times more efficient than the blackbody at only T = 1777 K. According to the Stefan–Boltzmann law, the eciency is improved by a factor of the blackbody at only T = 1777 K. According to the Stefan–Boltzmann law, the efficiency is improved from the viewpoint of power consumption. Additionally, from Figure 10, the radiation intensity at (2500/1700) = 3.9; i.e., the metasurface is 3.9 times more ecient than the blackbody from the viewpoint by a factor of (2500/1700) = 3.9; i.e., the metasurface is 3.9 times more efficient than the blackbody the plasmonic resonant wavelength is more than 10 times greater than that of the blackbody at T = of power consumption. Additionally, from Figure 10, the radiation intensity at the plasmonic resonant from the viewpoint of power consumption. Additionally, from Figure 10, the radiation intensity at 1777 K. wavelength is more than 10 times greater than that of the blackbody at T = 1777 K. the plasmonic resonant wavelength is more than 10 times greater than that of the blackbody at T = 1777 K. Figure 10. Simulated thermal radiation spectra in constant power mode: radiation spectra from the Figure 10. Simulated thermal radiation spectra in constant power mode: radiation spectra from the MDM metasurface (red line) composed of HfN and HfO at T = 2500 K with d = 40 nm, P = 80 nm, MDM metasurface (red line) composed of HfN and HfO2 at T = 2500 K with d = 40 nm, P = 80 nm, Tg T Figure 10. = 60 nm, T Simu = 20 lated thermal radiation nm, and the referencespec blackbody tra in consta (blue nt line) power m at T = 1777 ode: r K. adiation spectra from the = 60 nm, Td = 20 nm, and the reference blackbody (blue line) at T = 1777 K. MDM metasurface (red line) composed of HfN and HfO2 at T = 2500 K with d = 40 nm, P = 80 nm, Tg 5.2. Fabrication = 60 nm, Td = 20 nm, and the reference blackbody (blue line) at T = 1777 K. 5.2. Fabrication To demonstrate the thermal radiation control by a refractory plasmonic metasurface, we fabricated To demonstrate the thermal radiation control by a refractory plasmonic metasurface, we 5.2. Fabrication MDM metasurfaces based on HfN. In this study, we designed the metasurface to be a perfect absorber fabricated MDM metasurfaces based on HfN. In this study, we designed the metasurface to be a in the mid-IR range (~4 m) instead of in the visible as the ﬁrst step towards the fabrication of a To demonstrate the thermal radiation control by a refractory plasmonic metasurface, we perfect absorber in the mid-IR range (~4 µm) instead of in the visible as the first step towards the fabricated MDM metasurfaces based on HfN. In this study, we designed the metasurface to be a perfect absorber in the mid-IR range (~4 µm) instead of in the visible as the first step towards the Photonics 2019, 6, x FOR PEER REVIEW 11 of 20 Photonics 2019, 6, 105 11 of 20 fabrication of a “plasmonic” thermal radiation light source. To design and optimize the size “plasmonic” thermal radiation light source. To design and optimize the size parameters for a perfect parameters for a perfect absorber operating at ~4 µm, we performed numerical simulations using the absorber operating at ~4 m, we performed numerical simulations using the commercially available commercially available finite-difference time-domain method (FDTD) software (Lumerical Inc., ﬁnite-dierence time-domain method (FDTD) software (Lumerical Inc., Vancouver, BC, Canada) for Vancouver, BC, Canada) for the metasurface composed of HfN and SiO2 as shown in Appendix F. the metasurface composed of HfN and SiO as shown in Appendix F. From Figure A6, the designed From Figure A6, the designed value of diameter d = 1.2 µm was determined for achieving the value of diameter d = 1.2 m was determined for achieving the absorption peak of 4 m. absorption peak of 4 µm. Figure 11 shows the MDM metasurface sample with d = 1.14 m, P = 2.0 m, T = 130 nm, and Figure 11 shows the MDM metasurface sample with d = 1.14 µm, P = 2.0 µm, Tg g = 130 nm, and Td T= 20 = 0 nm 200 nm. . ThThe e me metasurface tasurface wa was s fafabricated bricated on ona a115 5 × 115 5 m mm m s squar quare e q quartz uartz substrate substrate us using ing R RFF sputtering and electron beam (EB) lithography. The details of the fabrication process are described sputtering and electron beam (EB) lithography. The details of the fabrication process are described in in Appendix D Appendix .D The SE . The SEM M ima image ge ofof met meta-atoms a-atoms isis show shown n in in FFigur igure e111 1c.c. Add Additionally itionally, we , we fabr fabricated icated a a blackbody reference sample by spraying a blackbody spray (TA410KS, Ichinen TASCO Co., Ltd., blackbody reference sample by spraying a blackbody spray (TA410KS, Ichinen TASCO Co., Ltd., Osaka, Osaka, J Japan) apan) t too t the he 1 155 × 15 15 m mm msquar squar eequartz quartzsubstrate, substrate, as as shown shown in F in Figur igue re11 11 a.a. Thi Thiss b blackbody lackbody reference has an average absorptivity of ~0.989 at = 3–10 m. reference has an average absorptivity of α ~0.989 at λ = 3–10 µm. Figure 11. Refractory MDM metasurface and blackbody reference: (a) a photograph of the blackbody Figure 11. Refractory MDM metasurface and blackbody reference: (a) a photograph of the blackbody reference sample, (b) the MDM metasurface sample composed of HfN and SiO2 on a quartz substrate reference sample, (b) the MDM metasurface sample composed of HfN and SiO on a quartz substrate with d = 1.14 µm, P = 2.0 µm, Tg = 130 nm, and Td = 200 nm. The patterned area is 10 × 10 mm, and (c) with d = 1.14 m, P = 2.0 m, T = 130 nm, and T = 200 nm. The patterned area is 10 10 mm, and g d SEM image of plasmonic resonators. (c) SEM image of plasmonic resonators. 5.3. Measurements 5.3. Measurements Before we measured the thermal radiation, we measured the spectral reﬂectivity R() of the Before we measured the thermal radiation, we measured the spectral reflectivity R(λ) of the metasurface at = 3–12 m using a confocal infrared microscope (HYPERION2000, Bruker Inc., metasurface at λ = 3–12 µm using a confocal infrared microscope (HYPERION2000, Bruker Inc., Billerica, MA, USA) and a Fourier transform infrared (FTIR) spectrometer (VERTEX 70v, Bruker Inc., Billerica, MA, USA) and a Fourier transform infrared (FTIR) spectrometer (VERTEX 70v, Bruker Inc., Billerica, MA, USA) at room temperature. IR light partially shielded by slits was focused on a sample Billerica, MA, USA) at room temperature. IR light partially shielded by slits was focused on a sample through an 15 (NA: 0.4) Schwarzschild objective lens. The reﬂected light from the sample was through an ×15 (NA: 0.4) Schwarzschild objective lens. The reflected light from the sample was corrected through the objective using a detector and converted through a Fourier transform to calculate corrected through the objective using a detector and converted through a Fourier transform to reﬂectivity. Spectral absorptivity, A(), can be calculated by A() = 1 R() if the sample is opaque. calculate reflectivity. Spectral absorptivity, A(λ), can be calculated by A(λ) = 1 − R(λ) if the sample is The thermal radiation spectra were measured using an FTIR spectrometer (FT/IR 6000, JASCO opaque. Co., Tokyo, Japan) at = 3–12 m. The setup of the thermal radiation measurement is shown in The thermal radiation spectra were measured using an FTIR spectrometer (FT/IR 6000, JASCO Appendix E. To avoid oxidation of the surface, the sample was set in a vacuum chamber connected to Co., Tokyo, Japan) at λ = 3–12 µm. The setup of the thermal radiation measurement is shown in the FTIR spectrometer and heated on a ceramic heater. A DC power supply was used to control the Appendix E. To avoid oxidation of the surface, the sample was set in a vacuum chamber connected temperature. The temperature was measured by a thermocouple placed on the surface of the sample. to the FTIR spectrometer and heated on a ceramic heater. A DC power supply was used to control The radiation spectra were measured for both the metasurface and the blackbody sample at 573 K. the temperature. The temperature was measured by a thermocouple placed on the surface of the Hence, all measurements were done under the constant temperature of 573 K. sample. The radiation spectra were measured for both the metasurface and the blackbody sample at 573 K. Hence, all measurements were done under the constant temperature of 573 K. 5.4. Results and Discussion 5.4. Results and Discussion The measured absorptivity spectrum of the MDM metasurface at room temperature is shown in Figure 12a. We observed a single resonant peak at 4.11 m. To identify the physical origin of the The measured absorptivity spectrum of the MDM metasurface at room temperature is shown in peak, we calculated the spectral absorptivity (see the dashed line in Figure 12a) and ﬁeld distribution Figure 12a. We observed a single resonant peak at 4.11 µm. To identify the physical origin of the peak, by the FDTD method. The measured spectrum is in good agreement with the simulated spectrum. we calculated the spectral absorptivity (see the dashed line in Figure 12a) and field distribution by Figure 12b shows a cross-sectional view of the spatial distribution of the electric ﬁeld normal to the the FDTD method. The measured spectrum is in good agreement with the simulated spectrum. Figure 12b shows a cross-sectional view of the spatial distribution of the electric field normal to the Photonics 2019, 6, 105 12 of 20 Photonics 2019, 6, x FOR PEER REVIEW 12 of 20 incident electric ﬁeld at 4.11 m around a meta-atom (plasmonic cavity). Here, we can conﬁrm that incident electric field at 4.11 µm around a meta-atom (plasmonic cavity). Here, we can confirm that the gap plasmon is excited to an FP resonant mode between two metal layers. Hence, the peak in the gap plasmon is excited to an FP resonant mode between two metal layers. Hence, the peak in absorptivity around 4 m is attributed to the plasmonic resonance inside a single plasmonic cavity. absorptivity around 4 µm is attributed to the plasmonic resonance inside a single plasmonic cavity. Note that the resonant peak position is robust against incident angle for both p- and s-polarizations Note that the resonant peak position is robust against incident angle for both p- and s-polarizations as shown in Appendix G. The measured FWHM of the peak (~2 m) is higher than the simulated as shown in Appendix G. The measured FWHM of the peak (~2 µm) is higher than the simulated value (~1.5 m) while the peak position and the peak value are red-shifted slightly and decreased, value (~1.5 µm) while the peak position and the peak value are red-shifted slightly and decreased, respectively. The dierence in the FWHM is attributed to the unexpected loss increase in the real respectively. The difference in the FWHM is attributed to the unexpected loss increase in the real materials. The dierence in the peak value is probably due to the o-axial arrangement of the incident materials. The difference in the peak value is probably due to the off-axial arrangement of the incident light through the Schwarzschild objective lens of the infrared microscope. From this measurement, light through the Schwarzschild objective lens of the infrared microscope. From this measurement, we were able to conﬁrm that the sample was correctly fabricated and operating as designed for a we were able to confirm that the sample was correctly fabricated and operating as designed for a perfect absorber. perfect absorber. Figure 12. Absorptivity spectra and electric ﬁeld distribution of the MDM metasurface composed of Figure 12. Absorptivity spectra and electric field distribution of the MDM metasurface composed of HfN and SiO with P = 2.0 m, d = 1.14 m, T = 130 nm, and T = 200 nm: (a) measured (solid line) HfN and SiO2 2with P = 2.0 µm, d = 1.14 µm, Tg = 130 g nm, and Td = 200 d nm: (a) measured (solid line) and and simulated (dashed line) absorptivity spectra at room temperature, and (b) normalized electric ﬁeld simulated (dashed line) absorptivity spectra at room temperature, and (b) normalized electric field distribution around the meta-atom for the resonance at 4.11 m. distribution around the meta-atom for the resonance at 4.11 µm. Next, we performed a thermal radiation experiment. Figure 13a shows the thermal radiation Next, we performed a thermal radiation experiment. Figure 13a shows the thermal radiation spectra at 573 K for the MDM metasurface and reference blackbody sample. We observe that the spectra at 573 K for the MDM metasurface and reference blackbody sample. We observe that the radiation intensity is suppressed at > 5 m compared with the blackbody level. Such suppression radiation intensity is suppressed at λ > 5 µm compared with the blackbody level. Such suppression is caused by the lower absorptivity (emissivity) at > 5 m, as seen in Figure 12a. Additionally, we is caused by the lower absorptivity (emissivity) at λ > 5 µm, as seen in Figure 12a. Additionally, we can derive the spectral emissivity, " (), of the metasurface from Figure 13a. From Kirchho’s law, can derive the spectral emissivity, ε (λ), of the metasurface from Figure 13a. From Kirchhoff’s law, this must be equal to () shown in Figure 12a if the temperature of a sample is identical. Figure 13b this must be equal to α (λ) shown in Figure 12a if the temperature of a sample is identical. Figure 13b shows the measured spectral emissivity at 573 K of the MDM metasurface. The resonant peak value shows the measured spectral emissivity at 573 K of the MDM metasurface. The resonant peak value of " = ~1 at 4.1 m with FWHM of ~3 m is obtained from Figure 13a. This is consistent with the of ε = ~1 at 4.1 µm with FWHM of ~3 µm is obtained from Figure 13a. This is consistent with the simulated results for absorptivity in Figure 12a (see also the solid line in Figure 13b). These results simulated results for absorptivity in Figure 12a (see also the solid line in Figure 13b). These results suggest that perfect absorption/emission occurred as designed, and the cavity loss was increased due suggest that perfect absorption/emission occurred as designed, and the cavity loss was increased due to the temperature increase. The FWHM of the measured peak actually is broader than the calculated to the temperature increase. The FWHM of the measured peak actually is broader than the calculated result of ~1.5 m. This is evidence of the loss increase caused by the thermal eect. result of ~1.5 µm. This is evidence of the loss increase caused by the thermal effect. Finally, we note that the measured radiation spectrum in Figure 13a is not an intrinsic radiation spectrum, but it includes the transmission function of the optical system in the spectrometer (see Appendix E). Hence, it is necessary to separate it out so we can estimate the intrinsic radiation spectrum from the sample. Since we obtained " (), as shown in Figure 13b, we can determine the intrinsic radiation spectrum of the sample by calculating the product of " () and Planck’s law (Equation (1)). Figure 14 shows the presumed spectrum of the intrinsic radiation as well as the blackbody radiation at 573 K. It is clearly observed that thermal radiation from the metasurface is signiﬁcantly suppressed at longer wavelength region at > 5 m while the surface temperature of the sample is 573 K. Here, we Photonics 2019, 6, x FOR PEER REVIEW 13 of 20 Photonics 2019, 6, 105 13 of 20 point out a crucial fact that the area under the spectral curve of the metasurface is much smaller than that of the blackbody. This indicates that the radiative power from the metasurface is signiﬁcantly suppressed compared with the blackbody resulting in the achievement of an ecient IR emitter, i.e., we are able to heat a sample quite eciently by a small amount of power. Such behavior is typical for constant-temperature-mode measurements, which is dierent from the constant power mode. Photonics 2019, 6, x FOR PEER REVIEW 13 of 20 Figure 13. Experimental thermal radiation spectra for the MDM metasurface: (a) thermal radiation spectra for the MDM metasurface (red line) and blackbody reference sample (solid line) at 573 K. (b) Experimental spectral emissivity at 573 K (red line) derived from (a) and simulated absorptivity at room temperature (solid line). Finally, we note that the measured radiation spectrum in Figure 13a is not an intrinsic radiation spectrum, but it includes the transmission function of the optical system in the spectrometer (see Appendix E). Hence, it is necessary to separate it out so we can estimate the intrinsic radiation spectrum from the sample. Since we obtained ε (λ), as shown in Figure 13b, we can determine the intrinsic radiation spectrum of the sample by calculating the product of ε (λ) and Planck’s law (Equation (1)). Figure 14 shows the presumed spectrum of the intrinsic radiation as well as the blackbody radiation at 573 K. It is clearly observed that thermal radiation from the metasurface is significantly suppressed at longer wavelength region at λ > 5 µm while the surface temperature of the sample is 573 K. Here, we point out a crucial fact that the area under the spectral curve of the metasurface is much smaller than that of the blackbody. This indicates that the radiative power from the metasurface is significantly suppressed compared with the blackbody resulting in the Figure 13. Experimental thermal radiation spectra for the MDM metasurface: (a) thermal radiation Figure 13. Experimental thermal radiation spectra for the MDM metasurface: (a) thermal radiation achievement of an efficient IR emitter, i.e., we are able to heat a sample quite efficiently by a small spectra for the MDM metasurface (red line) and blackbody reference sample (solid line) at 573 K. spectra for the MDM metasurface (red line) and blackbody reference sample (solid line) at 573 K. (b) amount of power. Such behavior is typical for constant-temperature-mode measurements, which is (b) Experimental spectral emissivity at 573 K (red line) derived from (a) and simulated absorptivity at Experimental spectral emissivity at 573 K (red line) derived from (a) and simulated absorptivity at different from the constant power mode. room temperature (solid line). room temperature (solid line). Finally, we note that the measured radiation spectrum in Figure 13a is not an intrinsic radiation spectrum, but it includes the transmission function of the optical system in the spectrometer (see Appendix E). Hence, it is necessary to separate it out so we can estimate the intrinsic radiation spectrum from the sample. Since we obtained ε (λ), as shown in Figure 13b, we can determine the intrinsic radiation spectrum of the sample by calculating the product of ε (λ) and Planck’s law (Equation (1)). Figure 14 shows the presumed spectrum of the intrinsic radiation as well as the blackbody radiation at 573 K. It is clearly observed that thermal radiation from the metasurface is significantly suppressed at longer wavelength region at λ > 5 µm while the surface temperature of the sample is 573 K. Here, we point out a crucial fact that the area under the spectral curve of the metasurface is much smaller than that of the blackbody. This indicates that the radiative power from the metasurface is significantly suppressed compared with the blackbody resulting in the achievement of an efficient IR emitter, i.e., we are able to heat a sample quite efficiently by a small amount of power. Such behavior is typical for constant-temperature-mode measurements, which is different from the constant power mode. Figure 14. Calculated thermal radiation spectra at 573 K: The radiation spectrum from the MDM Figure 14. Calculated thermal radiation spectra at 573 K: The radiation spectrum from the MDM metasurface (red line) is calculated from the measured emissivity shown in Figure 13b. The theoretical metasurface (red line) is calculated from the measured emissivity shown in Figure 13b. The theoretical blackbody radiation spectrum (Equation (1)) at 573 K (solid line) is plotted for reference. blackbody radiation spectrum (Equation (1)) at 573 K (solid line) is plotted for reference. 6. Conclusions We fabricated a prototype of microcavity lamp by a nanoimprint method that is suitable for mass production and demonstrated to control visible-light spectrum using a refractory metasurface made of Ta with an optical microcavity implemented into an incandescent light bulb. It was conﬁrmed that thermal radiation intensity from the microcavity ﬁlament was increased 1.8 times compared to the ﬂat ﬁlament under the constant power input. Then, we fabricated and demonstrated the thermal radiation control in mid-IR range by using an MDM plasmonic metasurface composed of a refractory plasmonic cavity made of HfN. A single narrow resonant peak was observed at designed wavelength Figure 14. Calculated thermal radiation spectra at 573 K: The radiation spectrum from the MDM metasurface (red line) is calculated from the measured emissivity shown in Figure 13b. The theoretical blackbody radiation spectrum (Equation (1)) at 573 K (solid line) is plotted for reference. Photonics 2019, 6, 105 14 of 20 as well as the suppression of thermal radiation in wide mid-IR range under the condition of constant surface temperature. We revaluated a thermal radiation light source as an ecient light source from the perspective of energy conversion. For a future energy-saving society, it is vital to reconsider thermal radiation sources as energy-saving technology. Author Contributions: J.T. conceived the idea of incandescent light bulbs based on refractory metasurface. H.T. and A.K. performed the numerical simulations. H.T. and K.K. performed the experiments. J.T. analyzed the experimental data and wrote the initial draft of the manuscript. J.T. supervised the project. All the authors discussed the results and contributed to the writing of the manuscript. Funding: This research was funded in part by the Photonics Advanced Research Center (PARC) from the Ministry of Education, Culture, Sports, Science and Technology, Japan (MEXT) and the JSPS Core-to-Core Program, and A. Advanced Research Networks (Advanced Nanophotonics in the Emerging Fields of Nano-imaging, Spectroscopy, Nonlinear Optics, Plasmonics/Metamaterials, and Devices). Acknowledgments: We would like to thank Yosuke Ueba and Yusuke Nagasaki for useful discussions. A part of this work was supported by the “Nanotechnology Platform Project (Nanotechnology Open Facilities in Osaka University)” of the Ministry of Education, Culture, Sports, Science, and Technology, Japan [No.: F-17-OS-0011, S-17-OS-0011]. Conﬂicts of Interest: The authors declare no conﬂicts of interest. The funders had no role in the design of the study, in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results. Appendix A The microcavity ﬁlaments were fabricated by a nanoimprint process suitable for mass production as described below. Figure A1 shows the fabrication process of a microcavity array onto a Ta substrate. At ﬁrst, a 3-inch Si wafer (thickness 380m) was prepared, and the electron beam (EB) resist (ZEP520A-7) spin-coated to a thickness of 300 nm using a spinner for 60 s at 4000 rpm. It was then baked for 5 min at 180 C. After that, a 20 20 mm square microcavity array pattern was drawn onto the EB resist using a 50-kV electron beam (EB) lithography system (F5112+VD01; ADVANTEST Co., Tokyo, Japan). After developing the resist, the Si wafer was dry-etched by an inductively coupled plasma (ICP) etching machine (EIS-700, ELIONIX Inc., Tokyo, Japan) to transfer the pattern to the substrate, resulting in a Si master mold with 300 300 nm square holes, 300 nm wall width, and approximately 200 nm depth. Then, the master mold was duplicated onto a photocrosslinkable resin ﬁlm under heat and high-pressure conditions, resulting in an intermediate resin membrane (IRM). These IRMs are used for mass production in the future so the master mold can be preserved. Next, we prepared polished Ta substrates (20 20 mm squares with a thickness of 100 m) for making ﬁlaments. A 30-nm-thickness Cr layer was deposited on the Ta substrate, and a photoresist (MUR-XR2-150, Maruzen Petrochemical Co., Ltd., Tokyo, Japan) was spin-coated on the substrate to a thickness of 215 nm by using a spinner at 3000 rpm. Then, the sample was placed in a vacuum chamber, and the IRM placed on a quartz cylinder was pressed onto the resist under UV light irradiation, resulting in stamping the pattern onto the resist residing on the substrate with the Cr layer. Since the IRM and the cylinder are transparent, it can be used as a template to transfer a pattern to a photoresist under UV irradiation. After removing the IRM, the Ta substrate was dry-etched through the resist and the Cr mask ﬁlm using an electron cyclotron resonance (ECR) ion shower machine (EIS-200ER, ELIONIX Inc., Tokyo, Japan) and ICP etching machine (EIS-700, ELIONIX Inc., Tokyo, Japan). The etching depths are 3.2 nm for the Cr layer and 18 nm for the Ta substrate. Finally, the resist pattern was transferred to the Ta substrate, as shown in Figure 4b. The resulting cavity size was wider than the mold (350 350 nm square holes, 250-nm wall width, and ~280-nm depth). Note that such a widening eect was caused by the dry-etching process and was calibrated by designing the EB lithography process. We fabricated three kinds of Ta substrate: (i) the microcavity on a single side, (ii) the microcavity on both sides, and (iii) a plane without patterning for reference. In the case of (i), we performed the Photonics 2019, 6, x FOR PEER REVIEW 15 of 20 After removing the IRM, the Ta substrate was dry-etched through the resist and the Cr mask film using an electron cyclotron resonance (ECR) ion shower machine (EIS-200ER, ELIONIX Inc., Tokyo, Japan) and ICP etching machine (EIS-700, ELIONIX Inc., Tokyo, Japan). The etching depths are 3.2 nm for the Cr layer and 18 nm for the Ta substrate. Finally, the resist pattern was transferred to the Ta substrate, as shown in Figure 4b. The resulting cavity size was wider than the mold (350 × 350 nm square holes, 250-nm wall width, and ~280-nm depth). Note that such a widening effect was Photonics 2019, 6, 105 15 of 20 caused by the dry-etching process and was calibrated by designing the EB lithography process. We fabricated three kinds of Ta substrate: (i) the microcavity on a single side, (ii) the microcavity on both sides, and (iii) a plane without patterning for reference. In the case of (i), we performed the process once. For (ii), we repeated the process twice. Finally, the Ta substrates were cut into narrow process once. For (ii), we repeated the process twice. Finally, the Ta substrates were cut into narrow 500-m strips as shown in Figure 4a. 500-µm strips as shown in Figure 4a. Figure A1. Nanoimprint process for fabricating microcavity ﬁlaments. Figure A1. Nanoimprint process for fabricating microcavity filaments. Appendix B Appendix B Figure A2 shows the relative permittivity spectra of conventional refractory metals (W, Ta, and Figure A2 shows the relative permittivity spectra of conventional refractory metals (W, Ta, and Mo) [41]. The real part of the permittivity for W and Mo are both positive in the visible range (λ < 0.8 Mo) [41]. The real part of the permittivity for W and Mo are both positive in the visible range µm). The real part of the permittivity of Ta switches from negative to positive below 0.6 µm. The ( < 0.8 m). The real part of the permittivity of Ta switches from negative to positive below 0.6 m. imaginary part of the permittivity of Ta is ~1/2 that of W and Mo. The imaginary part of the permittivity of Ta is ~1/2 that of W and Mo. Photonics 2019, 6, x FOR PEER REVIEW 16 of 20 Figure A2. Spectral relative permittivities of conventional refractory metals (W, Ta, and Mo): (a) real Figure A2. Spectral relative permittivities of conventional refractory metals (W, Ta, and Mo): (a) real and (b) imaginary part of the relative permittivity [41]. and (b) imaginary part of the relative permittivity [41]. Appendix C Figure A3 shows the relative permittivity spectra of conventional plasmonic metals (Au, Ag, and Al) [41], and refractory plasmonic materials (TiN and HfN) [43]. The real part of the permittivity of HfN is negative within the entire visible range and is very close to that of Au at λ < 0.6 µm. The imaginary part of the permittivity of HfN is ~1/2 that of TiN in the visible range. Figure A3. Spectral relative permittivity of conventional plasmonic metals (Au, Ag, and Al) [41] and refractory plasmonic materials (TiN and HfN) [43]: (a) real and (b) imaginary part of the relative permittivities. Appendix D Figure A4 shows the fabrication process for the refractory MDM metasurface. First, a HfN layer and a 130-nm-thick SiO2 layer were deposited on a quartz substrate (15 × 15 mm) using an RF sputtering system (SVC-700LRF, SANYU Electron, Tokyo, Japan). In fabricating the HfN layer, we used an HfN target (Toshima Manufacturing Co., Ltd., Saitama, Japan) under an Ar gas flow rate of −4 25 sccm at 2 × 10 Pa. Next, hexamethyldisilazane (HMDS) was spin-coated using a spinner for 90 s at 5000 rpm. Then, a photoresist (TSMR-8900) was spin-coated to a thickness of 700 nm using a spinner for 90 s at 5000 rpm. The metasurface patterns were exposed using a mask-less UV lithography system (DL-1000, Nanosystem Solutions Inc., Tokyo, Japan) then developed. Next, a 200- Photonics 2019, 6, x FOR PEER REVIEW 16 of 20 Photonics 2019, 6, 105 16 of 20 Figure A2. Spectral relative permittivities of conventional refractory metals (W, Ta, and Mo): (a) real and (b) imaginary part of the relative permittivity [41]. Appendix C Appendix C Figure A3 shows the relative permittivity spectra of conventional plasmonic metals (Au, Ag, and Figure A3 shows the relative permittivity spectra of conventional plasmonic metals (Au, Ag, and Al) [41], and refractory plasmonic materials (TiN and HfN) [43]. The real part of the permittivity of HfN Al) [41], and refractory plasmonic materials (TiN and HfN) [43]. The real part of the permittivity of is negative within the entire visible range and is very close to that of Au at < 0.6 m. The imaginary HfN is negative within the entire visible range and is very close to that of Au at λ < 0.6 µm. The part of the permittivity of HfN is ~1/2 that of TiN in the visible range. imaginary part of the permittivity of HfN is ~1/2 that of TiN in the visible range. Figure A3. Spectral relative permittivity of conventional plasmonic metals (Au, Ag, and Al) [41] and Figure A3. Spectral relative permittivity of conventional plasmonic metals (Au, Ag, and Al) [41] refractory plasmonic materials (TiN and HfN) [43]: (a) real and (b) imaginary part of the relative and refractory plasmonic materials (TiN and HfN) [43]: (a) real and (b) imaginary part of the permittivities. relative permittivities. Appendix D Appendix D Figure A4 shows the fabrication process for the refractory MDM metasurface. First, a HfN layer Figure A4 shows the fabrication process for the refractory MDM metasurface. First, a HfN and a 130-nm-thick SiO2 layer were deposited on a quartz substrate (15 × 15 mm) using an RF layer and a 130-nm-thick SiO layer were deposited on a quartz substrate (15 15 mm) using an RF sputtering system (SVC-700LRF, SANYU Electron, Tokyo, Japan). In fabricating the HfN layer, we sputtering system (SVC-700LRF, SANYU Electron, Tokyo, Japan). In fabricating the HfN layer, we used an HfN target (Toshima Manufacturing Co., Ltd., Saitama, Japan) under an Ar gas flow rate of used an HfN target (Toshima Manufacturing Co., Ltd., Saitama, Japan) under an Ar gas ﬂow rate −4 25 sccm at 2 × 10 Pa. Next, hexamethyldisilazane (HMDS) was spin-coated using a spinner for 90 s of 25 sccm at 2 10 Pa. Next, hexamethyldisilazane (HMDS) was spin-coated using a spinner at 5000 rpm. Then, a photoresist (TSMR-8900) was spin-coated to a thickness of 700 nm using a for 90sp sinner fo at 5000r rpm. 90 s at Then, 5000 rp a photor m. The m esistet(TSMR-8900) asurface patterns wer was spin-coated e exposed u tosing a thickness a mask-less of 700 UV nm lithography system (DL-1000, Nanosystem Solutions Inc., Tokyo, Japan) then developed. Next, a 200- using a spinner for 90 s at 5000 rpm. The metasurface patterns were exposed using a mask-less UV lithography system (DL-1000, Nanosystem Solutions Inc., Tokyo, Japan) then developed. Next, a Photonics 2019, 6, x FOR PEER REVIEW 17 of 20 200-nm-thick HfN layer was deposited by RF sputtering. Finally, the HfN layer was lifted o using nm-thick HfN layer was deposited by RF sputtering. Finally, the HfN layer was lifted off using N- N-methyl-2-pyrrolidone (NMP). methyl-2-pyrrolidone (NMP). Figure A4. Fabricating the refractory MDM metasurface. Figure A4. Fabricating the refractory MDM metasurface. Appendix E Figure A5 shows the experimental setup for measuring the thermal radiation spectrum. The optical system was placed in a vacuum chamber connected to an FTIR spectrometer (FT/IR 6000, JASCO Co., Tokyo, Japan) through a tunnel tube. The vacuum chamber and the FTIR were pumped 2 2 −1 to 2.0 × 10 Pa and 1.4 × 10 Pa, respectively. The spectral resolution was set to 4 cm , and a DLATGS detector was used for the measurement. The device is shown in Figure 12 and was placed on a micro- ceramic heater (MS-1000, Sakaguchi E.H VOC Corp., Tokyo, Japan). The temperature of the sample was measured by a K-type sheath thermocouple (T350251H, Sakaguchi E.H VOC Corp., Tokyo, Japan) placed on the surface of the sample. The measurements were performed at 573 K. Figure A5. Experimental setup for measuring the thermal radiation spectrum. The sample is set on a ceramic heater in a vacuum chamber that is connected to the FTIR spectrometer through a tunnel tube. Appendix F Figure A6 shows simulated spectral absorptivity to the diameter d of an MDM metasurface composed of HfN and SiO2 (see Figure 9) with P = 2.0 µm, Tg = 130 nm, and Td = 200 nm. The peak Photonics 2019, 6, x FOR PEER REVIEW 17 of 20 nm-thick HfN layer was deposited by RF sputtering. Finally, the HfN layer was lifted off using N- methyl-2-pyrrolidone (NMP). Photonics 2019, 6, 105 17 of 20 Figure A4. Fabricating the refractory MDM metasurface. Appendix E Figur Appendix E e A5 shows the experimental setup for measuring the thermal radiation spectrum. The optical system was placed in a vacuum chamber connected to an FTIR spectrometer (FT/IR 6000, Figure A5 shows the experimental setup for measuring the thermal radiation spectrum. The JASCO Co., Tokyo, Japan) through a tunnel tube. The vacuum chamber and the FTIR were pumped to optical system was placed in a vacuum chamber connected to an FTIR spectrometer (FT/IR 6000, 2 2 1 2.0 10 Pa and 1.4 10 Pa, respectively. The spectral resolution was set to 4 cm , and a DLATGS JASCO Co., Tokyo, Japan) through a tunnel tube. The vacuum chamber and the FTIR were pumped 2 2 −1 detector to 2.was 0 × 10used Pa and for 1.the 4 × 10 measur Pa, respecti ement. vely The . The spectra device is l resol shown ution in wa Figur s set to e 4 12 cm and , an was d a DL placed ATGSon a detector was used for the measurement. The device is shown in Figure 12 and was placed on a micro- micro-ceramic heater (MS-1000, Sakaguchi E.H VOC Corp., Tokyo, Japan). The temperature of the ceramic heater (MS-1000, Sakaguchi E.H VOC Corp., Tokyo, Japan). The temperature of the sample sample was measured by a K-type sheath thermocouple (T350251H, Sakaguchi E.H VOC Corp., Tokyo, was measured by a K-type sheath thermocouple (T350251H, Sakaguchi E.H VOC Corp., Tokyo, Japan) placed on the surface of the sample. The measurements were performed at 573 K. Japan) placed on the surface of the sample. The measurements were performed at 573 K. Figure A5. Experimental setup for measuring the thermal radiation spectrum. The sample is set on a Figure A5. Experimental setup for measuring the thermal radiation spectrum. The sample is set on a ceramic heater in a vacuum chamber that is connected to the FTIR spectrometer through a tunnel tube. ceramic heater in a vacuum chamber that is connected to the FTIR spectrometer through a tunnel tube. Appendix F Appendix F Figure A6 shows simulated spectral absorptivity to the diameter d of an MDM metasurface Photonics 2019, 6, x FOR PEER REVIEW 18 of 20 Figure A6 shows simulated spectral absorptivity to the diameter d of an MDM metasurface composed of HfN and SiO (see Figure 9) with P = 2.0 m, T = 130 nm, and T = 200 nm. The peak 2 g d composed of HfN and SiO2 (see Figure 9) with P = 2.0 µm, Tg = 130 nm, and Td = 200 nm. The peak position of absorption caused by gap plasmon mode in the circular cavity can be changed from 3.0 to position of absorption caused by gap plasmon mode in the circular cavity can be changed from 3.0 to 7.0 µm by changing d. 7.0 m by changing d. Figure A6. Simulated spectral absorptivity/emissivity map to the diameter of an MDM metasurface Figure A6. Simulated spectral absorptivity/emissivity map to the diameter of an MDM metasurface composed of HfN and SiO with P = 2.0 m, T = 130 nm, and T = 200 nm. 2 g d composed of HfN and SiO2 with P = 2.0 µm, Tg = 130 nm, and Td = 200 nm. Appendix G Figure A7 shows simulated spectral absorptivity to the incident angle to an MDM metasurface composed of HfN and SiO2 (see Figure 9). The single absorption peak caused by gap plasmon mode in the circular cavity is observed at ~4.5 µm for both polarizations, which is not strongly dependent on incident angle. The strong angle-dependent steep absorption is caused by diffraction at 2.5–4.0 µm only for p-polarization as shown in (a). Note that the designed value of d = 1.2 µm is slightly greater than that of the experimental value (d = 1.14 µm). Figure A7. Simulated spectral absorptivity/emissivity maps to the incident angle to an MDM metasurface composed of HfN and SiO2 with P = 2.0 µm, d = 1.2 µm, Tg = 130 nm, and Td = 200 nm: (a) p-polarization and (b) s-polarization. References 1. Takahara, J.; Ueba, Y.; Nagatsuma, T. Thermal radiation control by microcavity and ecological incandescent lamps. Jpn. J. Opt. 2010, 39, 482–488. Photonics 2019, 6, x FOR PEER REVIEW 18 of 20 position of absorption caused by gap plasmon mode in the circular cavity can be changed from 3.0 to 7.0 µm by changing d. Figure A6. Simulated spectral absorptivity/emissivity map to the diameter of an MDM metasurface Photonics 2019, 6, 105 18 of 20 composed of HfN and SiO2 with P = 2.0 µm, Tg = 130 nm, and Td = 200 nm. Appendix G Appendix G Figure A7 shows simulated spectral absorptivity to the incident angle to an MDM metasurface Figure A7 shows simulated spectral absorptivity to the incident angle to an MDM metasurface composed of HfN and SiO (see Figure 9). The single absorption peak caused by gap plasmon mode in composed of HfN and SiO 22 (see Figure 9). The single absorption peak caused by gap plasmon mode the circular cavity is observed at ~4.5 m for both polarizations, which is not strongly dependent on in the circular cavity is observed at ~4.5 µm for both polarizations, which is not strongly dependent incident angle. The strong angle-dependent steep absorption is caused by diraction at 2.5–4.0 m on incident angle. The strong angle-dependent steep absorption is caused by diffraction at 2.5–4.0 only for p-polarization as shown in (a). Note that the designed value of d = 1.2 m is slightly greater µm only for p-polarization as shown in (a). Note that the designed value of d = 1.2 µm is slightly than that of the experimental value (d = 1.14 m). greater than that of the experimental value (d = 1.14 µm). Figure A7. Simulated spectral absorptivity/emissivity maps to the incident angle to an MDM Figure A7. 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Incandescent Light Bulbs Based on a Refractory Metasurface

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Incandescent Light Bulbs Based on a Refractory Metasurface

Incandescent Light Bulbs Based on a Refractory Metasurface

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