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Abstract
hv photonics Article Ultra-Low-Loss and Broadband All-Silicon Dielectric Waveguides for WR-1 Band (0.75–1.1 THz) Modules Ratmalgre Koala * , Ryoma Maru, Kei Iyoda, Li Yi, Masayuki Fujita * and Tadao Nagatsuma Graduate School of Engineering Sciences, Osaka University, Toyonaka City, 560-8531, Japan; u248100f@alumni.osaka-u.ac.jp (R.M.); u986100d@ecs.osaka-u.ac.jp (K.I.); yi@ee.es.osaka-u.ac.jp (L.Y.); nagatsuma@ee.es.osaka-u.ac.jp (T.N.) * Correspondence: u665239a@ecs.osaka-u.ac.jp (R.K.); fujita@ee.es.osaka-u.ac.jp (M.F.) Abstract: This study presents ultra-low-loss and broadband all-silicon dielectric waveguides for the WR-1 band (0.75–1.1 THz). The waveguides are built in high-resistivity silicon (10 kW-cm) and integrated with supportive frames fabricated from the same silicon wafer in a single etch process to achieve a compact design. We pursued low-loss, broadband, substrateless, unclad and effective medium waveguides. Smaller propagation losses of 0.3 dB/cm and 0.1 dB/cm were achieved for the unclad and effective medium waveguides, respectively. The 3 dB bandwidth was not encountered in the frequency range of interest and was as broad as 350 GHz. An unclad waveguide was employed to devise a Y-junction to demonstrate its practical applications in terahertz imaging. An integrated circuit card was successfully scanned. In addition, we developed unclad waveguide, effective medium waveguide, and Y-junction modules. The modules incorporated an input/output interface compatible with a standard WR-1 flange (254 m 127 m). Unlike the conventional hollow waveguide modules, the unclad waveguide and effective medium waveguide modules reported total loss improvements of 6 dB and 8 dB, respectively, across the operation band. Our results provided a systematic way of achieving low-loss, compact, and versatile modules in the WR-1 band based on all-dielectric-waveguide platforms. Citation: Koala, R.; Maru, R.; Iyoda, Keywords: terahertz; dielectric waveguide; photonics; module; communication; imaging K.; Yi, L.; Fujita, M.; Nagatsuma, T. Ultra-Low-Loss and Broadband All-Silicon Dielectric Waveguides for WR-1 Band (0.75–1.1 THz) Modules. 1. Introduction Photonics 2022, 9, 515. https:// The terahertz (THz) range has recently attracted a lot of interest among researchers, doi.org/10.3390/photonics9080515 with a substantial amount of research effort being dedicated to exploring the frequencies Received: 28 June 2022 from 100 GHz to 10 THz that are covered by the terahertz range. Recent efforts have Accepted: 21 July 2022 been accelerated by nanophotonics [1], which have enabled a broad range of applications, Published: 24 July 2022 including high-data-rate wireless communications [2] and high-resolution imaging [3]. Until recently, existing systems were designed to operate in the lower end of the terahertz Publisher’s Note: MDPI stays neutral band, spanning frequencies up to 0.3 THz [4] and 0.6 THz [5]. This has restricted the with regard to jurisdictional claims in assessment of the full potential of the THz range. The development of more sophisticated published maps and institutional affil- and capable THz systems requires current research to focus on higher frequencies targeting iations. the frequencies in the WR-1 band (0.75–1.1 THz). These ultrahigh frequencies could contribute to accelerating future applications such as space exploration [6], non-invasive and non-destructive super-resolution imaging [7,8], sensing [9,10], and ultrafast wireless Copyright: © 2022 by the authors. communications [11]. At higher frequencies, higher data rates can be achieved owing to Licensee MDPI, Basel, Switzerland. increased bandwidth [12]. Indeed, higher frequencies lead to higher channel capacities, This article is an open access article as stated by Shannon’s theorem. Hence, data rates as high as one terabits/s in wireless distributed under the terms and communications can be achievable [13]. A noticeable advantage of higher frequencies conditions of the Creative Commons for THz components and systems built on all-silicon (Si) materials is the reduction in the Attribution (CC BY) license (https:// propagation loss determined by the absorption loss of the Si material. With increase in creativecommons.org/licenses/by/ frequency, the absorption loss due to free carriers in Si decreases [14]. A few issues still 4.0/). Photonics 2022, 9, 515. https://doi.org/10.3390/photonics9080515 https://www.mdpi.com/journal/photonics Photonics 2022, 9, 515 2 of 19 remain unaddressed in terms of taking advantage of the THz range. The lack of high-power sources and high-sensitivity detectors at high frequencies is a noticeable hindrance. For instance, the output power of commercially available signal generators decreases from ~5 dBm at 0.3 THz to 23 dBm at 1 THz; the sensitivity of a zero bias diode decreases from ~1.8 V/W at 0.3 THz to 0.75 V/W at 1 THz [1] because of the inherent difficulty in generating THz waves with artificial sources [15]. It is desirable to seek efficient integration with diverse components and sources to compensate for the decrease in available power, which will require THz-range interconnects. Waveguides are fundamental components in electronics that can serve as interconnects with external power sources, signal probers, and various components essential for the gen- eration and manipulation of THz waves. Hollow waveguides have become standard owing to extensive research on metallic rectangular and circular waveguides in the microwave region, where they yield good performance. These waveguides rely on guided waves based on the metallic media principle; that is, the waves are confined within the metallic walls of the waveguides. Hollow metallic waveguides have been the preferred interconnects in the THz range. However, at THz frequencies, the ohmic loss associated with the metal increases. In addition, metallic hollow waveguides are unsuitable to integrate with most THz-range components that are made of thin planar Si because of their non-planar profile as well as their relatively large physical size compared to the wavelength in the THz range. In addition, metal-based interconnects such as hollow waveguides have led to THz sys- tems that are bulky, with numerous individual components that exacerbate insertion and ohmic losses. Meanwhile, the development of low-loss THz-range waveguides has been accelerated by the progress in Nanophotonics. All-Si waveguide platforms are current focus of research because they have reported losses as low as 0.1 dB/cm owing to the low absorption loss of high-resistivity Si material. Various all-Si waveguides exist depending on the technology employed for their implementation. Photonic crystal waveguides are fundamental compo- nents built by perforating an array of through-holes into a silicon slab. These waveguides rely on the photonic bandgap effect to confine a THz wave in the waveguide track and achieve a reported low loss <0.1 dB/cm [16–18]. Photonic crystal waveguides have enabled many applications, including hybrid integration with active components [4,19,20] and the realization of efficient communication links employing THz fibers with achieved data rates of 10 Gbps at 0.33 THz [21]. However, these waveguides suffer from a limited bandwidth of approximately 20 GHz [16]. Subsequent to the photonic crystal waveguides, effective medium (EM) waveguides were implemented based on the effective-medium principle. EM waveguides have reported low loss and a broader bandwidth of >120 GHz [22], en- abling novel applications in high-data-rate wireless communications. Despite the increased bandwidth, EM waveguides are difficult to implement because of the small size and high density of holes. More recent waveguides have further reduced waveguide loss while maintaining the broader bandwidth of effective medium waveguides [22] by excavating a portion of the effective medium section of the waveguide, leaving only the waveguide track [23]. These are unclad waveguides and they have reported extremely low losses, and have recently been employed for novel applications in hybrid integration and terahertz- range communications. Another advantage of the unclad waveguides is their versatility. Indeed, essential components, such as bends and splits, can be easily implemented using unclad waveguides with very little loss [23]. In this study, we propose ultra-low-loss and broadband unclad and EM dielectric waveguide interconnects for the WR-1 band, covering THz frequencies in 0.75–1.1 THz. The proposed waveguides were used to implement a packaged, unclad waveguide module and an EM waveguide module with an input/output (I/O) interface compatible with a standard WR-1 flange. In addition, we have developed a Y-junction based on an unclad waveguide to demonstrate THz imaging applications. Photonics 2022, 9, 515 3 of 19 2. Materials and Method—All-Si Dielectric Waveguides 2.1. Design Figure 1a,b shows the designs of the unclad waveguide and the EM waveguide, respectively. Both waveguides proposed in this study rely on EM theory, which allows for the engineering of novel materials whose properties are inherited from the constituent composite materials. The EM waveguide has two main sections: the waveguide core and the EM section. The EM section was realized by introducing an array of through-holes in an 80 m-thick silicon slab with a relative permittivity # of 11.68, which corresponds Si to a refractive index of 3.418 and a resistivity of >10 kW-cm. The holes have a diameter D = 35 m and were perforated following an equilateral lattice with a period a = 45 m. Introducing the array of through-holes makes it possible to obtain a novel material with a refractive index between that of intrinsic silicon and that of air. The resulting refractive index is strongly dependent on a and D, and the values of a and D were obtained after a careful parameter sweep on these values. The waveguide core is therefore cladded in-plane by the EM section and out-of-plane by air, creating an index contrast between the waveguide core and cladding, which can be confined by total internal reflection (TIR). Consequently, the waveguides support a transverse electric (TE) mode that is parallel to the slab. This mode is associated with an in-plane electric field in which the relative permittivity can be approximated using Maxwell–Garnett approximations following the equation [24]: (# + # ) + (# # )z 0 Si 0 Si # = # (1) x Si (# + # ) (# # )z 0 Si 0 Si where # and # are the permittivities of air and Si, respectively, and z represents the filling 0 Si factor of the air in silicon. The unclad waveguide is built upon the EM waveguide owing to the difficulty as- sociated with the fabrication of EM waveguides. Owing to the small size of the holes, higher-precision machining is required to manufacture subwavelength hole diameters. This motivated the removal, or at least the reduction in the cladding section with small holes, to achieve substrateless designs that are entirely cladded by air. This allows for a simpler design with reduced complexity and ease of fabrication. Nevertheless, for ease of handling and practicality, a small portion of EM cladding was maintained. The EM section of the unclad waveguide is identical to that of the EM waveguides, that is, a = 45 m and D = 35 m. The waveguides were also built on 80 m-thick high-resistivity intrinsic silicon. Both the EM and unclad waveguides have an EM section that is 1.3 mm-long in the case of the unclad waveguide, and a length of L that corresponds to the length of the entire waveguide in the case of the EM waveguide. In both cases, the EM section is 2 mm-wide. For both waveguides, the waveguide core is 100 m-wide and of thickness 80 m. Frames are implemented in both waveguides for practicality and ease of handling, as well as to facilitate integration with metallic packaging. Alignment grooves of 0.5 0.5 mm are implemented into the frames to help align the waveguides within the metal packaging. Each frame is 1.5 mm wide, which renders the total width of each waveguide to 5 mm. The waveguides are terminated at both ends with 1.2 mm-long linear tapers. The waveguides were evaluated by a three-dimensional finite-integral time-domain electromagnetic simula- tion (CST Studio Suite 2021), and the electric field distributions at the center frequency of the WR-1 band (0.925 THz) are shown in Figure 2a,b for the unclad waveguide and EM waveguide, respectively. The electric-field distributions reveal strong confinement of THz waves within the waveguide core. Photonics 2022, 9, x FOR PEER REVIEW 4 of 21 Photonics 2022, 9, 515 4 of 19 Si frames Si unclad waveguide Alignment grooves Metallic hollow waveguide y x 1.3 mm 264 μm (a) 127 μm 2 mm 4 mm 500 μm Photonics 2022, 9, x FOR PEER REVIEW 5 of 21 500 μm (b) waveguide, respectively. The electric-field distributions reveal strong confinement of THz Figure 1. Designs of proposed dielectric waveguides: (a) Unclad waveguide; (b) EM waveguide; Figure 1. Designs of proposed dielectric waveguides: (a) Unclad waveguide; (b) EM waveguide; W waves within the waveguide core. W = 100 m, D = 35 m, a = 45 m and T = 80 m. = 100 μm, D = 35 μm, a = 45 μm and T = 80 μm. The unclad waveguide is built upon the EM waveguide owing to the difficulty asso- ciated with the fabrication of EM waveguides. Owing to the small size of the holes, higher- precision machining is required to manufacture subwavelength hole diameters. This mo- tivated the removal, or at least the reduction in the cladding section with small holes, to achieve substrateless designs that are entirely cladded by air. This allows for a simpler design with reduced complexity and ease of fabrication. Nevertheless, for ease of handling and practicality, a small portion of EM cladding was maintained. The EM section of the unclad waveguide is identical to that of the EM waveguides, that is, a = 45 μm and D = 35 Figure 2. Electric field distribution at 0.925 THz: (a) Unclad waveguide; (b) EM waveguide. μm. The waveguides were also built on 80 μm-thick high-resistivity intrinsic silicon. Both Figure 2. Electric field distribution at 0.925 THz: (a) Unclad waveguide; (b) EM waveguide. the EM and unclad waveguides have an EM section that is 1.3 mm-long in the case of the The waveguides previously reported for the WR-2.8 band (0.26–0.39 THz) achieved unclad waveguide, and a length of L that corresponds to the length of the entire wave- The waveguides previously reported for the WR-2.8 band (0.26–0.39 THz) achieved good performance, with hole pitches of 100 m and 120 m, as reported in [22,23], re- guide in the case of the EM waveguide. In both cases, the EM section is 2 mm-wide. For good performance, with hole pitches of 100 μm and 120 μm, as reported in [22,23], respec- spectively, while maintaining a minimum manufacturable limitation of 10 m for hole both waveguides, the waveguide core is 100 μm-wide and of thickness 80 μm. Frames are tively, while maintaining a minimum manufacturable limitation of 10 μm for hole diam- diameters of 90 m and 110 m, respectively. Following a simple scaling of [22,23] to implemented in both waveguides for practicality and ease of handling, as well as to facil- eters of 90 μm and 110 μm, respectively. Following a simple scaling of [22,23] to (0.75–1.1 (0.75–1.1 THz) would render a hole pitch of ~35.7 m and ~42.8 m, respectively; the itate integration with metallic packaging. Alignment grooves of 0.5 × 0.5 mm are imple- THz) would render a hole pitch of ~35.7 μm and ~42.8 μm, respectively; the scaling hole scaling hole diameter would be 32.1 m and 39.2 m. This would lead to 3.2 m and mented into the frames to help align the waveguides within the metal packaging. Each diameter would be 32.1 μm and 39.2 μm. This would lead to 3.2 μm and 3.6 μm clearance 3.6 m clearance between two consecutive holes. However, such small distances are not frame is 1.5 mm wide, which renders the total width of each waveguide to 5 mm. The between two consecutive holes. However, such small distances are not manufacturable. manufacturable. Therefore, additional considerations were made to increase the minimum waveguides are terminated at both ends with 1.2 mm-long linear tapers. The waveguides Therefore, additional considerations were made to increase the minimum distance be- distance between two adjacent holes. This could be achieved by decreasing the refractive were evaluated by a three-dimensional finite-integral time-domain electromagnetic sim- tween two adjacent holes. This could be achieved by decreasing the refractive index. The index. The EM theory requires that the hole pitch should be chosen to be smaller than the ulation (CST Studio Suite 2021), and the electric field distributions at the center frequency EM theory requires that the hole pitch should be chosen to be smaller than the quarter quarter wavelength in the EM. Given that the minimum wavelength in WR-1 is ~270 m, of the WR-1 band (0.925 THz) are shown in Figure 2a,b for the unclad waveguide and EM wavelength in the EM. Given that the minimum wavelength in WR-1 is ~270 μm, the quar- the quarter wavelength in EM is ~20 m at 1.1 THz with a refractive index of 3.418. Choos- ter wavelength in EM is ~20 μm at 1.1 THz with a refractive index of 3.418. Choosing the correct values of D and a will help in changing the refractive index, as D and a define the filling factor. According to Equation (1), the refractive index decreases with the filling fac- tor, as illustrated in Figure 3. For example, a refractive index of 2, which corresponds to D = 35 μm and a = 45 μm, would yield a quarter wavelength in EM ~35 μm. This should satisfy the minimum manufacturability condition. However, considering air cladding in the plane perpendicular to the waveguide, the 3D index should be further reduced. A study of the waveguide performance for different values of W and T, as shown in Figure 4, revealed that the waveguide width W has a larger impact on the improvement of the transmittance. Figure 4a,b show that the thickness of the waveguide has no impact on the transmittance, whereas Figure 4c reveals that the transmittance is improved from ~1 dB at 0.75 THz when the waveguide width is increased from 80 μm to 100 μm for the unclad waveguide. Figure 4d reveals similar improvement of ~1 dB at 0.75 THz. Figure 3. Refractive index of the EM section as a function of the hole diameter. Photonics 2022, 9, x FOR PEER REVIEW 5 of 21 waveguide, respectively. The electric-field distributions reveal strong confinement of THz waves within the waveguide core. dB dB ⁻40 ⁻40 (a) (b) Figure 2. Electric field distribution at 0.925 THz: (a) Unclad waveguide; (b) EM waveguide. The waveguides previously reported for the WR-2.8 band (0.26–0.39 THz) achieved good performance, with hole pitches of 100 μm and 120 μm, as reported in [22,23], respec- tively, while maintaining a minimum manufacturable limitation of 10 μm for hole diam- eters of 90 μm and 110 μm, respectively. Following a simple scaling of [22,23] to (0.75–1.1 THz) would render a hole pitch of ~35.7 μm and ~42.8 μm, respectively; the scaling hole diameter would be 32.1 μm and 39.2 μm. This would lead to 3.2 μm and 3.6 μm clearance between two consecutive holes. However, such small distances are not manufacturable. Therefore, additional considerations were made to increase the minimum distance be- tween two adjacent holes. This could be achieved by decreasing the refractive index. The EM theory requires that the hole pitch should be chosen to be smaller than the quarter Photonics 2022, 9, 515 5 of 19 wavelength in the EM. Given that the minimum wavelength in WR-1 is ~270 μm, the quar- ter wavelength in EM is ~20 μm at 1.1 THz with a refractive index of 3.418. Choosing the correct values of D and a will help in changing the refractive index, as D and a define the ing the correct values of D and a will help in changing the refractive index, as D and a filling factor. According to Equation (1), the refractive index decreases with the filling fac- define the filling factor. According to Equation (1), the refractive index decreases with tor, as illustrated in Figure 3. For example, a refractive index of 2, which corresponds to D the filling factor, as illustrated in Figure 3. For example, a refractive index of 2, which = 35 μm and a = 45 μm, would yield a quarter wavelength in EM ~35 μm. This should corresponds to D = 35 m and a = 45 m, would yield a quarter wavelength in EM ~35 m. satisfy the minimum manufacturability condition. However, considering air cladding in This should satisfy the minimum manufacturability condition. However, considering air the plane perpendicular to the waveguide, the 3D index should be further reduced. A cladding in the plane perpendicular to the waveguide, the 3D index should be further study of the waveguide performance for different values of W and T, as shown in Figure reduced. A study of the waveguide performance for different values of W and T, as shown 4, revealed that the waveguide width W has a larger impact on the improvement of the in Figure 4, revealed that the waveguide width W has a larger impact on the improvement transmittance. Figure 4a,b show that the thickness of the waveguide has no impact on the of the transmittance. Figure 4a,b show that the thickness of the waveguide has no impact transmittance, whereas Figure 4c reveals that the transmittance is improved from ~1 dB on the transmittance, whereas Figure 4c reveals that the transmittance is improved from at 0.75 THz when the waveguide width is increased from 80 μm to 100 μm for the unclad ~1 dB at 0.75 THz when the waveguide width is increased from 80 m to 100 m for the waveguide. Figure 4d reveals similar improvement of ~1 dB at 0.75 THz. unclad waveguide. Figure 4d reveals similar improvement of ~1 dB at 0.75 THz. Photonics 2022, 9, x FOR PEER REVIEW 6 of 21 Figure 3. Refractive index of the EM section as a function of the hole diameter. Figure 3. Refractive index of the EM section as a function of the hole diameter. (a) (b) (c) (d) Figure 4. Analysis of the impact of T and W on the performance of 1 cm-long unclad waveguide and Figure 4. Analysis of the impact of T and W on the performance of 1 cm-long unclad waveguide and EM waveguide: (a) Transmittance of unclad waveguide for T = 70 m (green), T = 80 m (black), EM waveguide: (a) Transmittance of unclad waveguide for T = 70 μm (green), T = 80 μm (black), T = 90 T = 90μm (red) and m (red) and WW is fixed at 100 is fixed at 100μ m; m; (b (b ) Transmittance of EM waveg ) Transmittance of EM wavegui uide for variab de for variable le T T with with fixed W = 100 μm; (c) Transmittance of unclad waveguide for W = 80 μm (red), W = 90 μm (green), fixed W = 100 m; (c) Transmittance of unclad waveguide for W = 80 m (red), W = 90 m (green), W = 100 μm (black) and T is fixed at 80 μm; (d) Transmittance of EM waveguide for variable W with W = 100 m (black) and T is fixed at 80 m; (d) Transmittance of EM waveguide for variable W with fixed T = 80 μm. fixed T = 80 m. 2. 2.2. 2. Fabri Fabrication cation an and d EExperimental xperimental Validation Validation We fabricated unclad and EM waveguides with different lengths L (1, 2, 3, 4, and 5 cm). We fabricated unclad and EM waveguides with different lengths L (1, 2, 3, 4, and 5 For the fabrication of the waveguides, we prepared an 80 m-thick, 4-inch float-zone silicon cm). For the fabrication of the waveguides, we prepared an 80 μm-thick, 4-inch float-zone wafer with the resistivity >10 kWcm. The samples were fabricated by photolithography silicon wafer with the resistivity >10 kΩ∙cm. The samples were fabricated by photolithog- and deep reactive ion etching (DRIE) using the manufacturing process of microelectrome- raphy and deep reactive ion etching (DRIE) using the manufacturing process of microe- chanical systems foundry with a minimum guaranteed dimension of 10 microns. The lectromechanical systems foundry with a minimum guaranteed dimension of 10 microns. resulting unclad and EM waveguides are shown in Figure 5a and 5b, respectively. Mi- The resulting unclad and EM waveguides are shown in Figure 5a and 5b, respectively. crographs are shown to highlight key sailing features such as EM sections and coupling Micrographs are shown to highlight key sailing features such as EM sections and coupling linear tapers. linear tapers. 1 cm 1 cm 50 μm 50 μm 50 μm 50 μm (a) (b) Figure 5. Fabricated waveguide samples with micrographs as inset: (a) Unclad waveguides; (b) EM waveguides. We measured the power transmission of these waveguides using the experimental setup shown in Figure 6. As shown in Figure 6a, the power transmission was probed by inserting linear tapers of the waveguides into metallic hollow waveguides. For the WR-1 band, the physical dimensions of the rectangular hollow waveguide are 127 μm × 254 μm, Photonics 2022, 9, x FOR PEER REVIEW 6 of 21 (a) (b) (c) (d) Figure 4. Analysis of the impact of T and W on the performance of 1 cm-long unclad waveguide and EM waveguide: (a) Transmittance of unclad waveguide for T = 70 μm (green), T = 80 μm (black), T = 90 μm (red) and W is fixed at 100 μm; (b) Transmittance of EM waveguide for variable T with fixed W = 100 μm; (c) Transmittance of unclad waveguide for W = 80 μm (red), W = 90 μm (green), W = 100 μm (black) and T is fixed at 80 μm; (d) Transmittance of EM waveguide for variable W with fixed T = 80 μm. 2.2. Fabrication and Experimental Validation We fabricated unclad and EM waveguides with different lengths L (1, 2, 3, 4, and 5 cm). For the fabrication of the waveguides, we prepared an 80 μm-thick, 4-inch float-zone silicon wafer with the resistivity >10 kΩ∙cm. The samples were fabricated by photolithog- raphy and deep reactive ion etching (DRIE) using the manufacturing process of microe- lectromechanical systems foundry with a minimum guaranteed dimension of 10 microns. The resulting unclad and EM waveguides are shown in Figure 5a and 5b, respectively. Photonics 2022, 9, 515 6 of 19 Micrographs are shown to highlight key sailing features such as EM sections and coupling linear tapers. 1 cm 1 cm 50 μm 50 μm 50 μm 50 μm (a) (b) Figure 5. Fabricated waveguide samples with micrographs as inset: (a) Unclad waveguides; Figure 5. Fabricated waveguide samples with micrographs as inset: (a) Unclad waveguides; (b) EM (b) EM waveguides. waveguides. We measured the power transmission of these waveguides using the experimental We measured the power transmission of these waveguides using the experimental setup shown in Figure 6. As shown in Figure 6a, the power transmission was probed by setup shown in Figure 6. As shown in Figure 6a, the power transmission was probed by inserting linear tapers of the waveguides into metallic hollow waveguides. For the WR-1 inserting linear tapers of the waveguides into metallic hollow waveguides. For the WR-1 band, the physical dimensions of the rectangular hollow waveguide are 127 m 254 m, band, the physical dimensions of the rectangular hollow waveguide are 127 μm × 254 μm, which targets the frequencies in 0.75–1.1 THz. The small dimensions of this waveguide can hinder accurate measurements in experiments and may pose a challenge in general for higher frequencies. To overcome this issue, we employed commercially available microscopes (DINO Lite), as shown in Figure 6b, to obtain a better view of the opening of the metallic hollow waveguides for the insertion of Si tapers. In addition, we have devised supporting jigs to help position the waveguide devices, assuring vertical alignment and 0-tilt, which has helped reduce imperfect alignment, the main cause of coupling losses. The metallic hollow waveguides interface with commercially available signal generators coupled with a multiplier to deliver THz signals in the 0.75–1.1 THz range; that is, the signal generator delivers millimeter waves into a 27-times multiplier which in turn delivers the THz waves into the waveguides. A THz-range mixer was employed at the detection side to perform frequency down-conversion, producing microwave signals that were then processed by a spectrum analyzer. Figure 7 shows the measured transmittances for the unclad (red) and EM (blue) waveguides. Each waveguide was measured five times, and for each measurement, the linear taper of the waveguide was removed from the hollow metallic waveguide on both the transmitter and receiver sides and then re-inserted. The standard deviation is used to derive error bars, which indicate the extent to which each measurement data point differs from the overall average. The error bar suggests minor differences between each measurement, which could be attributed to the positioning jigs that help secure the position of each sample between consecutive measurements. A shift in transmittance can be observed in both designs as the length of the waveguides, L, changes. For the unclad waveguides, the transmittance is shifted from a minimum of 5 dB—Figure 7i—for the longest waveguide (5 cm) to 0 dB—Figure 7a—for the shortest (1 cm). In the case of EM waveguides, the power transmission was found to be ~1 dB better than that of unclad waveguides. Specifically, the transmittance shifted from 4 dB (Figure 7j) for the longest waveguide (5 cm) to ~0 dB (Figure 7b) for the shortest (1 cm). In some cases, the transmittance was found to be >0 dB, which means that the power transmission is better when employing all-Si waveguides than when probing the reference measurement employing hollow metallic waveguides. This phenomenon has been observed previously in dielectric waveguides of this type in the WR2.8 band, and this phenomenon can be exacerbated in the WR-1 band [23]. However, Photonics 2022, 9, x FOR PEER REVIEW 7 of 21 which targets the frequencies in 0.75–1.1 THz. The small dimensions of this waveguide can hinder accurate measurements in experiments and may pose a challenge in general for higher frequencies. To overcome this issue, we employed commercially available mi- croscopes (DINO Lite), as shown in Figure 6b, to obtain a better view of the opening of the metallic hollow waveguides for the insertion of Si tapers. In addition, we have devised supporting jigs to help position the waveguide devices, assuring vertical alignment and 0-tilt, which has helped reduce imperfect alignment, the main cause of coupling losses. The metallic hollow waveguides interface with commercially available signal generators Photonics 2022, 9, 515 7 of 19 coupled with a multiplier to deliver THz signals in the 0.75–1.1 THz range; that is, the signal generator delivers millimeter waves into a 27-times multiplier which in turn deliv- ers the THz waves into the waveguides. A THz-range mixer was employed at the detec- this phenomenon can be possibly mitigated by averaging the measurements to derive the tion side to perform frequency down-conversion, producing microwave signals that were propagation and coupling losses. then processed by a spectrum analyzer. Waveguide Multiplier (×27) Mixer LO IF WR-1 hollow Signal metallic Spectrum Jig waveguide analyzer generator (a) Multiplier Mixer WR-1 hollow Microscopes metallic waveguides (b) 2 cm Figure 6. Experimental setup for the measurement of the transmittance: (a) Schematic of the setup; Figure 6. Experimental setup for the measurement of the transmittance: (a) Schematic of the setup; ((b b)) Photograph Photograph of the of the setup. setup. We probed five waveguides of different lengths for both the unclad and EM designs Figure 7 shows the measured transmittances for the unclad (red) and EM (blue) to derive propagation and coupling losses using the difference in power transmission. waveguides. Each waveguide was measured five times, and for each measurement, the Specifically, the transmittance for each waveguide length was plotted as a function of the linear taper of the waveguide was removed from the hollow metallic waveguide on both frequency. Thereafter, for each frequency point, the transmittance data points were fitted to the transmitter and receiver sides and then re-inserted. The standard deviation is used to a curve using the least-squares method. The slope and intercept of the resulting curve are derive error bars, which indicate the extent to which each measurement data point differs then used to estimate the propagation and coupling losses; that is, the propagation loss per from the overall average. The error bar suggests minor differences between each meas- unit length corresponds to the slope of the curve and the coupling loss to the Y-intercept. urement, which could be attributed to the positioning jigs that help secure the position of The deduction of the propagation and coupling losses is illustrated in Figure 8a,b for the each sample between consecutive measurements. A shift in transmittance can be observed unclad and EM waveguides, respectively. in both designs as the length of the waveguides, L, changes. For the unclad waveguides, Figure 9 shows the measured and simulated propagation losses for both the unclad the transmittance is shifted from a minimum of −5 dB—Figure 7i—for the longest wave- waveguides (red) and effective-medium waveguides (blue). For the unclad waveguides, guide (5 cm) to 0 dB—Figure 7a—for the shortest (1 cm). In the case of EM waveguides, the simulated and measured propagation losses were in good agreement for the lower the power transmission was found to be ~1 dB better than that of unclad waveguides. frequencies of up to ~0.9 THz. For the frequencies >0.9 THz, the measured propagation loss Specifically, the transmittance shifted from 4 dB (Figure 7j) for the longest waveguide (5 was found to be ~0.3 dB on average higher than simulated propagation loss. The results cm) to ~0 dB (Figure 7b) for the shortest (1 cm). In some cases, the transmittance was found are shown in Figure 9a. This could be attributed to the difference in silicon properties and to be >0 dB, which means that the power transmission is better when employing all-Si the limitations of the fabrication foundries. The propagation loss of the EM waveguide is waveguides than when probing the reference measurement employing hollow metallic presented in Figure 9b, and the measured and simulated losses are observed to be in good waveguides. This phenomenon has been observed previously in dielectric waveguides of agreement. The coupling loss of the unclad waveguide is shown in Figure 9c. The measured this type in the WR2.8 band, and this phenomenon can be exacerbated in the WR-1 band coupling loss was greater than the simulated coupling loss. The difference is negligible for lower frequencies up to 0.8 THz and higher frequencies >1.05 THz with a difference of ~0.2 dB. The difference in coupling is exacerbated around the center frequency of 0.925 THz, reaching ~0.4 dB as opposed to the good agreement for the EM waveguide, as shown in Figure 9d. This can be ascribed to the coupling losses at different interfaces of the unclad waveguide. Indeed, both the unclad and EM waveguides have linear tapers that serve as coupling interfaces with the metallic hollow waveguides, but the unclad waveguide has additional interfaces at the point where the EM section is interrupted, revealing the waveguide core as a single wire. This transition from EM to air creates an additional coupling loss at both ends of the waveguide, which can be higher for the fabricated samples depending on the fabrication accuracy. With a difference in coupling loss of ~0.4 dB, we can attribute a ~0.2 dB loss to each EM-unclad transition interface. In addition, the results Photonics 2022Photonics , 9, x FO2022 R PEER , 9, 515 REVIEW 8 of 8 of 21 19 revealed the very broadband performance of these waveguides, as the 3-dB bandwidth [23]. However, this phenomenon can be possibly mitigated by averaging the measure- was not encountered in the entire frequency range of interest, suggesting a bandwidth of ments to derive the propagation and coupling losses. ~350 GHz. (a) (b) (c) (d) (f) (e) (g) (h) (i) (j) Figure 7. Measured transmittance of the fabricated waveguides: (a) 1 cm unclad waveguide; (b) 1 cm EM waveguide; (c) 2 cm unclad waveguide; (d) 2 cm EM waveguide; (e) 3 cm unclad waveguide; (f) 3 cm EM waveguide; (g) 4 cm unclad waveguide; (h) 4 cm EM waveguide; (i) 5 cm unclad Figure 7. Measured transmittance of the fabricated waveguides: (a) 1 cm unclad waveguide; (b) 1 waveguide; (j) 5 cm EM waveguide. cm EM waveguide; (c) 2 cm unclad waveguide; (d) 2 cm EM waveguide; (e) 3 cm unclad waveguide; (f) 3 cm EM waveguide; (g) 4 cm unclad waveguide; (h) 4 cm EM waveguide; (i) 5 cm unclad wave- guide; (j) 5 cm EM waveguide. Photonics 2022, 9, x FOR PEER REVIEW 9 of 21 We probed five waveguides of different lengths for both the unclad and EM designs to derive propagation and coupling losses using the difference in power transmission. Specifically, the transmittance for each waveguide length was plotted as a function of the frequency. Thereafter, for each frequency point, the transmittance data points were fitted to a curve using the least-squares method. The slope and intercept of the resulting curve are then used to estimate the propagation and coupling losses; that is, the propagation loss per unit length corresponds to the slope of the curve and the coupling loss to the Y- intercept. The deduction of the propagation and coupling losses is illustrated in Figure 8a,b for the unclad and EM waveguides, respectively. Photonics 2022, 9, 515 9 of 19 (a) (b) Intercept = Coupling loss Slope = Propagation loss Photonics 2022, 9, x FOR PEER REVIEW 10 of 21 Photonics 2022, 9, x FOR PEER REVIEW 10 of 21 Figure 8. Measured total loss at 0.925 THz as a function of waveguide length: (a) Total loss of unclad Figure 8. Measured total loss at 0.925 THz as a function of waveguide length: (a) Total loss of unclad waveguide; (b) Total loss of EM waveguide. The propagation loss is def waveguide; ined using the slop (b) Total e. The loss of cou EM pling waveguide. loss is defined The pr us opagation ing the inter loss cept. is defined using the slope. The (a) (b) coupling loss is defined using the intercept. Figure 9 shows the measured and simulated propagation losses for both the unclad (a) (b) waveguides (red) and effective-medium waveguides (blue). For the unclad waveguides, the simulated and measured propagation losses were in good agreement for the lower frequencies of up to ~0.9 THz. For the frequencies >0.9 THz, the measured propagation loss was found to be ~0.3 dB on average higher than simulated propagation loss. The re- sults are shown in Figure 9a. This could be attributed to the difference in silicon properties (c) (d) and the limitations of the fabrication foundries. The propagation loss of the EM wave- guide is presented in Figure 9b, and the measured and simulated losses are observed to (c) (d) be in good agreement. The coupling loss of the unclad waveguide is shown in Figure 9c. The measured coupling loss was greater than the simulated coupling loss. The difference is negligible for lower frequencies up to 0.8 THz and higher frequencies >1.05 THz with a difference of ~0.2 dB. The difference in coupling is exacerbated around the center fre- quency of 0.925 THz, reaching ~0.4 dB as opposed to the good agreement for the EM wave- guide, as shown in Figure 9d. This can be ascribed to the coupling losses at different in- terfaces of the unclad waveguide. Indeed, both the unclad and EM waveguides have lin- Figure 9. Measured (solid) and simulated (dashed) propagation loss and coupling loss: Figure 9. Measured (solid) and simulated (dashed) propagation loss and coupling loss: (a) Propaga- ear tapers that serve as coupling interfaces with the metallic hollow waveguides, but the (a) Propagation loss of unclad waveguides; (b) Propagation loss of EM waveguide; (c) Figure 9. Measured (solid) and simulated (dashed) propagation loss and coupling loss: tion loss of unclad waveguides; (b) Propagation loss of EM waveguide; (c) Coupling loss of unclad unclad waveguide has additional interfaces at the point where the EM section is inter- Coupling loss of unclad waveguide; (d) Coupling loss of EM waveguide. (a) Propag waveguide; ation los (ds ) of Coupling uncladloss wave of EM guid waveguide. es; (b) Propagation loss of EM waveguide; (c) rupted, revealing the waveguide core as a single wire. This transition from EM to air cre- Coupling loss of unclad waveguide; (d) Coupling loss of EM waveguide. ates an additional coupling loss at both ends of the waveguide, which can be higher for 3. 3. All-Silicon All-Silicon Di Dielectric electric W Waveguide aveguide M Modules odules the fabricated samples depending on the fabrication accuracy. With a difference in cou- 3.1. Concept 3.1. Concept 3. All-Silicon Dielectric Waveguide Modules pling loss of ~0.4 dB, we can attribute a ~0.2 dB loss to each EM-unclad transition interface. The main concept of the proposed modules is illustrated in Figure 10. An all-Si device The main concept of the proposed modules is illustrated in Figure 10. An all-Si device In addition, the results revealed the very broadband performance of these waveguides, as 3.1. Concept that operates as the central component was encapsulated in metallic packaging. The Si that operates as the central component was encapsulated in metallic packaging. The Si the 3-dB bandwidth was not encountered in the entire frequency range of interest, sug- The main concept of the proposed modules is illustrated in Figure 10. An all-Si device device is terminated by transition interfaces that serve as interfaces with metallic packaging device is terminated by transition interfaces that serve as interfaces with metallic packag- gesting a bandwidth of ~350 GHz. that operates as the central component was encapsulated in metallic packaging. The Si while ensuring a low insertion loss. ing while ensuring a low insertion loss. device is terminated by transition interfaces that serve as interfaces with metallic packag- ing while ensuring a low insertion loss. Interconnection Metal packaging Interface #1 Interconnection Metal packaging Transition Transition Interface #1 interface #1 interface #4 Transition Transition interface #1 interface #4 Interconnection Interconnection Interface #2 Interface #4 Si device Interconnection Interconnection Interface #2 Interface #4 Si device Transition Transition interface #2 interface #3 Transition Transition Interconnection interface #2 interface #3 Interface #3 Interconnection Interface #3 Figure 10. Concept of the proposed module, highlighting the transition and connection interfaces. Figure 10. Concept of the proposed module, highlighting the transition and connection interfaces. Figure 10. Concept of the proposed module, highlighting the transition and connection interfaces. The transition interface also serves to secure the device onto packaging, ensuring safety and perfect alignment. Transition interfaces play a crucial role in reducing the im- The transition interface also serves to secure the device onto packaging, ensuring pact of metallic packaging on the inherent behavior of the device. In practice, transition safety and perfect alignment. Transition interfaces play a crucial role in reducing the im- interfaces can be realized by employing photonic crystals [25] and effective mediums [26], pact of metallic packaging on the inherent behavior of the device. In practice, transition as well as by gradual hole density [27]. Either technique can be considered while consid- interfaces can be realized by employing photonic crystals [25] and effective mediums [26], ering various performance indices of the waveguide, such as reflection, bandwidth, and as well as by gradual hole density [27]. Either technique can be considered while consid- fabrication limitations on the minimal manufacturable hole size. Transition interfaces usu- ering various performance indices of the waveguide, such as reflection, bandwidth, and ally incorporate all-Si tapered structures for smooth transitions. Among various taper fabrication limitations on the minimal manufacturable hole size. Transition interfaces usu- shapes, linear tapers have proven to allow for the gradual transmission of THz waves ally incorporate all-Si tapered structures for smooth transitions. Among various taper with <0.2 dB coupling loss of [17] in the WR-2.8 band. For the WR-1 band, simulated shapes, linear tapers have proven to allow for the gradual transmission of THz waves with <0.2 dB coupling loss of [17] in the WR-2.8 band. For the WR-1 band, simulated Photonics 2022, 9, 515 10 of 19 The transition interface also serves to secure the device onto packaging, ensuring safety and perfect alignment. Transition interfaces play a crucial role in reducing the impact of metallic packaging on the inherent behavior of the device. In practice, transition interfaces can be realized by employing photonic crystals [25] and effective mediums [26], as well as by gradual hole density [27]. Either technique can be considered while considering various performance indices of the waveguide, such as reflection, bandwidth, and fabrication limi- tations on the minimal manufacturable hole size. Transition interfaces usually incorporate all-Si tapered structures for smooth transitions. Among various taper shapes, linear tapers have proven to allow for the gradual transmission of THz waves with <0.2 dB coupling loss of [17] in the WR-2.8 band. For the WR-1 band, simulated coupling efficiency was found to be as high as >90% with <0.2 dB coupling loss. Linear tapers can also be implemented with a specific hole arrangement to improve matching at the interface where the taper contacts other THz-range components [28]. The transition interfaces are, in turn, terminated by interconnection interfaces that serve to interconnect the module with other terahertz-range components such as THz sources and detectors. In this study, the interconnection interface was an I/O interface compatible with the WR-1 standard flange. 3.2. Unclad Waveguide and Effective-Medium Waveguide Modules The modules were realized by devising metallic packaging to house the unclad and EM waveguides, as shown in Figure 1. Metallic packaging has an I/O interface that is compatible with the WR-1 standard hollow waveguide. The design of the module is illustrated in Figure 11. The packing is 56.2 mm 26.0 mm 26.0 mm and is made of copper with gold plating. The module comprises a base and a lid. A 2.3 mm-deep trench was carved out of the base, leaving a void wide enough to prevent the packing from impacting the inherent function of the waveguide core. The base was also designed to house the waveguide while ensuring perfect alignment with the integrated WR-1 hollow waveguide track. This was achieved by incorporating a shelf into the base on which the waveguide rested. The shelf had a depth of 40 m, which is half the thickness of the waveguide. This is because the lid is identical to the base; therefore, there is a 40 m shelf in the lid as well. The width of the shelf matched that of the waveguides, including the frames, and was 5 mm. The modules also incorporate knobs that match the alignment grooves of the waveguide, thereby helping to fix the waveguide inside the module. For the assembly, each waveguide was carefully placed on the base such that the tapers sat in the hollow waveguide section of the metallic packaging. This is achieved by employing high-resolution microscopes to improve alignment accuracy. Employing microscopes was necessary because the dimensions in this band are extremely small. Once the waveguide was secured to the base, the lid was affixed with screws. The resulting module is extremely compact, lightweight, and can easily be connected to other terahertz-range components via the WR-1 band I/O interface, using screws to ensure perfect alignment, which is crucial for reducing insertion loss. We devised two metallic packaging and assembled an unclad waveguide module and an EM waveguide module, as shown in Figure 11a and Figure 11b, respectively. Micrographs are also shown, and it can be seen that the knobs of the base fit perfectly into the grooves of the waveguides. Additional micrographs are presented to show the positioning of the linear tapers within WR-1 standard hollow metallic waveguides. We also devised a 5 cm-long standard hollow metallic waveguide module, as shown in Figure 11c, to investigate its performance in comparison with the unclad waveguide module and EM waveguide module. Photonics 2022, 9, x FOR PEER REVIEW 12 of 21 Photonics 2022, 9, 515 11 of 19 Metallic walls Unclad waveguide Lid Waveguide track 0.5 mm Si taper Base 5 mm 5 mm (a) Lid EM waveguide Metallic walls Waveguide track Fixing screws 0.5 mm Si taper 5 mm 5 mm Base (b) Metallic walls Lid Waveguide track 0.5 mm 5 mm Base (c) Hollow metallic waveguide Module Multiplier Mixer 2 cm (d) Figure 11. Fabricated modules and experimental setup: (a) Unclad waveguide module; (b) EM Figure 11. Fabricated modules and experimental setup: (a) Unclad waveguide module; (b) EM waveguide module; (c) Hollow metallic waveguide; (d) Experimental setup. waveguide module; (c) Hollow metallic waveguide; (d) Experimental setup. The power transmission of the three modules was probed using a similar system, as shown This is bec in Figu ause the re 6. A lid is identical to th photograph of the experimental e base; therefore, th setup is shown ere is in a 40 Figur μm shel e 11d.f in the Figure 12 shows the simulated (dashed line) and measured (solid line) transmittances of lid as well. The width of the shelf matched that of the waveguides, including the frames, the modules. The transmittance of the unclad waveguide module is shown in red, that of and was 5 mm. The modules also incorporate knobs that match the alignment grooves of the waveguide, thereby helping to fix the waveguide inside the module. For the assembly, each waveguide was carefully placed on the base such that the tapers sat in the hollow waveguide section of the metallic packaging. This is achieved by employing high-resolu- tion microscopes to improve alignment accuracy. Employing microscopes was necessary because the dimensions in this band are extremely small. Once the waveguide was Photonics 2022, 9, x FOR PEER REVIEW 13 of 21 secured to the base, the lid was affixed with screws. The resulting module is extremely compact, lightweight, and can easily be connected to other terahertz-range components via the WR-1 band I/O interface, using screws to ensure perfect alignment, which is crucial for reducing insertion loss. We devised two metallic packaging and assembled an unclad waveguide module and an EM waveguide module, as shown in Figure 11a and Figure 11b, respectively. Micrographs are also shown, and it can be seen that the knobs of the base fit perfectly into the grooves of the waveguides. Additional micrographs are pre- sented to show the positioning of the linear tapers within WR-1 standard hollow metallic waveguides. We also devised a 5 cm-long standard hollow metallic waveguide module, as shown in Figure 11c, to investigate its performance in comparison with the unclad waveguide module and EM waveguide module. The power transmission of the three modules was probed using a similar system, as shown in Figure 6. A photograph of the experimental setup is shown in Figure 11d. Figure 12 shows the simulated (dashed line) and measured (solid line) transmittances of the mod- ules. The transmittance of the unclad waveguide module is shown in red, that of the EM waveguide module in blue, and that of the metallic hollow waveguide in black. The meas- Photonics 2022, 9, 515 12 of 19 ured and simulated transmittances of the EM waveguide module are in good agreement. In the case of the unclad waveguide module, the measured transmittance was in good agreement with the simulated transmittance for frequencies up to 0.95 THz. The transmit- the EM waveguide module in blue, and that of the metallic hollow waveguide in black. tance is ~−2 dB for The both measur simulated ed and simulated and metransmittances asured across 0.75– of the EM 1.1 waveguide THz. For module frequenci are ines good agreement. In the case of the unclad waveguide module, the measured transmittance >0.95 THz, the difference between the measured transmittance and simulated transmit- was in good agreement with the simulated transmittance for frequencies up to 0.95 THz. tance increases from 0 dB at ~0.955 THz to a maximum of ~0.8 dB at 1.05 THz, and finally, The transmittance is ~ 2 dB for both simulated and measured across 0.75–1.1 THz. For a 0.4 dB difference at 1.1 THz. For the hollow waveguide module, the trends of both sim- frequencies >0.95 THz, the difference between the measured transmittance and simulated ulated and measured transmittance are the same, with a performance difference of 0.4 dB transmittance increases from 0 dB at ~0.955 THz to a maximum of ~0.8 dB at 1.05 THz, and finally, a 0.4 dB difference at 1.1 THz. For the hollow waveguide module, the trends of at 0.75 THz, which slowly decreases to 0 dB at ~1.0 THz. both simulated and measured transmittance are the same, with a performance difference of 0.4 dB at 0.75 THz, which slowly decreases to 0 dB at ~1.0 THz. EM module Unclad module Hollow waveguide module Figure 12. Measured (solid) and simulated (dashed) transmittance of fabricated modules. The modules are 56.2 mm-long. The unclad and EM modules incorporate each 50 mm unclad and 50 mm Figure 12. Measured (solid) and simulated (dashed) transmittance of fabricated modules. The mod- EM waveguide, respectively. ules are 56.2 mm-long. The unclad and EM modules incorporate each 50 mm unclad and 50 mm EM waveguide, respectively. Probing the power transmission of these modules yielded results that were very close to the theoretical approximations. The unclad module has a ~6 dB loss improvement Probing the po over wer tr theansm hollow ission o waveguide f these module module ands y a ~2 ielded results th dB loss compared at to were ver the EM y module. close As discussed previously, this is due to the increased coupling loss of the unclad waveguide at to the theoretical approximations. The unclad module has a ~6 dB loss improvement over the interface where the EM section is interrupted. The EM waveguide module has very little the hollow waveguide module and a ~2 dB loss compared to the EM module. As discussed loss and exhibits a ~8 dB loss improvement over the hollow waveguide module. Both the previously, this is due to the increased coupling loss of the unclad waveguide at the inter- unclad waveguide module and EM waveguide module demonstrate superior performance face where the EM section is interrupted. The EM waveguide module has very little loss compared to the metallic hollow waveguide module. and exhibits a ~8 dB loss improvement over the hollow waveguide module. Both the un- 3.3. Y-Junction Module clad waveguide module and EM waveguide module demonstrate superior performance Having validated the functionality of the developed modules, we developed passive compared to the metallic hollow waveguide module. components for the realization of integrated terahertz systems. We implemented a Y- junction, which is an elementary passive component. Y-junctions are versatile components 3.3. Y-Junction Module that can serve as both splitters and combiners. The design of the proposed Y-junction is illustrated in Figure 13. This component was implemented with a 2 mm-radius 90 circular bend. The bend was symmetrically reflected to implement the third port of the component. The Y-junction is built on the unclad waveguide described in Section 2. Unclad waveg- uides are versatile, resemble silicon wires, and can be bent easily with very little loss [23]. However, the implementation of bend structures in EM and photonic crystal platforms has resulted in more losses [29,30]. More recent research has focused on employing VPC to decrease bending losses [2]. The EM section of the Y-junction was similar to that of the unclad waveguide. That is, the lattice is a = 45 m and the hole diameter is D = 35 m. For the waveguide core, the Y-branch was implemented with an 80 m-wide waveguide track instead of 100 m, as for the unclad waveguide, to increase the clearance between the waveguide track and the walls of the WR-1 standard metallic hollow waveguide module. The inner dimensions of the waveguide were 127 m 254 m. To excite TE modes, the shorter side must be oriented vertically to the unclad waveguide plane. Consequently, Photonics 2022, 9, x FOR PEER REVIEW 14 of 21 Having validated the functionality of the developed modules, we developed passive components for the realization of integrated terahertz systems. We implemented a Y-junc- tion, which is an elementary passive component. Y-junctions are versatile components that can serve as both splitters and combiners. The design of the proposed Y-junction is illustrated in Figure 13. This component was implemented with a 2 mm-radius 90°circular bend. The bend was symmetrically reflected to implement the third port of the compo- nent. The Y-junction is built on the unclad waveguide described in Section II. Unclad waveguides are versatile, resemble silicon wires, and can be bent easily with very little loss [23]. However, the implementation of bend structures in EM and photonic crystal platforms has resulted in more losses [29,30]. More recent research has focused on em- ploying VPC to decrease bending losses [2]. The EM section of the Y-junction was similar to that of the unclad waveguide. That is, the lattice is a = 45 μm and the hole diameter is D = 35 μm. For the waveguide core, the Y-branch was implemented with an 80 μm-wide waveguide track instead of 100 μm, as for the unclad waveguide, to increase the clearance Photonics 2022, 9, 515 13 of 19 between the waveguide track and the walls of the WR-1 standard metallic hollow wave- guide module. The inner dimensions of the waveguide were 127 μm × 254 μm. To excite TE modes, the shorter side must be oriented vertically to the unclad waveguide plane. having a 100 m waveguide width leads to a ~20 m margin left for the insertion of the Consequently, having a 100 μm waveguide width leads to a ~20 μm margin left for the unclad waveguide into the hollow waveguide via the taper. Therefore, to design the Y- insertion of the unclad waveguide into the hollow waveguide via the taper. Therefore, to junction, the waveguide track width was chosen to be smaller. Protective frames were also design the Y-junction, the waveguide track width was chosen to be smaller. Protective implemented to ensure ease of handling during the experiments and to facilitate integration frames were also implemented to ensure ease of handling during the experiments and to with metallic packaging. The thickness of the Y-junction was kept the same (80 m) to facilitate integration with metallic packaging. The thickness of the Y-junction was kept the allow for fabrication from the same wafer using DRIE. same (80 μm) to allow for fabrication from the same wafer using DRIE. 7.8 μm Effective medium 7.8 μm 3.1 μm Unclad waveguide bend Linear taper Figure 13. Design Figure 13. of propo Des sed ign Y of proposed -branch with Y-branch with the effective medium the effecti section ve mediu as m inset. section W = as ins 80 e m, t. W = 80 μm, T = 80 μm, D = 45 μm, P = 45 μm. T = 80 m, D = 45 m, P = 45 m. We fabricated the Y-junction and assembled the Y-junction module with packaging We fabricated the Y-junction and assembled the Y-junction module with packaging similar to that described in Section 3. The module was 27 mm 27 mm 27 mm and similar to that described in section III. The module was 27 mm × 27 mm × 27 mm and was was made of copper with gold plating. The assembly was performed in a similar manner, made of copper with gold plating. The assembly was performed in a similar manner, em- employing tweezers to place the all-Si component onto the base of the packaging, and ploying tweezers to place the all-Si component onto the base of the packaging, and then then the lid was affixed using screws. The assembled module is shown in Figure 14a, with the lid was affixed using screws. The assembled module is shown in Figure 14a, with a a micrograph of the Si taper within the hollow metallic waveguide. Figure 14a also shows micrograph of the Si taper within the hollow metallic waveguide. Figure 14a also shows the module ports, following which we probed the transmission power using the setup the module ports, following which we probed the transmission power using the setup shown in Figure 6. Figure 14b–e shows the experimental setup for measuring transmittance shown in Figure 6. Figure 14b–e shows the experimental setup for measuring transmit- S21 and isolation S31, respectively. The results of these measurements are shown in tance S21 and isolation S31, respectively. The results of these measurements are shown in Figure 15. Both the simulated (dashed line) and measured (solid line) transmittance and Figure 15. Both the simulated (dashed line) and measured (solid line) transmittance and isolation are presented. The measured and simulated transmittances (red) are in good agreement, with very little discrepancy between the lower and upper ends of the entire band of interest. The measured isolation was found to be better than the simulated isolation. Theoretically, the lower the isolation, the better. When isolation is lower, very little power deviates from Port 1 to Port 2, causing most of the power to be transmitted. The isolation in the measurement is found to be approximately 26 dB at 0.75 THz, against 20 dB in simulation. However, for frequencies >0.85 THz, the simulated and measured isolations are in acceptable agreement, mostly around 25 dB. The discrepancy for the lower frequency is attributed to minor fabricator errors in the unclad section leading to Port 3 of the module. As shown in Figure 4c,d, the unclad waveguide performance is strongly dependent on the waveguide width. A waveguide width of 100 m has very little loss, with a transmittance of ~ 0.3 dB at 0.75 THz. In comparison, a waveguide width of 80 m has a transmittance of ~1.8 dB at 0.75 THz. This significant difference could affect the overall performance of the Y-junction. Looking at the trends for the transmittance for different values of W, it can be deduced that the transmittance worsens as the waveguide width decreases. As such, a waveguide width W < 80 m would cause the transmittance to be < 2 dB. Because Port 3 Photonics 2022, 9, 515 14 of 19 Photonics 2022, 9, x FOR PEER REVIEW 15 of 21 of the very small dimensions of the Y-junction, fabrication errors occurred, resulting in an increased measured loss at Port 3. In addition, as established through measurements, isolation are presented. The measured and simulated transmittances (red) are in good unclad waveguides have increased loss caused by additional coupling loss at the interface agreement, with very little discrepancy between the lower and upper ends of the entire where the EM is interrupted, revealing the waveguide core. For the straight waveguide, band of interest. The measured isolation was found to be better than the simulated isola- two such interfaces exist, accounting for a loss of ~0.4 dB. In case of the Y-junction, there tion. Theoretically, the lower the isolation, the better. When isolation is lower, very little are three such interfaces, which theoretically should account for a total coupling loss of power deviates from Port 1 to Port 2, causing most of the power to be transmitted. The 0.6 dB. Considering the circular bending, these losses could have increased in the fabricated isolation in the measurement is found to be approximately −26 dB at 0.75 THz, against −20 device. This phenomenon stresses the requirement for more accurate machining and is dB in simulation. a major hindrance to the currently available techniques. Base Lid 27 mm 27 mm Port 2 27 mm Lid 0.5 mm (a) WR-1 hollow LO Mixer Spectrum WR-1 hollow metallic waveguide metallic waveguide analyzer IF Multiplier Spectrum (×27) analyzer Signal generator Signal 2 cm (c) generator (b) Spectrum WR-1 hollow analyzer metallic waveguide Mixer IF Multiplier LO (×27) Hollow metallic waveguide Signal Spectrum Signal generator analyzer generator 2 cm (d) (e) Figure 14. Y-branch measurement setup: (a) Fabricated module; (b) Block diagram for the measure- Figure 14. Y-branch measurement setup: (a) Fabricated module; (b) Block diagram for the measure- ment of S21; (c) Photograph of experiments setup for the measurement of S21; (d) Diagram for the ment of S21; (c) Photograph of experiments setup for the measurement of S21; (d) Diagram for the measurement of S31; (e) Photograph of the experimental set up for the measurement of S31. measurement of S31; (e) Photograph of the experimental set up for the measurement of S31. Port 1 Photonics 2022, 9, x FOR PEER REVIEW 16 of 21 Photonics 2022, 9, 515 15 of 19 Figure 15. Measured S21 (red) and S31 (black) of proposed Y-branch (dashed lines) and Y-branch Figure 15. Measured S21 (red) and S31 (black) of proposed Y-branch (dashed lines) and Y-branch module (solid lines). module (solid lines). 4. Imaging Application THz waves are well-suited for imaging applications. THz waves can penetrate a wide variety of non-conducting materials such as wood, paper, cardboard, plastic, and ceramic. However, for frequencies >0.85 THz, the simulated and measured isolations are in This has allowed for key novel applications in healthcare for single-strand DNA detec- tion [31] and in security for enhanced non-invasive detection of concealed weapons [7]. acceptable agreement, mostly around −25 dB. The discrepancy for the lower frequency is Other notable applications have been realized in industrial inspections for the detection at of tr defects ibut [ed 32]. to However minor , imaging fabric applications ator error have s been in t restricted he unc bylad the sec physically tion leading to Port 3 of the module. large size of the systems because of the need for components such as splitters and lenses. As shown in Figure 4c,d, the unclad waveguide performance is strongly dependent on the Employing a large number of individual components increases the system complexity and imposes a more stringent alignment for optical paths. More practical imaging systems waveguide width. A waveguide width of 100 μm has very little loss, with a transmittance may require a reduction in the physical size and complexity as more compact systems can of ~−0.3 dB at 0.75 THz. In comparison, a waveguide width of 80 μm has a transmittance allow for large-scale hybrid integration and open the door to novel applications. Recent research has sought hybrid integration with flat optics, allowing for the realization of of ~1.8 dB at 0.75 THz. This significant difference could affect the overall performance of all-Si lenses [33,34], beam splitters [35], and filters [36]. This has significantly reduced the size and complexity of imaging systems, resulting in compact designs that can be the Y-junction. Looking at the trends for the transmittance for different values of W, it can handheld [37], enabling applications at higher terahertz frequencies and novel applications be deduced that the transmittance worsens as the waveguide width decreases. As such, a such as drone-borne technology. Having realized and demonstrated the WR-1 band Y-junction module, we attempted waveguide width W < 80 μm would cause the transmittance to be <−2 dB. Because of the to demonstrate the practical imaging applications of this module. Targeting the frequencies in the 0.75–1.1 THz range is a step towards high-resolution imaging applications. To very small dimensions of the Y-junction, fabrication errors occurred, resulting in an in- this end, we employed the configuration shown in Figure 16a. At the transmitter side, creased measured loss at Port 3. In addition, as established through measurements, unclad a ~35.4 GHz millimeter wave signal that is modulated using a 100 kHz signal was injected into a 27 multiplier to deliver THz waves in the WR-1 band. The terahertz signal was then waveguides have increased loss caused by additional coupling loss at the interface where injected into the Y-junction module through Port 1 and radiated into free space through the EM is interrupted, revealing the waveguide core. For the straight waveguide, two such a horn antenna attached to Port 2. Two parabolic mirrors, one that serves as a collimating mirror and the other as a galvanometer mirror, were employed. The radiated signal from interfaces exist, accounting for a loss of ~0.4 dB. In case of the Y-junction, there are three the horn is collimated by the first mirror, which ricochets off the second mirror to reach the target. The target is attached to a motorized stage that allowed both x-axis and z- such interfaces, which theoretically should account for a total coupling loss of 0.6 dB. Con- axis scanning. On the receiver side, the residual signal from the target was detected by sidering the circular bending, these losses could have increased in the fabricated device. a Schottky barrier diode (SBD) and measured by a digital multimeter (DMM) after lock-in detection. Figure 16b shows a photograph of the imaging experimental setup with the This phenomenon stresses the requirement for more accurate machining and is a major Y-junction module as an inset. The imaging results are shown in Figure 17. Figure 17a shows the imaging results for a commercially available test target with paper on top to hindrance to the currently available techniques. reproduce a real-life obstacle. The test target was successfully scanned at a resolution of 0.28 mm. Upon successful confirmation of the imaging resolution of the system, imaging 4. Imaging Application THz waves are well-suited for imaging applications. THz waves can penetrate a wide variety of non-conducting materials such as wood, paper, cardboard, plastic, and ceramic. This has allowed for key novel applications in healthcare for single-strand DNA detection [31] and in security for enhanced non-invasive detection of concealed weapons [7]. Other notable applications have been realized in industrial inspections for the detection of de- fects [32]. However, imaging applications have been restricted by the physically large size of the systems because of the need for components such as splitters and lenses. Employing a large number of individual components increases the system complexity and imposes a more stringent alignment for optical paths. More practical imaging systems may require a reduction in the physical size and complexity as more compact systems can allow for large-scale hybrid integration and open the door to novel applications. Recent research has sought hybrid integration with flat optics, allowing for the realization of all-Si lenses [33,34], beam splitters [35], and filters [36]. This has significantly reduced the size and complexity of imaging systems, resulting in compact designs that can be handheld [37], enabling applications at higher terahertz frequencies and novel applications such as drone-borne technology. Having realized and demonstrated the WR-1 band Y-junction module, we attempted to demonstrate the practical imaging applications of this module. Targeting the frequen- cies in the 0.75–1.1 THz range is a step towards high-resolution imaging applications. To Photonics 2022, 9, 515 16 of 19 Photonics 2022, 9, x FOR PEER REVIEW 18 of 21 of an integrated circuit (IC) card was successful. The results are presented in Figure 17b, revealing that the internal circuit lies beneath the cover of the IC card. The imaging results indicate that THz waves can penetrate nonconducting materials. Imaging target Motorized stage Paper Parabolic mirrors Horn antenna Incident Reflected 2 wave wave Multiplier (×27) 3 1 DMM SBD Lock-in amplifier Signal generator (a) Sample holder Y-branch module SBD 2 cm Multiplier Parabolic mirrors 2 cm (b) Figure 16. Imaging experimental setup: (a) Block diagram of the setup; (b) Photograph of the Figure 16. Imaging experimental setup: (a) Block diagram of the setup; (b) Photograph of the meas- measurement showing the Y-branch module as inset. urement showing the Y-branch module as inset. Photonics 2022, 9, x FOR PEER REVIEW 19 of 21 Photonics 2022, 9, 515 17 of 19 mm mm (a) mm mm (b) Figure 17. Imaging results at 0.957 THz: (a) Imaging of test target without paper; (b); (b) Imaging Figure 17. Imaging results at 0.957 THz: (a) Imaging of test target without paper; (b); (b) Imaging results of an IC card. results of an IC card. 5. Conclusions 5. Conclusions We have presented WR-1 (0.75–1.1 THz) band dielectric waveguide modules that We have presented WR-1 (0.75–1.1 THz) band dielectric waveguide modules that employ ultra-low-loss and broadband substrateless unclad waveguides and EM waveg- employ ultra-low-loss and broadband substrateless unclad waveguides and EM wave- uides. Experimental validations of these modules revealed superior performance in terms guides. Experimental validations of these modules revealed superior performance in of loss improvement compared to that of standard metallic hollow waveguides. We also terms of loss improvement compared to that of standard metallic hollow waveguides. We developed a Y-branch module that was employed to successfully demonstrate imaging also developed a Y-branch module that was employed to successfully demonstrate imag- applications at 0.975 THz. ing applications at 0.975 THz. In this study, we have highlighted the process of realizing such waveguide modules In this study, we have highlighted the process of realizing such waveguide modules to establish a new standard for interconnects in THz systems to replace the long-standing, to establish a new standard for interconnects in THz systems to replace the long-standing, lossy, hollow waveguide module. The proposed module has the potential of hybrid lossy, hollow waveguide module. The proposed module has the potential of hybrid inte- integration with various dielectric components that are widely employed in the terahertz gration with various dielectric components that are widely employed in the terahertz range. We have demonstrated this in the case of the Y-junction; however, the packaging range. We have demonstrated this in the case of the Y-junction; however, the packaging technique discussed in this study can be extended to interferometers, providing a sturdy, lightweight technique diplatform scussed in for this study c more sophisticated an be extende systems d to interferome that can be ter the s, provid centering of a sturdy the next, generation lightweighof t p applications latform for in more sophi the THz range. sticated Building system upon s tha the t ca success n be the of cent hybrid er o integration f the next between all-Si platforms and active devices such as resonant tunneling diodes and mixer generation of applications in the THz range. Building upon the success of hybrid integra- cir tion cuits, between this technique all-Si plat is for not ms and limited acto tive devic passivee components s such as reson only anbut t tunn also eling maydiodes realizeand all compact modules of active components. mixer circuits, this technique is not limited to passive components only but also may re- alize all compact modules of active components. Author Contributions: Conceptualization, R.K. and M.F.; methodology, R.K. and M.F.; software, R.K.; validation, R.K., R.M., K.I., M.F. and T.N.; formal analysis, R.K. and M.F.; investigation, R.K., Author Contributions: Conceptualization, R.K. and M.F.; methodology, R.K. and M.F.; software, R.M. and K.I.; resources, L.Y., M.F. and T.N.; data curation, R.K.; writing—original draft preparation, R.K.; validation, R.K., R.M., K.I., M.F. and T.N.; formal analysis, R.K. and M.F.; investigation, R.K., R.K.; writing—review and editing, L.Y., M.F. and T.N.; visualization, R.K.; supervision, M.F. and T.N.; R.M. and K.I.; resources, L.Y., M.F. and T.N.; data curation, R.K.; writing—original draft prepara- project administration, M.F. and T.N.; funding acquisition, M.F. All authors have read and agreed to tion, R.K.; writing—review and editing, L.Y., M.F. and T.N.; visualization, R.K.; supervision, M.F. the published version of the manuscript. and T.N.; project administration, M.F. and T.N.; funding acquisition, M.F. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the Core Research for Evolutional Science and Technology (CREST) program of the Japan Science and Technology Agency (JPMJCR21C4), in part by KAK- Photonics 2022, 9, 515 18 of 19 ENHI Japan (20H00249), and in part by the National Institute of Information and Communications Technology (NICT), Japan, commissioned research (03001). Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Acknowledgments: The authors acknowledge the contributions of Norihiko Shibata for their fruitful discussions. 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Journal
Photonics – Multidisciplinary Digital Publishing Institute
Published: Jul 24, 2022
Keywords: terahertz; dielectric waveguide; photonics; module; communication; imaging