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Unidirectional Optical Kerr Transmittance in Hierarchical Carbon/Platinum Nanostructures

Unidirectional Optical Kerr Transmittance in Hierarchical Carbon/Platinum Nanostructures hv photonics Article Unidirectional Optical Kerr Transmittance in Hierarchical Carbon/Platinum Nanostructures 1 , 2 3 2 Samuel Morales-Bonilla , Cecilia Mercado-Zúñiga , Juan Pablo Campos-López , 4 5 5 , César Carrillo-Delgado , Claudia Lizbeth Martínez-González and Carlos Torres-Torres * Ingeniería en Sistemas Automotrices, Escuela Superior de Ingeniería Mecánica y Eléctrica Unidad Culhuacán, Instituto Politécnico Nacional, Ciudad de México 04440, Mexico; smoralesbo@ipn.mx División de Ingeniería en Mecatrónica, Universidad Politécnica del Valle de México, Tultitlán, Estado de México 54910, Mexico; campos.dono@gmail.com Depto. Ing. Materiales, Tecnológico de Estudios Superiores de Coacalco, Coacalco de Berriozábal, Estado de México 55700, Mexico; cecilia@tesco.edu.mx Depto. de Ingeniería Robótica, Universidad Politécnica del Bicentenario, Silao, Guanajuato 36283, Mexico; ccarrillop@upbicentenario.edu.mx Sección de Estudios de Posgrado e Investigación, Escuela Superior de Ingeniería Mecánica y Eléctrica Unidad Zacatenco, Instituto Politécnico Nacional, Ciudad de México 07738, Mexico; clmartinezg@ipn.mx * Correspondence: ctorrest@ipn.mx or crstorres@yahoo.com.mx Received: 2 July 2020; Accepted: 28 July 2020; Published: 30 July 2020 Abstract: A strong contrast in the third-order nonlinear optical e ects exhibited by hierarchical nanostructures explored in a bidirectional optical circuit is reported. The samples were integrated by multiwall carbon nanotubes and platinum-decorated carbon nanotubes synthetized by an aerosol pyrolysis technique and followed by a chemical vapor deposition method. Coupled and decoupled third-order nonlinear optical properties of the nanocomposites were studied. A nanosecond two-wave mixing experiment at 532 nm wavelength was conducted to analyze the optical Kerr e ect in the samples. Multi-photonic interactions were evaluated by a single-beam transmittance as a function of input irradiance and volume fraction of the nanoparticles integrated in the nanohybrids. A two-photon absorption process was identified as the main physical mechanism responsible for the anisotropy in the observed optical nonlinearities. Random carbon nanotube networks in film form were put on top of platinum-decorated carbon nanotubes in order to build up a bilayer sample featuring optical selectivity. The switching of optical signals in propagation through the samples was obtained by an orientation-selectable optical transmittance. Unidirectional optically controlled laser pulses dependent on irradiance and polarization in a two-wave mixing was proposed with potential nanophotonic and nanoelectronic applications. The design of signal processing functions driven by nanohybrid platforms can be contemplated. Keywords: nonlinear optics; two-wave mixing; sensors; carbon nanotubes; platinum nanoparticles 1. Introduction In the last years, the progress in nanotechnology has promoted outstanding solutions for developing new alternatives in material science and engineering. A growing number of di erent nanoparticles have been demonstrated to be useful in multifunctional applications regarding their remarkable nonlinear optical and mechanical characteristics [1–7]. In this aspect, the advanced features of carbon-based nanomaterials pointed out unique geometrical structures, high-speed electronic phenomena, photonic nonlinearities, and superlative mechanical e ects [8–11]. Nanostructures can behave di erently from macrostructures due to the presence of significant size e ects that are not present at the macro scale, i.e., surface stresses, strain gradients, Photonics 2020, 7, 54; doi:10.3390/photonics7030054 www.mdpi.com/journal/photonics Photonics 2020, 7, 54 2 of 15 and non-locality [12]. Particularly, carbon nanotubes (CNT) take advantage of their size and shape to present an exceptional mechanical response under the influence of external agents and resonance e ects that may be useful to study pretty small deformations at the order of nanometers [13]. A substantial e ort has been made in order to analyze the structural nature composed by carbon atoms linked in hexagonal shapes and containing several hexagonal shapes conformed in concentric nanotubes [14]. When the length of CNT is several times larger than their inner diameter, they behave as electrical dipoles for a specific wavelength, and then, a strong selectivity for optical absorption can be originated [15]. In this direction, multiwall CNT (MWCNT) can absorb approximately 3 times the amount of light absorbed by single-wall CNT (SWCNT) because of the presence of more electrons available for absorption per particle [16]. Furthermore, the electrical and optical behavior exhibited by CNT can be enhanced by the modification in their chirality and alignment [17]. Moreover, hierarchical carbon nanostructures have been attractive as a platform for the fabrication of plasmonic-based sensors. The light-to-physical conversion eciencies of CNT have been tailored to be significantly larger for specific quantum processes derived from metal decoration methods [18]. In addition, polarization and irradiance conditions can be chosen to produce multivalent e ects that can be explained by simple fractional models in real applications [19]. By using chemical deposition methods, a good control on the parameters of synthesis in noble metal-decorated carbon nanostructures has been obtained [20]. Exhaustive studies have been carried out in the nonlinear optical properties of noble metal nanoparticles integrated in nanocomposites [21]. Nevertheless, the optical properties in platinum nanoparticles (Pt NPs) are still weakly reported [22]. Even though the peak in the absorption band of the Surface Plasmon Resonance (SPR) of platinum is at 215 nm [23], it is still observed to have saturable absorption at lower fluences, which are usually linked to SPR in gold nanoparticles (532 nm) [24]. Fuel cells, electronic signal modulation, photonic sensing, and nanomechanical actuators are some of the other fascinating applications that can be contemplated using carbon-based nanohybrids [25]. The nonlinear optical response exhibited by carbon/metal nanostructures can play a key role in a wide number of scientific disciplines related to photoelectrical devices [26] and all-optical instrumentation systems [27], among others. Therefore, considering the arguments mentioned above, this research has been devoted to further investigate the vectorial e ects produced by nonlinear optical phenomena in CNT. The influence of platinum decoration in the studied CNT allowed us to present hierarchical nanostructures with orientation-selectable nonlinear optical response. A modification in the optical and mechanical e ects exhibited by CNT samples was numerically and experimentally analyzed. The nanosecond third-order nonlinear optical behavior of the samples was evaluated by considering discrete groups of energy automatically modulated by o -resonance pulses in two-wave mixing experiments. The potential use of carbon-based nanostructures promises the development of good candidates for designing sensors and instrumentation nanodevices [28,29]. The nonlinear optical e ects exhibited by MWCTs have been suggested for optical instrumentation [30] and signal processing [31]. Remarkably, the tuning of refractive and absorptive e ects by intense light can be useful for flexible optical platforms [32]. Moreover, optical nonlinearities can be contemplated for implementing all-optical systems sensitive to the vectorial nature in the superposition of optical waves [33]. The significance of this study pointed out third-order nonlinear optics in carbon/metal nanohybrids for interferometrically controlled functions [34,35]. 2. Materials and Methods 2.1. Sample Preparation For the preparation of the CNT, an aerosol pyrolysis method was employed [35]. The aerosol was ultrasonically generated, and an argon flow of 2.5 l/min was employed. Toluene and ferrocene in a carbon solution were used in the synthesis. The resulting solution in saturation condition was Photonics 2020, 7, 54 3 of 15 deposited in quartz tube at 800 C in order to produce a stable transport in the precipitation e ect and to guarantee the growing of the nanotubes. In order to include platinum nanoparticles in the CNT samples, a chemical vapor deposition technique was used. The CNT samples located in a horizontal quartz tube reactor at about 5–7 Torr received two sequential thermal processes. As a part of the process, a mixture of Pt precursor [(CH –COCHCO–CH ) Pt; Aldrich 97%] was incorporated by grinding with agate mortar and pestle 3 3 2 for 10 min. The resulting mixture under Ar gas flow (100 cm /min) was heated at 180 C for 10 min and then at 400 C for 10 min. The morphology and structure of the samples was analyzed by Transmission Electron Microscopy (TEM; JEM 220FS) and Scanning Electronic Microscopy (SEM; SEM ULTRA 55 FEG System from ZEISS with Secondary Electron and Backscattering Detector). Compositional analysis was carried out by Energy-Dispersive X-ray Spectroscopy (EDS; JEOL JSM-6701F). The UV-vis spectra of the samples suspended in ethanol was recorded by a USB 2000+XR1-ES spectrometer assisted by a DH-2000 light source with a 300–900 nm wavelength of emission. The nonlinear optical experiments were performed with samples in film form with approximately 50 m thickness and deposited on quartz substrates. The MWCNT film was put on top of the Pt decorated MWCNT (Pt-MWCNT) in order to integrate a bilayer sample for the nonlinear optical measurements. 2.2. Nanosecond Transmittance and Bidirectional Two-Wave Mixing Experiments A Neodymium-Yttrium Garnet (Nd-YAG) laser system Continuum Model SL II-10 emitting pulses of 4 nanosecond in single-shot mode at 532 nm wavelength was used as an optical source to study the nonlinear optical response of the samples. Single-beam experiments were carried out in order to observe any nonlinear optical absorption exhibited by the samples. A vectorial two-wave mixing method [36] was used to analyze the nonlinear refraction and nonlinear absorption in the samples irradiated by the nanosecond pulses. A schematic illustration associated with the implementation of the two-wave mixing can be observed in Figure 1. L1–3 represent the lenses that focused the spot size in the sample to be approximately 0.1 mm. The beam splitters BS1–2 separate the principal beam into the pump beam and two probe beams with linear polarization. The beams were reflected by mirrors M1–9 to follow di erent optical paths. A half-wave plate, /2, was used to rotate the polarization of the pump beam during the experiment, while the polarization of both probe beams was fixed. The modulation of the polarization of each probe beam by the participation of the pump beam was separately analyzed by the polarizers, A1–3, with its transmission axis in the orthogonal position with respect to the initial polarization of the probe beam. It means that the probe beam traveling from right to left (forward direction) was blocked during the measurement of the probe beam traveling from left to right (backward direction), and vice versa. The transmitted irradiance was measured by photodetectors PD1–3. The two-wave mixing experiments were carried out in both backward and forward directions in respect to the incidence in the bilayer sample with the MWCNT in the right part of the experimental setup and the Pt-MWCNT in the left part. This was made in order to see if a change in the transmittance of the probe beams depends on the zone of the sample that interacts first with the beam, MWCNT, or Pt-MWCNT. We noticed an increase in the probe transmittance in the two-wave mixing experiment by using geometrical angles between interacting beams close to 1 , as it has been previously reported for Kerr media [37]. However, an increase in the error bar was also promoted as a result of an increase in the scattering emerging from our sample in the Raman-Nath di raction regime. With these considerations, the geometrical angle between beams was systematically settled at 10 according to our evaluation of the relation of signal-to-noise rate in the experimental setup. The stability of the pulse at the output of our laser system employed in the experiments was about3%. Regarding that the transmittance in the two-wave mixing is a third-order nonlinear optical process, there is an error bar that is larger as a consequence of the nonlinearity involved in the studied optical processes. The error bar in our transmittance experiments and two-wave mixing is approximately10%. Each point in the experimental data related to nonlinear transmittance experiments and two-wave mixing experiments corresponds to an average of 10 shots. Photonics 2020, 7, 54 4 of 15 Photonics 2020, 10, x FOR PEER REVIEW 4 of 16 Figure 1. Schematic illustration of the two-wave mixing experiment. Figure 1. Schematic illustration of the two-wave mixing experiment. The numerical estimation of the transmitted irradiances was approximated by using the wave The numerical estimation of the transmitted irradiances was approximated by using the wave equation [38]: equation [38]: 2 2 n ! n  r 2E = E (1) (1) EE   2  2  where the electric fields in propagation through the samples are represented by the circular components where the electric fields in propagation through the samples are represented by the circular of the right and left electric fields E+ and E, respectively. The optical frequency of the light is !, components of the right and left electric fields E+ and E−, respectively. The optical frequency of the the index of refraction is n, and the speed of the light is c. We consider a refractive index dependent on light is ω, the index of refraction is n, and the speed of the light is c. We consider a refractive index irradiance that can be approximated as follows [38]: dependent on irradiance that can be approximated as follows [38]: (3) (3) (3) 2 2 2 2 2 2 2 2 ( 3) ( 3) ( 3) n = n + 4  jE j + ( +  )jE j (2) n  n  4  E  (  ) E (2) 0  1122 1122 1212   0 1122 1122 1212 the weak-field refractive index is represented in Equation (2) as n , and the independent components the weak-field refractive index is represented in Equation (2) as n , and the independent (3) (3) (3) (3) (3) of the third-order optical susceptibility tensor  are  and  . The calibration of the two-wave (3)   components of the third-order optical susceptibility tensor 1122 χ are1212 and . The calibration of 1122 1212 mixing experiment was conducted by using a CS sample with the magnitude of its third-order the two-wave mixing experiment was conducted by using a CS2 sample with the magnitude of its (3) 12 nonlinear optical susceptibility,  = 1.9  10 esu [38], contained in a quartz cuvette with 1 (3) −12 third-order nonlinear optical susceptibility, = 1.9 × 10 esu [38], contained in a quartz cuvette mm length. (3) with 1 mm length. The real and imaginary parts of the complex magnitude associated with  can be given by [38], (3) The real and imaginary parts of the complex magnitude associated with  can be given by n c n c (3) 0 0 [38], = n + i (3) 2 2 7.91 10 n c n c (3) (3)  ni where  is the optical wavelength, n is the nonlinear refractive index, and the nonlinear optical 1111 2 7.9110  absorption coecient. where Theλ mathematical is the optical description wavelength,for n2 i the s thtransmitted e nonlinear re irradiance, fractive index, I, as a and function β the non of the linepr ar opagation optical absorption coefficient. distance L, with incident irradiance I through a nonlinear optical absorptive media is: The mathematical description for the transmitted irradiance, I, as a function of the propagation I exp( L) distance L, with incident irradiance I0 through a non o linear opt o ical absorptive media is: I(L) = , (4) 1 + I L o e f f IL exp( ) oo IL ( ) , (4) 1IL with as the optical absorption coecient al low irradiance, and L as the e ective length given by o eff 0 e the mathematical expression: with α0 as the optical absorption coefficient al low irradiance, and Leff as the effective length given by (1 exp( L)) L = . (5) the mathematical expression: e f f Photonics 2020, 10, x FOR PEER REVIEW 5 of 16 1 exp  L    L  eff (5) In order to analyze the contribution of different elements integrated in a hybrid nanostructure, Photonics 2020, 7, 54 5 of 15 (3) the magnitude of  can be approximated taking into account the relation in volume fraction, ρ, in the nanostructures as follows, In order to analyze the contribution of di erent elements integrated in a hybrid nanostructure, (3) (3) (3)   1   (3)   (6) ms m s the magnitude of  can be approximated taking into account the relation in volume fraction, , in the nanostructures as follows, (3) (3) (3) (3) = (1 ) +  (6) where ms  represents the nonlinear third-order suscepti m bility s of the integrated nanohybrids with m+s (3) (3) (3) (3) where   represents the nonlinear third-order susceptibility of the integrated nanohybrids with m and s as the correspondent values of the uncoupled nanostructures. m m+s (3) and  as the correspondent values of the uncoupled nanostructures. 3. Results and Discussion 3. Results and Discussion Figure 2a depicts a typical TEM image of the CNT samples in bright field mode confirming the Figure 2a depicts a typical TEM image of the CNT samples in bright field mode confirming the multiwall nature of the nanotubes. The darker array in the micrograph represents the inner diameter multiwall nature of the nanotubes. The darker array in the micrograph represents the inner diameter (d) of a representative CNT, and the surrounding layers represent multiwall layers that form the (d) of a representative CNT, and the surrounding layers represent multiwall layers that form the outer outer diameter (D). The distance between the inner and outer diameter is the thickness (t) of the diameter (D). The distance between the inner and outer diameter is the thickness (t) of the MWCNT. MWCNT. Figure 2b shows an isolated pristine CNT in a SEM image. The length L of a tube can be Figure 2b shows an isolated pristine CNT in a SEM image. The length L of a tube can be seen. Figure 2c seen. Figure 2c shows an SEM micrograph of a representative Pt-decorated MWCNT sample; the shows an SEM micrograph of a representative Pt-decorated MWCNT sample; the bright points in the bright points in the image correspond to the Pt nanoparticles incorporated in the walls of the tubes. image correspond to the Pt nanoparticles incorporated in the walls of the tubes. Figure 2d shows the Figure 2d shows the EDS analysis revealing the presence of Pt in the metal-decorated CNT samples EDS analysis revealing the presence of Pt in the metal-decorated CNT samples and confirming the and confirming the concentration of the metal in the nanostructures. concentration of the metal in the nanostructures. (a) (b) (d) (c) Figure 2. (a) TEM image of a typical carbon nanotubes (CNT) concerning the studied sample; (b) SEM image of an isolated pristine CNT; (c) SEM image of platinum-decorated multiwall CNT (Pt-MWCNT); (d) Energy-Dispersive X-ray Spectroscopy (EDS) in a representative section of Pt nanoparticles supported on a CNT bundle. Similar optical absorption spectra were acquired during the evaluation of the MWCNT and Pt-MWCNT samples in a liquid suspension; both spectra are comparatively equal. A concentration of 1 Photonics 2020, 10, x FOR PEER REVIEW 6 of 16 Figure 2. (a) TEM image of a typical carbon nanotubes (CNT) concerning the studied sample; (b) SEM image of an isolated pristine CNT; (c) SEM image of platinum-decorated multiwall CNT (Pt- MWCNT); (d) Energy-Dispersive X-ray Spectroscopy (EDS) in a representative section of Pt nanoparticles supported on a CNT bundle. Similar optical absorption spectra were acquired during the evaluation of the MWCNT and Pt- Photonics 2020, 7, 54 6 of 15 MWCNT samples in a liquid suspension; both spectra are comparatively equal. A concentration of 1 mg of nanostructures in 5 mL of ethanol was selected in order to clearly see the peak in the absorption mg of nanostructures in 5 mL of ethanol was selected in order to clearly see the peak in the absorption band of the samples. Figure 3 plots the typical UV-vis absorption spectrum of the studied Pt-MWCNT band of the samples. Figure 3 plots the typical UV-vis absorption spectrum of the studied Pt-MWCNT samples where it is possible to see close to the 270 nm wavelength the resonant response associated samples where it is possible to see close to the 270 nm wavelength the resonant response associated with the π–π bond of the carbon nanostructures. This UV region also corresponds to the wavelengths with the – bond of the carbon nanostructures. This UV region also corresponds to the wavelengths where the absorption peak of the Localized Surface Plasmon Resonance exhibited by Pt nanoparticles where the absorption peak of the Localized Surface Plasmon Resonance exhibited by Pt nanoparticles could emerge [39]. could emerge [39]. Figure 3. Typical UV-vis absorbance spectrum of the studied samples. Figure 3. Typical UV-vis absorbance spectrum of the studied samples. An ablation threshold of 110 mJ/cm in the samples was experimentally measured at a 532 nm An ablation threshold of 110 mJ/cm in the samples was experimentally measured at a 532 nm wavelength with 4 ns pulse duration in single-shot mode. We verified that thermal damage can be wavelength with 4 ns pulse duration in single-shot mode. We verified that thermal damage can be derived by heat propagation up 50 C in the samples conducted by a Thermo-Scientific CIMAREC derived by heat propagation up 50 °C in the samples conducted by a Thermo-Scientific CIMAREC system (model SP131635), which was assisted by an infrared pyrometer (Master Instruments model system (model SP131635), which was assisted by an infrared pyrometer (Master Instruments model MI-1326S). However, no important changes in temperature were detected in the samples irradiated MI-1326S). However, no important changes in temperature were detected in the samples irradiated by o -resonance optical pulses in single-shot mode at 532 nm wavelength below 20 MW/cm . Then, by off-resonance optical pulses in single-shot mode at 532 nm wavelength below 20 MW/cm . Then, we analyzed if a mechanical action induced by high-irradiance pulses could be promoted in the we analyzed if a mechanical action induced by high-irradiance pulses could be promoted in the nanotubes. It is known that the physical properties of CNT are sensitive due to their diameter, nanotubes. It is known that the physical properties of CNT are sensitive due to their diameter, length, length, and chirality. These values have a strong influence on the electronic properties of CNT. It has and chirality. These values have a strong influence on the electronic properties of CNT. It has been been shown that CNT can be either metallic or semi-conducting materials [40]. With this in mind, shown that CNT can be either metallic or semi-conducting materials [40]. With this in mind, we we assumed that the geometry of CNT can be defined by the internal tube diameter (d), the external assumed that the geometry of CNT can be defined by the internal tube diameter (d), the external tube tube diameter (D), a longitudinal length (L), a total thickness (t), and a circular cross-section. The ratio diameter (D), a longitudinal length (L), a total thickness (t), and a circular cross-section. The ratio L/D L/D is called slenderness ratio of the tube. The lowest length L of CNT was close to 180 nm according to is called slenderness ratio of the tube. The lowest length L of CNT was close to 180 nm according to our statistical TEM observations. For simplicity, we used a 12 carbon atoms configuration at each layer our statistical TEM observations. For simplicity, we used a 12 carbon atoms configuration at each in our numerical simulations. It is worth mentioning that the mechanical behavior of CNT under the layer in our numerical simulations. It is worth mentioning that the mechanical behavior of CNT influence of external factors has been described by many techniques; but essentially, these techniques under the influence of external factors has been described by many techniques; but essentially, these correspond to ab initio processes, quantum mechanics, and Finite Element Method (FEM) calculations techniques correspond to ab initio processes, quantum mechanics, and Finite Element Method (FEM) by using discrete and continuous models [41,42]. In this paper, in order to describe the specific geometry calculations by using discrete and continuous models [41,42]. In this paper, in order to describe the properties, we took into account the main considerations described by Fan, et al. [43]. We used the specific geometry properties, we took into account the main considerations described by Fan, et al. discrete FEM method taking into account each element as a mechanical beam in order to form a zig-zag [43]. We used the discrete FEM method taking into account each element as a mechanical beam in carbon layer. We considered 1129 beam elements type B33 to determine the final geometry of the order to form a zig-zag carbon layer. We considered 1129 beam elements type B33 to determine the internal cylinder [44]. The software used in the simulations was ABAQUS. L/D was the principal geometrical parameter to evaluate the deformation of the tube. Thus, carbon atoms were considered as nodes, and the distances between every two atoms were treated as mechanical beams with an initial distance equivalent to the covalent bond. Thus, the carbon–carbon bond length ( ) was c-c assumed to be 0.142 nm [45]. However, no established values are available for the wall thickness of Photonics 2020, 10, x FOR PEER REVIEW 7 of 16 final geometry of the internal cylinder [44]. The software used in the simulations was ABAQUS. L/D was the principal geometrical parameter to evaluate the deformation of the tube. Thus, carbon atoms were considered as nodes, and the distances between every two atoms were treated as mechanical Photonics 2020, 7, 54 7 of 15 beams with an initial distance equivalent to the covalent bond. Thus, the carbon–carbon bond length (αc-c) was assumed to be 0.142 nm [45]. However, no established values are available for the wall an thickness isolated of nanot an isol ube. ated The nanot values ube. of Th thickness e values suggested of thickness by su available ggested literatur by availe abl varied e litersignificantly ature varied significantly from 0.066 nm [46] to 0.68 nm [47]. In view of such a wide range of suggested thickness, from 0.066 nm [46] to 0.68 nm [47]. In view of such a wide range of suggested thickness, our study was our complemented study was comusing plemeTEM nted observations using TEM ob tosdeduce ervations this tovalue. deduc Our e thobservations is value. Our indicate observation that a s indicate that a MWCNT had about 44 walls with a total thickness of 15 nm (see Figure 2a), thus, an MWCNT had about 44 walls with a total thickness of 15 nm (see Figure 2a), thus, an appropriate wall approp thickness riate wall from thica knes CNT s .from Taking a CNT into . T account aking into our ac TEM count observations, our TEM obse we rvatio conclude ns, wethat conc the lude value that the value of e must be near 0.341 nm. This value is in agreement with the thickness 0.335 nm reported of e must be near 0.341 nm. This value is in agreement with the thickness 0.335 nm reported in the in the literature [48]. The effect of Van der Waals interaction over the vibration characteristics of literature [48]. The e ect of Van der Waals interaction over the vibration characteristics of MWCNT MWCNT is crucial for describing physical effects emerging from carbon nanostructures [49]. is crucial for describing physical e ects emerging from carbon nanostructures [49]. Particularly, Particularly, the fundamental mechanical properties exhibited by MWCNT can be analyzed by the fundamental mechanical properties exhibited by MWCNT can be analyzed by considering linear considering linear vibrations exhibited by concentric SWCNT with Van der Waals interactions vibrations exhibited by concentric SWCNT with Van der Waals interactions between each pair of between each pair of layers [50]. Concerning the cross circular section of MWCNT, we observed a layers [50]. Concerning the cross circular section of MWCNT, we observed a range of diameters from 1 range of diameters from 1 to 10 nm; thus, we considered an average diameter of 5 nm and a length L to 10 nm; thus, we considered an average diameter of 5 nm and a length L 180 nm. A unit scaling ≥ 180 nm. A unit scaling processor was used to reduce the numerical error due to the very small processor was used to reduce the numerical error due to the very small dimensions of the results. dimensions of the results. By doing this, we obtained similar results from Li, et al. [42]. By doing this, we obtained similar results from Li, et al. [42]. We also consider that our sample in film form could present a strain in the CNT when it is We also consider that our sample in film form could present a strain in the CNT when it is irradiated by a fluence close to the photodamage. The action of light over whole cylinders shut on irradiated by a fluence close to the photodamage. The action of light over whole cylinders shut on the global system, thus, an increment in sensitivity due to excitations of electrons can be expected per the global system, thus, an increment in sensitivity due to excitations of electrons can be expected unit of length. In any case, when the global system is modulated by a mechanical effect, changes in per unit of length. In any case, when the global system is modulated by a mechanical e ect, changes electromagnetic parameters must be also present. Samples integrated by CNT strongly absorb the in electromagnetic parameters must be also present. Samples integrated by CNT strongly absorb energy associated with vibration of electrons as a result of laser irradiance taking into account the the energy associated with vibration of electrons as a result of laser irradiance taking into account multilayer configuration; the energy transferred to each carbon layer modifies their geometric the multilayer configuration; the energy transferred to each carbon layer modifies their geometric configuration due to a strain effect. Besides, inhomogeneous electromagnetic irradiance is able to configuration due to a strain e ect. Besides, inhomogeneous electromagnetic irradiance is able to induce an additional internal motion. Remarkably, the range of these values depends on the optical induce an additional internal motion. Remarkably, the range of these values depends on the optical irradiance before reaching optical ablation. In this direction, we also evaluated the critical load, Pcr. irradiance before reaching optical ablation. In this direction, we also evaluated the critical load, P . cr In this sense, our concern will be with the amount of energy coming from the laser system as long as In this sense, our concern will be with the amount of energy coming from the laser system as long P does not exceed the critical value Pcr. Figure 4a describes the values of P and also Pcr versus L/D as P does not exceed the critical value P . Figure 4a describes the values of P and also P versus cr cr that are related to permanent mechanical deformation and optical ablation conditions. Within this L/D that are related to permanent mechanical deformation and optical ablation conditions. Within figure, we also sketch two scenarios, (1) a quasilinear behavior (green line) that delimits a plastic this figure, we also sketch two scenarios, (1) a quasilinear behavior (green line) that delimits a plastic deformation of two cylinders before reaching melting and plastic deformation conditions, and (2) a deformation of two cylinders before reaching melting and plastic deformation conditions, and (2) a plastic deformation in which ablation and P > Pcr could be expected. Figure 4b delimits the geometry plastic deformation in which ablation and P > P could be expected. Figure 4b delimits the geometry cr of an isolated carbon cylinder with a particular thickness. Figure 4c illustrates the cross circular of an isolated carbon cylinder with a particular thickness. Figure 4c illustrates the cross circular section section of MWCNT. We note from Figure 4 that P is inversely proportional to L/D. For a typical of MWCNT. We note from Figure 4 that P is inversely proportional to L/D. For a typical mechanical mechanical behavior in the sample, we can assume that critical stress (σcr) will occur across thickness. behavior in the sample, we can assume that critical stress ( ) will occur across thickness. cr (a) Figure 4. Cont. Photonics 2020, 10, x FOR PEER REVIEW 8 of 16 Photonics 2020, 7, 54 8 of 15 (b) (c) Figure 4. (a) Plot of slenderness ratio vs. load; (b) schematic representation of an isolated nanotube; Figure 4. (a) Plot of slenderness ratio vs. load; (b) schematic representation of an isolated nanotube; (c) outer diameter D and inner diameter d of MWCNT. (c) outer diameter D and inner diameter d of MWCNT. It should be noted that the condition L/D  2.3 can be related to the potential buckling in the It should be noted that the condition L/D ≥ 2.3 can be related to the potential buckling in the structure of the tube due to the action of nanosecond pulses with irradiances below the ablation structure of the tube due to the action of nanosecond pulses with irradiances below the ablation threshold of the samples. In this case, our concept of buckling refers to the excitation of electrons threshold of the samples. In this case, our concept of buckling refers to the excitation of electrons from from the inner diameter to the external diameter of MWCNT to produce permanent deformations and the inner diameter to the external diameter of MWCNT to produce permanent deformations and photodamage. It is worth mentioning that these opto-mechanical idealistic results obey the Euler ’s photodamage. It is worth mentioning that these opto-mechanical idealistic results obey the Euler’s critical load for buckling defined by [51]. critical load for buckling defined by [51]. P  E cr 2 =  E (7) cr (L/r) (7) Lr   In this case, the e ect of the value P /A becomes negligible for large values of L/D. Furthermore, cr In this case, the effect of the value Pcr/A becomes negligible for large values of L/D. Furthermore, we should keep in mind that the right position of loading P as a result of light propagation is seldom we should keep in mind that the right position of loading P as a result of light propagation is seldom known with a high degree of precision. However, in this idealistic approach, we initially evaluated the known with a high degree of precision. However, in this idealistic approach, we initially evaluated value of the load, P, for CNT in a rigid condition. It can be considered that increments in temperature the value of the load, P, for CNT in a rigid condition. It can be considered that increments in generate a reduction in the magnitude of P. Furthermore, if P is not aligned (the most probably temperature generate a reduction in the magnitude of P. Furthermore, if P is not aligned (the most scenario), the mechanical momentum might increase the sensitivity of the CNT. probably scenario), the mechanical momentum might increase the sensitivity of the CNT. From Figure 4a, we also observe that values P  62 nN and L/D  2.3 describe a quasi-linear From Figure 4a, we also observe that values P ≤ 62 nN and L/D ≤ 2.3 describe a quasi-linear mechanical behavior exhibited by the sample studied. On the contrast, if L/D > 2.3, it gives origin to a mechanical behavior exhibited by the sample studied. On the contrast, if L/D > 2.3, it gives origin to deformation and a possible melting process. This result can be related to interatomic forces between a deformation and a possible melting process. This result can be related to interatomic forces between carbon atoms in cylinder layers and a direct consequence of the geometrical parameters associated with carbon atoms in cylinder layers and a direct consequence of the geometrical parameters associated D and L. A proportional relation between Young´s modulus and the values of diameters D–d can be with D and L. A proportional relation between Young´s modulus and the values of diameters D–d expected in the sample. A high Young´s modulus will be present for a low nanotube internal diameter can be expected in the sample. A high Young´s modulus will be present for a low nanotube internal d; this is attributed to an increment in cross-sectional area (A) to transmit the load as a consequence of diameter d; this is attributed to an increment in cross-sectional area (A) to transmit the load as a an increment of thickness t from MWCNT. consequence of an increment of thickness t from MWCNT. From our TEM explorations concerning MWCNT, we estimated the following geometrical values: From our TEM explorations concerning MWCNT, we estimated the following geometrical D = 134 nm, d = 4 nm, L > 180 nm, and thickness 15 nm. The Young´s modulus from these results values: D = 134 nm, d = 4 nm, L > 180 nm, and thickness 15 nm. The Young´s modulus from these is in a range from 0.81 TPa to 1.28 TPa; thus, we assumed that our predictions are within reasonable results is in a range from 0.81 TPa to 1.28 TPa; thus, we assumed that our predictions are within agreement with previous results in carbon nanostructures [52]. On the other hand, our results from the reasonable agreement with previous results in carbon nanostructures [52]. On the other hand, our computational technique are also comparable to those provided by di erent models reported with results from the computational technique are also comparable to those provided by different models magnitudes from 1.0 to 1.24 TPa [53]. One advantage of this analysis is the simplicity of the working reported with magnitudes from 1.0 to 1.24 TPa [53]. One advantage of this analysis is the simplicity principle based on the classical structural mechanics and experimental observations. of the working principle based on the classical structural mechanics and experimental observations. Standard two-wave mixing measurements were conducted by using the setup illustrated in (3) Standard two-wave mixing measurements were conducted by using the setup illustrated in Figure 1 in order to evaluate the magnitude of  in the nanostructures integrated in film form. (3) Fi Figur gure e1 5in shows order tto he eva exper luate imental the ma results gnitude asof a function  in th of e the nanostr variation uctures ofintegr the volume ated in film fraction form. of nanoparticles of Pt decorating MWCNT in the sample studied in film form. The numerical fitting was Figure 5 shows the experimental results as a function of the variation of the volume fraction of (3) made by using Equation (1). The estimated value for the MWCNT was  = 2.2 10 esu, and for nanoparticles of Pt decorating MWCNT in the sample studied in film form. The numerical fitting was Photonics 2020, 10, x FOR PEER REVIEW 9 of 16 Photonics 2020, 7, 54 9 of 15 (3)  −9 made by using Equation (1). The estimated value for the MWCNT was m = 2.2 × 10 esu, and for (3) (3) 8 −8 the Pt nanoparticles, = 2.76 × 10 esu according to a decoupling approximation for the the Pt nanoparticles,  = 2.76 10 esu according to a decoupling approximation for the nonlinear non optical liner ar esponse optical re of composite sponse of comp media osite expr media essed expres by Equation sed by(6). Equation (6). ( (3 3)) Figure 5. Magnitude of  as a function of a change in the volume fraction of nanoparticles of Pt Figure 5. Magnitude of as a function of a change in the volume fraction of nanoparticles of Pt decorating MWCNT in a sample in film form. decorating MWCNT in a sample in film form. From Figure 5, it can be clearly observed that the incorporation of 30% of the volume fraction of From Figure 5, it can be clearly observed that the incorporation of 30% of the volume fraction of Pt Pt nanoparticles in the MWCNT can improve the nonlinearity by a factor of almost four. However, nanoparticles in the MWCNT can improve the nonlinearity by a factor of almost four. However, it is (3) it is remarkable that the estimated optical nonlinearity  of the metal nanoparticles seems to be one (3) (3) remarkable that the estimated optical nonlinearity s of the metal nanoparticles seems to be one order of magnitude higher than the same parameter exhibited by the MWCNT denoted as  . (3) (3) In order to resolve the imaginary part related to  , we explored an input–output experiment as a order of magnitude higher than the same parameter exhibited by the MWCNT denoted as . function of optical irradiance and volume fraction of the Pt nanoparticles incorporated in the MWCNT (3) In order to resolve the imaginary part related to , we explored an input–output experiment samples. The experimental results are depicted in Figure 6. The numerical fitting of obtained by as a function of optical irradiance and volume fraction of the Pt nanoparticles incorporated in the Equation (4) is shown in Table 1 together to the n coecients estimated by Equation (3) and the results (3) MWCNT samples. The experimental results are depicted in Figure 6. The numerical fitting of β of  measured in this research. Photonics 2020, 10, x FOR PEER REVIEW 10 of 16 obtained by Equation (4) is shown in Table 1 together to the n2 coefficients estimated by Equation (3) (3) and the results of  measured in this research. (3) (3) Figure 6. Magnitude of as a function of a change in the volume fraction of nanoparticles of Pt Figure 6. Magnitude of  as a function of a change in the volume fraction of nanoparticles of Pt decorating MWCNT in a sample in film form. decorating MWCNT in a sample in film form. Table 1. Nonlinear optical parameters evaluated in nanohybrids studied at a 532 nm and 4 ns pulses. ρ Related to Pt n2 β (3) Nanoparticles Incorporated  [esu] [m /W] [m/W] to [m /W] −15 −9 0 –1.46 × 10 - 2.21 × 10 −15 −9 −9 0.1 –4.03 × 10 3.1 × 10 6.10 × 10 −15 −9 −9 0.2 –5.62 × 10 5.9 × 10 8.53 × 10 −15 −9 −9 0.3 –6.08 × 10 10.9 × 10 9.25 × 10 It is interesting that the strong n2 exhibited by the pure MWCNT can be considered for envisioning all-optical modulation applications; however, the incorporation of the metal nanoparticles causes an increase in n2, but also, a detriment derived by β automatically emerges. With this implication, we designed a bilayer sample with Pt-MWCNT and pure MWCNT in order to obtain the advantage of a dissimilar bidirectional response for switching functions. Vectorial two-wave mixing experiments were carried out by using the setup illustrated in Figure 1 in order to identify the modification of the probe transmittance by the pump beam action. The experimental setup was calibrated by using a standard nonlinear CS2 sample with a nonlinear −14 2 refractive index n2 = 6 × 10 cm /W [38]. Figure 7a shows the evolution of a single-beam transmitted irradiance as a function of its incident irradiance; while Figure 7b plots the transmitted probe irradiance as a function of the angle between the planes of linear polarization of the incident beams. The best fitting of the experimental data were calculated by Equation (1) and the beam propagation method. Statistical measurements were conducted in order to estimate the third-order nonlinear optical parameters shown in Table 2 for the bilayer film in the forward and backward direction of irradiation for the bilayer film with an error bar of about ±10%. These results are in good agreement with previous publications [30,54]. Photonics 2020, 7, 54 10 of 15 Photonics 2020, 10, x FOR PEER REVIEW 11 of 16 Table 1. Nonlinear optical parameters evaluated in nanohybrids studied at a 532 nm and 4 ns pulses. Table 2. Nonlinear optical parameters evaluated in the samples studied at 532 nm and 4 ns pulses. Related to Pt Nanoparticles n 2 (3) Sample Incidence n2 (cm /W) β (cm/W) [esu] 2 2 Incorporated to [m /W] [m /W] [m/W] Forward −11 15 9 –1.12 × 10 - 0 1.46 10 - 2.21 10 MWCNT/Pt-MWCNT 15 9 9 0.1 4.03 10 3.1 10 6.10 10 Backward 15 9 9 0.2 5.62 10 5.9 10 −7 8.53 10 - 4.9 × 10 15 9 9 Pt-MWCNT/MWCNT 0.3 6.08 10 10.9 10 9.25 10 The data shown in Figure 7a represent a clear signature of a two-photon absorption process in It is interesting that the strong n exhibited by the pure MWCNT can be considered for envisioning the backward direction of the experiment as a decrease in the transmittance as a function on all-optical modulation applications; however, the incorporation of the metal nanoparticles causes irradiance [38], while the inhibition of the nonlinear optical absorption seems to occur if the sample an increase in n , but also, a detriment derived by automatically emerges. With this implication, is explored in the forward direction. Conversely, Figure 7b shows a clear change in the transmitted we designed a bilayer sample with Pt-MWCNT and pure MWCNT in order to obtain the advantage of probe beam by the pump beam action just in the forward direction of the two-wave mixing. It is a dissimilar bidirectional response for switching functions. worth mentioning that an unbalanced distribution in the composition or structure of the film could Vectorial two-wave mixing experiments were carried out by using the setup illustrated in be responsible for the dispersion and asymmetry in the nonlinear data plotted in Figure 7b [55]. From Figure 1 in order to identify the modification of the probe transmittance by the pump beam action. Figure 7b, it can be seen that the maximum transmittance of the probe beam can be obtained for an The experimental setup was calibrated by using a standard nonlinear CS sample with a nonlinear angle of polarization of 45°, which is in good agreement with the fact that parallel or orthogonal 14 2 refractive index n = 6 10 cm /W [38]. Figure 7a shows the evolution of a single-beam transmitted polarizations between the incident beams do not generate any induced birefringence. We observed irradiance as a function of its incident irradiance; while Figure 7b plots the transmitted probe irradiance in our screen a good contrast in an irradiance fringe pattern by using a microscope objective located as a function of the angle between the planes of linear polarization of the incident beams. The best fitting in the interaction region of the beams in absence of the sample in the experimental setup. The of the experimental data were calculated by Equation (1) and the beam propagation method. Statistical generation of a diffracting grating was expected; however, the absence of a self-diffraction signal was measurements were conducted in order to estimate the third-order nonlinear optical parameters verified in our experiment with nanosecond pulses. The main physical mechanism responsible of the shown in Table 2 for the bilayer film in the forward and backward direction of irradiation for the nonlinearity of index seems to be a dynamic thermal effect with instability in a modulation of an bilayer film with an error bar of about 10%. These results are in good agreement with previous induced birefringence and diffraction grating. publications [30,54]. (a) (b) Figure 7. (a) Single-beam transmittance as a function of the incident irradiance; (b) transmitted probe Figure 7. (a) Single-beam transmittance as a function of the incident irradiance; (b) transmitted probe irradiance as a function of the angle between planes of polarization of the incident beams. irradiance as a function of the angle between planes of polarization of the incident beams. Table 2. Nonlinear optical parameters evaluated in the samples studied at 532 nm and 4 ns pulses. It is clear from Figure 1 that different interferometric effects can be induced by the two-wave Sample Incidence n (cm /W) (cm/W) mixing in the forward or backward configuration associated to the superposition of the pump and probe beams. Then, we experimen Forward tally confirmed the observed nonlinear optical behavior in the 1.12 10 - MWCNT/Pt-MWCNT transmitted Kerr measurements just by switching the position of the bilayer sample in the Backward experiments in a standard two-wave mixing experiment - . 4.9 10 Pt-MWCNT/MWCNT The reproducibility of the data plotted in Figures 5–7 was guaranteed for a pulse repetition rate of 1 Hz. Comparative experiments in carbon MWCNT pointed out that the participation of dynamic The data shown in Figure 7a represent a clear signature of a two-photon absorption process in the thermal transport in photothermal effects induced by nanosecond pulses at a 532 nm wavelength backward direction of the experiment as a decrease in the transmittance as a function on irradiance [38], corresponds to a decay time of about 500 ms [56,57]. For further investigation of the contrast in the nonlinear Kerr transmittance with potential modulation of the nanosecond third-order nonlinear optical behavior of the bilayer sample, we Photonics 2020, 7, 54 11 of 15 while the inhibition of the nonlinear optical absorption seems to occur if the sample is explored in the forward direction. Conversely, Figure 7b shows a clear change in the transmitted probe beam by the pump beam action just in the forward direction of the two-wave mixing. It is worth mentioning that an unbalanced distribution in the composition or structure of the film could be responsible for the dispersion and asymmetry in the nonlinear data plotted in Figure 7b [55]. From Figure 7b, it can be seen that the maximum transmittance of the probe beam can be obtained for an angle of polarization of 45 , which is in good agreement with the fact that parallel or orthogonal polarizations between the incident beams do not generate any induced birefringence. We observed in our screen a good contrast in an irradiance fringe pattern by using a microscope objective located in the interaction region of the beams in absence of the sample in the experimental setup. The generation of a di racting grating was expected; however, the absence of a self-di raction signal was verified in our experiment with nanosecond pulses. The main physical mechanism responsible of the nonlinearity of index seems to be a dynamic thermal e ect with instability in a modulation of an induced birefringence and di raction grating. It is clear from Figure 1 that di erent interferometric e ects can be induced by the two-wave mixing in the forward or backward configuration associated to the superposition of the pump and probe beams. Then, we experimentally confirmed the observed nonlinear optical behavior in the transmitted Kerr measurements just by switching the position of the bilayer sample in the experiments in a standard two-wave mixing experiment. The reproducibility of the data plotted in Figures 5–7 was guaranteed for a pulse repetition rate of 1 Hz. Comparative experiments in carbon MWCNT pointed out that the participation of dynamic thermal transport in photothermal e ects induced by nanosecond pulses at a 532 nm wavelength corresponds to a decay time of about 500 ms [56,57]. For further investigation of the contrast in the nonlinear Kerr transmittance with potential Photonics 2020, 10, x FOR PEER REVIEW 12 of 16 modulation of the nanosecond third-order nonlinear optical behavior of the bilayer sample, we analyzed a standard two-wave mixing interaction by Equation (1). Figure 8 shows numerical results describing analyzed a standard two-wave mixing interaction by Equation (1). Figure 8 shows numerical results the Kerr transmittance in the bidirectional propagation of the beams in the bilayer film interacting in a describing the Kerr transmittance in the bidirectional propagation of the beams in the bilayer film two-wave mixing. interacting in a two-wave mixing. (a) (b) Figure 8. Numerical results of the propagation of a degenerated two-wave mixing in the studied bilayer Figure 8. Numerical results of the propagation of a degenerated two-wave mixing in the studied sample at 532 nm wavelength: (a) forward direction; the beams initially interacts in the MWCNT film bilayer sample at 532 nm wavelength: (a) forward direction; the beams initially interacts in the and then in the Pt-MWCNT; (b) backward direction; the beams initially interacts in the Pt-MWCNT MWCNT film and then in the Pt-MWCNT; (b) backward direction; the beams initially interacts in the film and then in the MWCNT. Pt-MWCNT film and then in the MWCNT. It can be stated that the maximum peak of irradiance in the sample can be obtained for parallel It can be stated that the maximum peak of irradiance in the sample can be obtained for parallel polarizations between the incident beams, because this condition promotes the maximum contrast polarizations between the incident beams, because this condition promotes the maximum contrast in in the irradiance pattern generated in the sample by constructive interference. From Figure 8a,b, the irradiance pattern generated in the sample by constructive interference. From Figure 8a,b, remarkable di erences in the Kerr transmittance obtained by the propagation of the probe beam in the remarkable differences in the Kerr transmittance obtained by the propagation of the probe beam in forward and backward direction can be clearly observed in the two-wave mixing interaction. These the forward and backward direction can be clearly observed in the two-wave mixing interaction. conditions are associated to the participation of the two-photon absorption in the Pt-MWCNT that These conditions are associated to the participation of the two-photon absorption in the Pt-MWCNT results in a depletion of the incident beams before an optical Kerr e ect can be excited in the MWCNT. that results in a depletion of the incident beams before an optical Kerr effect can be excited in the MWCNT. A noticeable self-focusing effect occurs in the MWCNT when this layer initially interacts with the two-wave mixing, and it promotes a modulation of the polarization that derives in a Kerr transmittance that can be observed in an optical Kerr gate. Regarding the results demonstrated in Figures 6–8, nonlinear optical Kerr functions induced by ultrafast optical irradiance or polarization effects can be considered by hierarchical nanostructures. Researchers have previously reported important changes in the mechanical, electronic and optical properties of hybrid nanostructures because of the inclusion of CNT [58,59]. In addition, CNT can produce flexural strength in advanced nanocomposites as a function of their concentration of the tubes [60] that could be employed for the instrumentation of optomechanical signals. The density of CNT-based samples receives an important contribution from their thermo-mechanic and photo- physical characteristics [61]. The electromagnetic properties in the hierarchical composites can be correlated to hierarchical structure in good agreement with estimations related to CNT samples [62]. We highlight in this work that the polarization and phase-change in nonlinear optical signals in propagation through CNT can be modulated by orientation-selectable optical effects. Immediate applications of the influence of third-order nonlinear optical properties on hierarchical nanostructures can be considered for developing multifunctional sensors and actuators that are able to control photonic signals. The possibility of low-dimensional interconnections modulated by the vectorial nature of light in quantum systems can be contemplated. An advantage of these findings is the simplicity of the working principle based on the vectorial nature of light and the selectivity of hierarchical nanostructures for controlling nonlinear optical phenomena. 4. Conclusions The impact of platinum decoration in the orientation-selectable nonlinear optical response of hierarchical carbon nanostructures was analyzed. A unidirectional modulation of optical polarization in a bilayer sample was demonstrated by an optical circuit with a bidirectional optical Kerr gate. A strong dependence on the polarization of the induced third-order phenomena in the nanostructures Photonics 2020, 7, 54 12 of 15 A noticeable self-focusing e ect occurs in the MWCNT when this layer initially interacts with the two-wave mixing, and it promotes a modulation of the polarization that derives in a Kerr transmittance that can be observed in an optical Kerr gate. Regarding the results demonstrated in Figures 6–8, nonlinear optical Kerr functions induced by ultrafast optical irradiance or polarization e ects can be considered by hierarchical nanostructures. Researchers have previously reported important changes in the mechanical, electronic and optical properties of hybrid nanostructures because of the inclusion of CNT [58,59]. In addition, CNT can produce flexural strength in advanced nanocomposites as a function of their concentration of the tubes [60] that could be employed for the instrumentation of optomechanical signals. The density of CNT-based samples receives an important contribution from their thermo-mechanic and photo-physical characteristics [61]. The electromagnetic properties in the hierarchical composites can be correlated to hierarchical structure in good agreement with estimations related to CNT samples [62]. We highlight in this work that the polarization and phase-change in nonlinear optical signals in propagation through CNT can be modulated by orientation-selectable optical e ects. Immediate applications of the influence of third-order nonlinear optical properties on hierarchical nanostructures can be considered for developing multifunctional sensors and actuators that are able to control photonic signals. The possibility of low-dimensional interconnections modulated by the vectorial nature of light in quantum systems can be contemplated. An advantage of these findings is the simplicity of the working principle based on the vectorial nature of light and the selectivity of hierarchical nanostructures for controlling nonlinear optical phenomena. 4. Conclusions The impact of platinum decoration in the orientation-selectable nonlinear optical response of hierarchical carbon nanostructures was analyzed. A unidirectional modulation of optical polarization in a bilayer sample was demonstrated by an optical circuit with a bidirectional optical Kerr gate. A strong dependence on the polarization of the induced third-order phenomena in the nanostructures allowed us to systematically control electronic signals in the nanotube networks integrating an electronic circuit. A two-wave mixing method was used to modulate interferometrically controlled laser pulses by third-order optical nonlinearities in the nanostructures. The switching of the optical signals was assisted by the rotation of the angle between the planes of linear polarization of the incident beams in the sample. The experiments were carried out far from the nanosecond ablation threshold of the samples in single-pulse mode. This work highlights the possibility of tailoring polarization-selectable e ects in nanosystems by nonlinear optical phenomena. Potential applications for developing signal processing functions driven by nonlinearities induced by light in low-dimensional systems can be considered. The collective behavior exhibited by carbon-based nanostructures with applications for designing nanophotonic platforms driven by nonlinear optical signals can be envisioned. Author Contributions: Analysis of optical experiments, S.M.-B.; preparation, decoration and morphology characterization of the samples, C.M.-Z.; mechanical studies, J.P.C.-L.; evolution of the nonlinear optical signals during experiments, C.C.-D.; design of hierarchical vectorial functions, C.L.M.-G.; elucidated and evaluated the nonlinear optical properties and designed the paper, C.T.-T. The manuscript was written through contribution of all authors. All authors have read and agreed to the published version of the manuscript. Funding: The authors kindly acknowledge the financial support from the Instituto Politécnico Nacional, COFAA-IPN, Universidad Politécnica del Valle de México, Universidad Politécnica del Bicentenario, Tecnológico de Estudios Superiores de Coacalco and from the Consejo Nacional de Ciencia y Tecnología (CB-2015-251201). Acknowledgments: The authors kindly acknowledge to Instituto Politécnico Nacional, Comisión de Operación y Fomento de Actividades Académicas del Instituto Politécnico Nacional, Universidad Politécnica del Valle de México, Universidad Politécnica del Bicentenario, Tecnológico de Estudios Superiores de Coacalco and CONACyT. The authors are also thankful to the Central Microscopy facilities of the CNMN-IPN. Conflicts of Interest: The authors declare no conflict of interest. Photonics 2020, 7, 54 13 of 15 References 1. Di Ventra, M.; Evoy, S.; Heflin, J. Carbon Nanotubes. In Introduction to Nanoscale Science and Technology; Kluwer Academic Publishers: Ottawa, ON, Canada, 2004; Volume 1, pp. 137–181. 2. Zhao, Q.; Wood, J.R.; Wagner, H.D. Stress fields around defects and fibers in a polymer using carbon nanotubes as sensors. Appl. Phys. Lett. 2001, 78, 1748–1750. [CrossRef] 3. Ma, L.; Wang, J.; Yip, J.; Ding, F. Mechanism of Transition-Metal Nanoparticle Catalytic Graphene Cutting. J. Phys. 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Unidirectional Optical Kerr Transmittance in Hierarchical Carbon/Platinum Nanostructures

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hv photonics Article Unidirectional Optical Kerr Transmittance in Hierarchical Carbon/Platinum Nanostructures 1 , 2 3 2 Samuel Morales-Bonilla , Cecilia Mercado-Zúñiga , Juan Pablo Campos-López , 4 5 5 , César Carrillo-Delgado , Claudia Lizbeth Martínez-González and Carlos Torres-Torres * Ingeniería en Sistemas Automotrices, Escuela Superior de Ingeniería Mecánica y Eléctrica Unidad Culhuacán, Instituto Politécnico Nacional, Ciudad de México 04440, Mexico; smoralesbo@ipn.mx División de Ingeniería en Mecatrónica, Universidad Politécnica del Valle de México, Tultitlán, Estado de México 54910, Mexico; campos.dono@gmail.com Depto. Ing. Materiales, Tecnológico de Estudios Superiores de Coacalco, Coacalco de Berriozábal, Estado de México 55700, Mexico; cecilia@tesco.edu.mx Depto. de Ingeniería Robótica, Universidad Politécnica del Bicentenario, Silao, Guanajuato 36283, Mexico; ccarrillop@upbicentenario.edu.mx Sección de Estudios de Posgrado e Investigación, Escuela Superior de Ingeniería Mecánica y Eléctrica Unidad Zacatenco, Instituto Politécnico Nacional, Ciudad de México 07738, Mexico; clmartinezg@ipn.mx * Correspondence: ctorrest@ipn.mx or crstorres@yahoo.com.mx Received: 2 July 2020; Accepted: 28 July 2020; Published: 30 July 2020 Abstract: A strong contrast in the third-order nonlinear optical e ects exhibited by hierarchical nanostructures explored in a bidirectional optical circuit is reported. The samples were integrated by multiwall carbon nanotubes and platinum-decorated carbon nanotubes synthetized by an aerosol pyrolysis technique and followed by a chemical vapor deposition method. Coupled and decoupled third-order nonlinear optical properties of the nanocomposites were studied. A nanosecond two-wave mixing experiment at 532 nm wavelength was conducted to analyze the optical Kerr e ect in the samples. Multi-photonic interactions were evaluated by a single-beam transmittance as a function of input irradiance and volume fraction of the nanoparticles integrated in the nanohybrids. A two-photon absorption process was identified as the main physical mechanism responsible for the anisotropy in the observed optical nonlinearities. Random carbon nanotube networks in film form were put on top of platinum-decorated carbon nanotubes in order to build up a bilayer sample featuring optical selectivity. The switching of optical signals in propagation through the samples was obtained by an orientation-selectable optical transmittance. Unidirectional optically controlled laser pulses dependent on irradiance and polarization in a two-wave mixing was proposed with potential nanophotonic and nanoelectronic applications. The design of signal processing functions driven by nanohybrid platforms can be contemplated. Keywords: nonlinear optics; two-wave mixing; sensors; carbon nanotubes; platinum nanoparticles 1. Introduction In the last years, the progress in nanotechnology has promoted outstanding solutions for developing new alternatives in material science and engineering. A growing number of di erent nanoparticles have been demonstrated to be useful in multifunctional applications regarding their remarkable nonlinear optical and mechanical characteristics [1–7]. In this aspect, the advanced features of carbon-based nanomaterials pointed out unique geometrical structures, high-speed electronic phenomena, photonic nonlinearities, and superlative mechanical e ects [8–11]. Nanostructures can behave di erently from macrostructures due to the presence of significant size e ects that are not present at the macro scale, i.e., surface stresses, strain gradients, Photonics 2020, 7, 54; doi:10.3390/photonics7030054 www.mdpi.com/journal/photonics Photonics 2020, 7, 54 2 of 15 and non-locality [12]. Particularly, carbon nanotubes (CNT) take advantage of their size and shape to present an exceptional mechanical response under the influence of external agents and resonance e ects that may be useful to study pretty small deformations at the order of nanometers [13]. A substantial e ort has been made in order to analyze the structural nature composed by carbon atoms linked in hexagonal shapes and containing several hexagonal shapes conformed in concentric nanotubes [14]. When the length of CNT is several times larger than their inner diameter, they behave as electrical dipoles for a specific wavelength, and then, a strong selectivity for optical absorption can be originated [15]. In this direction, multiwall CNT (MWCNT) can absorb approximately 3 times the amount of light absorbed by single-wall CNT (SWCNT) because of the presence of more electrons available for absorption per particle [16]. Furthermore, the electrical and optical behavior exhibited by CNT can be enhanced by the modification in their chirality and alignment [17]. Moreover, hierarchical carbon nanostructures have been attractive as a platform for the fabrication of plasmonic-based sensors. The light-to-physical conversion eciencies of CNT have been tailored to be significantly larger for specific quantum processes derived from metal decoration methods [18]. In addition, polarization and irradiance conditions can be chosen to produce multivalent e ects that can be explained by simple fractional models in real applications [19]. By using chemical deposition methods, a good control on the parameters of synthesis in noble metal-decorated carbon nanostructures has been obtained [20]. Exhaustive studies have been carried out in the nonlinear optical properties of noble metal nanoparticles integrated in nanocomposites [21]. Nevertheless, the optical properties in platinum nanoparticles (Pt NPs) are still weakly reported [22]. Even though the peak in the absorption band of the Surface Plasmon Resonance (SPR) of platinum is at 215 nm [23], it is still observed to have saturable absorption at lower fluences, which are usually linked to SPR in gold nanoparticles (532 nm) [24]. Fuel cells, electronic signal modulation, photonic sensing, and nanomechanical actuators are some of the other fascinating applications that can be contemplated using carbon-based nanohybrids [25]. The nonlinear optical response exhibited by carbon/metal nanostructures can play a key role in a wide number of scientific disciplines related to photoelectrical devices [26] and all-optical instrumentation systems [27], among others. Therefore, considering the arguments mentioned above, this research has been devoted to further investigate the vectorial e ects produced by nonlinear optical phenomena in CNT. The influence of platinum decoration in the studied CNT allowed us to present hierarchical nanostructures with orientation-selectable nonlinear optical response. A modification in the optical and mechanical e ects exhibited by CNT samples was numerically and experimentally analyzed. The nanosecond third-order nonlinear optical behavior of the samples was evaluated by considering discrete groups of energy automatically modulated by o -resonance pulses in two-wave mixing experiments. The potential use of carbon-based nanostructures promises the development of good candidates for designing sensors and instrumentation nanodevices [28,29]. The nonlinear optical e ects exhibited by MWCTs have been suggested for optical instrumentation [30] and signal processing [31]. Remarkably, the tuning of refractive and absorptive e ects by intense light can be useful for flexible optical platforms [32]. Moreover, optical nonlinearities can be contemplated for implementing all-optical systems sensitive to the vectorial nature in the superposition of optical waves [33]. The significance of this study pointed out third-order nonlinear optics in carbon/metal nanohybrids for interferometrically controlled functions [34,35]. 2. Materials and Methods 2.1. Sample Preparation For the preparation of the CNT, an aerosol pyrolysis method was employed [35]. The aerosol was ultrasonically generated, and an argon flow of 2.5 l/min was employed. Toluene and ferrocene in a carbon solution were used in the synthesis. The resulting solution in saturation condition was Photonics 2020, 7, 54 3 of 15 deposited in quartz tube at 800 C in order to produce a stable transport in the precipitation e ect and to guarantee the growing of the nanotubes. In order to include platinum nanoparticles in the CNT samples, a chemical vapor deposition technique was used. The CNT samples located in a horizontal quartz tube reactor at about 5–7 Torr received two sequential thermal processes. As a part of the process, a mixture of Pt precursor [(CH –COCHCO–CH ) Pt; Aldrich 97%] was incorporated by grinding with agate mortar and pestle 3 3 2 for 10 min. The resulting mixture under Ar gas flow (100 cm /min) was heated at 180 C for 10 min and then at 400 C for 10 min. The morphology and structure of the samples was analyzed by Transmission Electron Microscopy (TEM; JEM 220FS) and Scanning Electronic Microscopy (SEM; SEM ULTRA 55 FEG System from ZEISS with Secondary Electron and Backscattering Detector). Compositional analysis was carried out by Energy-Dispersive X-ray Spectroscopy (EDS; JEOL JSM-6701F). The UV-vis spectra of the samples suspended in ethanol was recorded by a USB 2000+XR1-ES spectrometer assisted by a DH-2000 light source with a 300–900 nm wavelength of emission. The nonlinear optical experiments were performed with samples in film form with approximately 50 m thickness and deposited on quartz substrates. The MWCNT film was put on top of the Pt decorated MWCNT (Pt-MWCNT) in order to integrate a bilayer sample for the nonlinear optical measurements. 2.2. Nanosecond Transmittance and Bidirectional Two-Wave Mixing Experiments A Neodymium-Yttrium Garnet (Nd-YAG) laser system Continuum Model SL II-10 emitting pulses of 4 nanosecond in single-shot mode at 532 nm wavelength was used as an optical source to study the nonlinear optical response of the samples. Single-beam experiments were carried out in order to observe any nonlinear optical absorption exhibited by the samples. A vectorial two-wave mixing method [36] was used to analyze the nonlinear refraction and nonlinear absorption in the samples irradiated by the nanosecond pulses. A schematic illustration associated with the implementation of the two-wave mixing can be observed in Figure 1. L1–3 represent the lenses that focused the spot size in the sample to be approximately 0.1 mm. The beam splitters BS1–2 separate the principal beam into the pump beam and two probe beams with linear polarization. The beams were reflected by mirrors M1–9 to follow di erent optical paths. A half-wave plate, /2, was used to rotate the polarization of the pump beam during the experiment, while the polarization of both probe beams was fixed. The modulation of the polarization of each probe beam by the participation of the pump beam was separately analyzed by the polarizers, A1–3, with its transmission axis in the orthogonal position with respect to the initial polarization of the probe beam. It means that the probe beam traveling from right to left (forward direction) was blocked during the measurement of the probe beam traveling from left to right (backward direction), and vice versa. The transmitted irradiance was measured by photodetectors PD1–3. The two-wave mixing experiments were carried out in both backward and forward directions in respect to the incidence in the bilayer sample with the MWCNT in the right part of the experimental setup and the Pt-MWCNT in the left part. This was made in order to see if a change in the transmittance of the probe beams depends on the zone of the sample that interacts first with the beam, MWCNT, or Pt-MWCNT. We noticed an increase in the probe transmittance in the two-wave mixing experiment by using geometrical angles between interacting beams close to 1 , as it has been previously reported for Kerr media [37]. However, an increase in the error bar was also promoted as a result of an increase in the scattering emerging from our sample in the Raman-Nath di raction regime. With these considerations, the geometrical angle between beams was systematically settled at 10 according to our evaluation of the relation of signal-to-noise rate in the experimental setup. The stability of the pulse at the output of our laser system employed in the experiments was about3%. Regarding that the transmittance in the two-wave mixing is a third-order nonlinear optical process, there is an error bar that is larger as a consequence of the nonlinearity involved in the studied optical processes. The error bar in our transmittance experiments and two-wave mixing is approximately10%. Each point in the experimental data related to nonlinear transmittance experiments and two-wave mixing experiments corresponds to an average of 10 shots. Photonics 2020, 7, 54 4 of 15 Photonics 2020, 10, x FOR PEER REVIEW 4 of 16 Figure 1. Schematic illustration of the two-wave mixing experiment. Figure 1. Schematic illustration of the two-wave mixing experiment. The numerical estimation of the transmitted irradiances was approximated by using the wave The numerical estimation of the transmitted irradiances was approximated by using the wave equation [38]: equation [38]: 2 2 n ! n  r 2E = E (1) (1) EE   2  2  where the electric fields in propagation through the samples are represented by the circular components where the electric fields in propagation through the samples are represented by the circular of the right and left electric fields E+ and E, respectively. The optical frequency of the light is !, components of the right and left electric fields E+ and E−, respectively. The optical frequency of the the index of refraction is n, and the speed of the light is c. We consider a refractive index dependent on light is ω, the index of refraction is n, and the speed of the light is c. We consider a refractive index irradiance that can be approximated as follows [38]: dependent on irradiance that can be approximated as follows [38]: (3) (3) (3) 2 2 2 2 2 2 2 2 ( 3) ( 3) ( 3) n = n + 4  jE j + ( +  )jE j (2) n  n  4  E  (  ) E (2) 0  1122 1122 1212   0 1122 1122 1212 the weak-field refractive index is represented in Equation (2) as n , and the independent components the weak-field refractive index is represented in Equation (2) as n , and the independent (3) (3) (3) (3) (3) of the third-order optical susceptibility tensor  are  and  . The calibration of the two-wave (3)   components of the third-order optical susceptibility tensor 1122 χ are1212 and . The calibration of 1122 1212 mixing experiment was conducted by using a CS sample with the magnitude of its third-order the two-wave mixing experiment was conducted by using a CS2 sample with the magnitude of its (3) 12 nonlinear optical susceptibility,  = 1.9  10 esu [38], contained in a quartz cuvette with 1 (3) −12 third-order nonlinear optical susceptibility, = 1.9 × 10 esu [38], contained in a quartz cuvette mm length. (3) with 1 mm length. The real and imaginary parts of the complex magnitude associated with  can be given by [38], (3) The real and imaginary parts of the complex magnitude associated with  can be given by n c n c (3) 0 0 [38], = n + i (3) 2 2 7.91 10 n c n c (3) (3)  ni where  is the optical wavelength, n is the nonlinear refractive index, and the nonlinear optical 1111 2 7.9110  absorption coecient. where Theλ mathematical is the optical description wavelength,for n2 i the s thtransmitted e nonlinear re irradiance, fractive index, I, as a and function β the non of the linepr ar opagation optical absorption coefficient. distance L, with incident irradiance I through a nonlinear optical absorptive media is: The mathematical description for the transmitted irradiance, I, as a function of the propagation I exp( L) distance L, with incident irradiance I0 through a non o linear opt o ical absorptive media is: I(L) = , (4) 1 + I L o e f f IL exp( ) oo IL ( ) , (4) 1IL with as the optical absorption coecient al low irradiance, and L as the e ective length given by o eff 0 e the mathematical expression: with α0 as the optical absorption coefficient al low irradiance, and Leff as the effective length given by (1 exp( L)) L = . (5) the mathematical expression: e f f Photonics 2020, 10, x FOR PEER REVIEW 5 of 16 1 exp  L    L  eff (5) In order to analyze the contribution of different elements integrated in a hybrid nanostructure, Photonics 2020, 7, 54 5 of 15 (3) the magnitude of  can be approximated taking into account the relation in volume fraction, ρ, in the nanostructures as follows, In order to analyze the contribution of di erent elements integrated in a hybrid nanostructure, (3) (3) (3)   1   (3)   (6) ms m s the magnitude of  can be approximated taking into account the relation in volume fraction, , in the nanostructures as follows, (3) (3) (3) (3) = (1 ) +  (6) where ms  represents the nonlinear third-order suscepti m bility s of the integrated nanohybrids with m+s (3) (3) (3) (3) where   represents the nonlinear third-order susceptibility of the integrated nanohybrids with m and s as the correspondent values of the uncoupled nanostructures. m m+s (3) and  as the correspondent values of the uncoupled nanostructures. 3. Results and Discussion 3. Results and Discussion Figure 2a depicts a typical TEM image of the CNT samples in bright field mode confirming the Figure 2a depicts a typical TEM image of the CNT samples in bright field mode confirming the multiwall nature of the nanotubes. The darker array in the micrograph represents the inner diameter multiwall nature of the nanotubes. The darker array in the micrograph represents the inner diameter (d) of a representative CNT, and the surrounding layers represent multiwall layers that form the (d) of a representative CNT, and the surrounding layers represent multiwall layers that form the outer outer diameter (D). The distance between the inner and outer diameter is the thickness (t) of the diameter (D). The distance between the inner and outer diameter is the thickness (t) of the MWCNT. MWCNT. Figure 2b shows an isolated pristine CNT in a SEM image. The length L of a tube can be Figure 2b shows an isolated pristine CNT in a SEM image. The length L of a tube can be seen. Figure 2c seen. Figure 2c shows an SEM micrograph of a representative Pt-decorated MWCNT sample; the shows an SEM micrograph of a representative Pt-decorated MWCNT sample; the bright points in the bright points in the image correspond to the Pt nanoparticles incorporated in the walls of the tubes. image correspond to the Pt nanoparticles incorporated in the walls of the tubes. Figure 2d shows the Figure 2d shows the EDS analysis revealing the presence of Pt in the metal-decorated CNT samples EDS analysis revealing the presence of Pt in the metal-decorated CNT samples and confirming the and confirming the concentration of the metal in the nanostructures. concentration of the metal in the nanostructures. (a) (b) (d) (c) Figure 2. (a) TEM image of a typical carbon nanotubes (CNT) concerning the studied sample; (b) SEM image of an isolated pristine CNT; (c) SEM image of platinum-decorated multiwall CNT (Pt-MWCNT); (d) Energy-Dispersive X-ray Spectroscopy (EDS) in a representative section of Pt nanoparticles supported on a CNT bundle. Similar optical absorption spectra were acquired during the evaluation of the MWCNT and Pt-MWCNT samples in a liquid suspension; both spectra are comparatively equal. A concentration of 1 Photonics 2020, 10, x FOR PEER REVIEW 6 of 16 Figure 2. (a) TEM image of a typical carbon nanotubes (CNT) concerning the studied sample; (b) SEM image of an isolated pristine CNT; (c) SEM image of platinum-decorated multiwall CNT (Pt- MWCNT); (d) Energy-Dispersive X-ray Spectroscopy (EDS) in a representative section of Pt nanoparticles supported on a CNT bundle. Similar optical absorption spectra were acquired during the evaluation of the MWCNT and Pt- Photonics 2020, 7, 54 6 of 15 MWCNT samples in a liquid suspension; both spectra are comparatively equal. A concentration of 1 mg of nanostructures in 5 mL of ethanol was selected in order to clearly see the peak in the absorption mg of nanostructures in 5 mL of ethanol was selected in order to clearly see the peak in the absorption band of the samples. Figure 3 plots the typical UV-vis absorption spectrum of the studied Pt-MWCNT band of the samples. Figure 3 plots the typical UV-vis absorption spectrum of the studied Pt-MWCNT samples where it is possible to see close to the 270 nm wavelength the resonant response associated samples where it is possible to see close to the 270 nm wavelength the resonant response associated with the π–π bond of the carbon nanostructures. This UV region also corresponds to the wavelengths with the – bond of the carbon nanostructures. This UV region also corresponds to the wavelengths where the absorption peak of the Localized Surface Plasmon Resonance exhibited by Pt nanoparticles where the absorption peak of the Localized Surface Plasmon Resonance exhibited by Pt nanoparticles could emerge [39]. could emerge [39]. Figure 3. Typical UV-vis absorbance spectrum of the studied samples. Figure 3. Typical UV-vis absorbance spectrum of the studied samples. An ablation threshold of 110 mJ/cm in the samples was experimentally measured at a 532 nm An ablation threshold of 110 mJ/cm in the samples was experimentally measured at a 532 nm wavelength with 4 ns pulse duration in single-shot mode. We verified that thermal damage can be wavelength with 4 ns pulse duration in single-shot mode. We verified that thermal damage can be derived by heat propagation up 50 C in the samples conducted by a Thermo-Scientific CIMAREC derived by heat propagation up 50 °C in the samples conducted by a Thermo-Scientific CIMAREC system (model SP131635), which was assisted by an infrared pyrometer (Master Instruments model system (model SP131635), which was assisted by an infrared pyrometer (Master Instruments model MI-1326S). However, no important changes in temperature were detected in the samples irradiated MI-1326S). However, no important changes in temperature were detected in the samples irradiated by o -resonance optical pulses in single-shot mode at 532 nm wavelength below 20 MW/cm . Then, by off-resonance optical pulses in single-shot mode at 532 nm wavelength below 20 MW/cm . Then, we analyzed if a mechanical action induced by high-irradiance pulses could be promoted in the we analyzed if a mechanical action induced by high-irradiance pulses could be promoted in the nanotubes. It is known that the physical properties of CNT are sensitive due to their diameter, nanotubes. It is known that the physical properties of CNT are sensitive due to their diameter, length, length, and chirality. These values have a strong influence on the electronic properties of CNT. It has and chirality. These values have a strong influence on the electronic properties of CNT. It has been been shown that CNT can be either metallic or semi-conducting materials [40]. With this in mind, shown that CNT can be either metallic or semi-conducting materials [40]. With this in mind, we we assumed that the geometry of CNT can be defined by the internal tube diameter (d), the external assumed that the geometry of CNT can be defined by the internal tube diameter (d), the external tube tube diameter (D), a longitudinal length (L), a total thickness (t), and a circular cross-section. The ratio diameter (D), a longitudinal length (L), a total thickness (t), and a circular cross-section. The ratio L/D L/D is called slenderness ratio of the tube. The lowest length L of CNT was close to 180 nm according to is called slenderness ratio of the tube. The lowest length L of CNT was close to 180 nm according to our statistical TEM observations. For simplicity, we used a 12 carbon atoms configuration at each layer our statistical TEM observations. For simplicity, we used a 12 carbon atoms configuration at each in our numerical simulations. It is worth mentioning that the mechanical behavior of CNT under the layer in our numerical simulations. It is worth mentioning that the mechanical behavior of CNT influence of external factors has been described by many techniques; but essentially, these techniques under the influence of external factors has been described by many techniques; but essentially, these correspond to ab initio processes, quantum mechanics, and Finite Element Method (FEM) calculations techniques correspond to ab initio processes, quantum mechanics, and Finite Element Method (FEM) by using discrete and continuous models [41,42]. In this paper, in order to describe the specific geometry calculations by using discrete and continuous models [41,42]. In this paper, in order to describe the properties, we took into account the main considerations described by Fan, et al. [43]. We used the specific geometry properties, we took into account the main considerations described by Fan, et al. discrete FEM method taking into account each element as a mechanical beam in order to form a zig-zag [43]. We used the discrete FEM method taking into account each element as a mechanical beam in carbon layer. We considered 1129 beam elements type B33 to determine the final geometry of the order to form a zig-zag carbon layer. We considered 1129 beam elements type B33 to determine the internal cylinder [44]. The software used in the simulations was ABAQUS. L/D was the principal geometrical parameter to evaluate the deformation of the tube. Thus, carbon atoms were considered as nodes, and the distances between every two atoms were treated as mechanical beams with an initial distance equivalent to the covalent bond. Thus, the carbon–carbon bond length ( ) was c-c assumed to be 0.142 nm [45]. However, no established values are available for the wall thickness of Photonics 2020, 10, x FOR PEER REVIEW 7 of 16 final geometry of the internal cylinder [44]. The software used in the simulations was ABAQUS. L/D was the principal geometrical parameter to evaluate the deformation of the tube. Thus, carbon atoms were considered as nodes, and the distances between every two atoms were treated as mechanical Photonics 2020, 7, 54 7 of 15 beams with an initial distance equivalent to the covalent bond. Thus, the carbon–carbon bond length (αc-c) was assumed to be 0.142 nm [45]. However, no established values are available for the wall an thickness isolated of nanot an isol ube. ated The nanot values ube. of Th thickness e values suggested of thickness by su available ggested literatur by availe abl varied e litersignificantly ature varied significantly from 0.066 nm [46] to 0.68 nm [47]. In view of such a wide range of suggested thickness, from 0.066 nm [46] to 0.68 nm [47]. In view of such a wide range of suggested thickness, our study was our complemented study was comusing plemeTEM nted observations using TEM ob tosdeduce ervations this tovalue. deduc Our e thobservations is value. Our indicate observation that a s indicate that a MWCNT had about 44 walls with a total thickness of 15 nm (see Figure 2a), thus, an MWCNT had about 44 walls with a total thickness of 15 nm (see Figure 2a), thus, an appropriate wall approp thickness riate wall from thica knes CNT s .from Taking a CNT into . T account aking into our ac TEM count observations, our TEM obse we rvatio conclude ns, wethat conc the lude value that the value of e must be near 0.341 nm. This value is in agreement with the thickness 0.335 nm reported of e must be near 0.341 nm. This value is in agreement with the thickness 0.335 nm reported in the in the literature [48]. The effect of Van der Waals interaction over the vibration characteristics of literature [48]. The e ect of Van der Waals interaction over the vibration characteristics of MWCNT MWCNT is crucial for describing physical effects emerging from carbon nanostructures [49]. is crucial for describing physical e ects emerging from carbon nanostructures [49]. Particularly, Particularly, the fundamental mechanical properties exhibited by MWCNT can be analyzed by the fundamental mechanical properties exhibited by MWCNT can be analyzed by considering linear considering linear vibrations exhibited by concentric SWCNT with Van der Waals interactions vibrations exhibited by concentric SWCNT with Van der Waals interactions between each pair of between each pair of layers [50]. Concerning the cross circular section of MWCNT, we observed a layers [50]. Concerning the cross circular section of MWCNT, we observed a range of diameters from 1 range of diameters from 1 to 10 nm; thus, we considered an average diameter of 5 nm and a length L to 10 nm; thus, we considered an average diameter of 5 nm and a length L 180 nm. A unit scaling ≥ 180 nm. A unit scaling processor was used to reduce the numerical error due to the very small processor was used to reduce the numerical error due to the very small dimensions of the results. dimensions of the results. By doing this, we obtained similar results from Li, et al. [42]. By doing this, we obtained similar results from Li, et al. [42]. We also consider that our sample in film form could present a strain in the CNT when it is We also consider that our sample in film form could present a strain in the CNT when it is irradiated by a fluence close to the photodamage. The action of light over whole cylinders shut on irradiated by a fluence close to the photodamage. The action of light over whole cylinders shut on the global system, thus, an increment in sensitivity due to excitations of electrons can be expected per the global system, thus, an increment in sensitivity due to excitations of electrons can be expected unit of length. In any case, when the global system is modulated by a mechanical effect, changes in per unit of length. In any case, when the global system is modulated by a mechanical e ect, changes electromagnetic parameters must be also present. Samples integrated by CNT strongly absorb the in electromagnetic parameters must be also present. Samples integrated by CNT strongly absorb energy associated with vibration of electrons as a result of laser irradiance taking into account the the energy associated with vibration of electrons as a result of laser irradiance taking into account multilayer configuration; the energy transferred to each carbon layer modifies their geometric the multilayer configuration; the energy transferred to each carbon layer modifies their geometric configuration due to a strain effect. Besides, inhomogeneous electromagnetic irradiance is able to configuration due to a strain e ect. Besides, inhomogeneous electromagnetic irradiance is able to induce an additional internal motion. Remarkably, the range of these values depends on the optical induce an additional internal motion. Remarkably, the range of these values depends on the optical irradiance before reaching optical ablation. In this direction, we also evaluated the critical load, Pcr. irradiance before reaching optical ablation. In this direction, we also evaluated the critical load, P . cr In this sense, our concern will be with the amount of energy coming from the laser system as long as In this sense, our concern will be with the amount of energy coming from the laser system as long P does not exceed the critical value Pcr. Figure 4a describes the values of P and also Pcr versus L/D as P does not exceed the critical value P . Figure 4a describes the values of P and also P versus cr cr that are related to permanent mechanical deformation and optical ablation conditions. Within this L/D that are related to permanent mechanical deformation and optical ablation conditions. Within figure, we also sketch two scenarios, (1) a quasilinear behavior (green line) that delimits a plastic this figure, we also sketch two scenarios, (1) a quasilinear behavior (green line) that delimits a plastic deformation of two cylinders before reaching melting and plastic deformation conditions, and (2) a deformation of two cylinders before reaching melting and plastic deformation conditions, and (2) a plastic deformation in which ablation and P > Pcr could be expected. Figure 4b delimits the geometry plastic deformation in which ablation and P > P could be expected. Figure 4b delimits the geometry cr of an isolated carbon cylinder with a particular thickness. Figure 4c illustrates the cross circular of an isolated carbon cylinder with a particular thickness. Figure 4c illustrates the cross circular section section of MWCNT. We note from Figure 4 that P is inversely proportional to L/D. For a typical of MWCNT. We note from Figure 4 that P is inversely proportional to L/D. For a typical mechanical mechanical behavior in the sample, we can assume that critical stress (σcr) will occur across thickness. behavior in the sample, we can assume that critical stress ( ) will occur across thickness. cr (a) Figure 4. Cont. Photonics 2020, 10, x FOR PEER REVIEW 8 of 16 Photonics 2020, 7, 54 8 of 15 (b) (c) Figure 4. (a) Plot of slenderness ratio vs. load; (b) schematic representation of an isolated nanotube; Figure 4. (a) Plot of slenderness ratio vs. load; (b) schematic representation of an isolated nanotube; (c) outer diameter D and inner diameter d of MWCNT. (c) outer diameter D and inner diameter d of MWCNT. It should be noted that the condition L/D  2.3 can be related to the potential buckling in the It should be noted that the condition L/D ≥ 2.3 can be related to the potential buckling in the structure of the tube due to the action of nanosecond pulses with irradiances below the ablation structure of the tube due to the action of nanosecond pulses with irradiances below the ablation threshold of the samples. In this case, our concept of buckling refers to the excitation of electrons threshold of the samples. In this case, our concept of buckling refers to the excitation of electrons from from the inner diameter to the external diameter of MWCNT to produce permanent deformations and the inner diameter to the external diameter of MWCNT to produce permanent deformations and photodamage. It is worth mentioning that these opto-mechanical idealistic results obey the Euler ’s photodamage. It is worth mentioning that these opto-mechanical idealistic results obey the Euler’s critical load for buckling defined by [51]. critical load for buckling defined by [51]. P  E cr 2 =  E (7) cr (L/r) (7) Lr   In this case, the e ect of the value P /A becomes negligible for large values of L/D. Furthermore, cr In this case, the effect of the value Pcr/A becomes negligible for large values of L/D. Furthermore, we should keep in mind that the right position of loading P as a result of light propagation is seldom we should keep in mind that the right position of loading P as a result of light propagation is seldom known with a high degree of precision. However, in this idealistic approach, we initially evaluated the known with a high degree of precision. However, in this idealistic approach, we initially evaluated value of the load, P, for CNT in a rigid condition. It can be considered that increments in temperature the value of the load, P, for CNT in a rigid condition. It can be considered that increments in generate a reduction in the magnitude of P. Furthermore, if P is not aligned (the most probably temperature generate a reduction in the magnitude of P. Furthermore, if P is not aligned (the most scenario), the mechanical momentum might increase the sensitivity of the CNT. probably scenario), the mechanical momentum might increase the sensitivity of the CNT. From Figure 4a, we also observe that values P  62 nN and L/D  2.3 describe a quasi-linear From Figure 4a, we also observe that values P ≤ 62 nN and L/D ≤ 2.3 describe a quasi-linear mechanical behavior exhibited by the sample studied. On the contrast, if L/D > 2.3, it gives origin to a mechanical behavior exhibited by the sample studied. On the contrast, if L/D > 2.3, it gives origin to deformation and a possible melting process. This result can be related to interatomic forces between a deformation and a possible melting process. This result can be related to interatomic forces between carbon atoms in cylinder layers and a direct consequence of the geometrical parameters associated with carbon atoms in cylinder layers and a direct consequence of the geometrical parameters associated D and L. A proportional relation between Young´s modulus and the values of diameters D–d can be with D and L. A proportional relation between Young´s modulus and the values of diameters D–d expected in the sample. A high Young´s modulus will be present for a low nanotube internal diameter can be expected in the sample. A high Young´s modulus will be present for a low nanotube internal d; this is attributed to an increment in cross-sectional area (A) to transmit the load as a consequence of diameter d; this is attributed to an increment in cross-sectional area (A) to transmit the load as a an increment of thickness t from MWCNT. consequence of an increment of thickness t from MWCNT. From our TEM explorations concerning MWCNT, we estimated the following geometrical values: From our TEM explorations concerning MWCNT, we estimated the following geometrical D = 134 nm, d = 4 nm, L > 180 nm, and thickness 15 nm. The Young´s modulus from these results values: D = 134 nm, d = 4 nm, L > 180 nm, and thickness 15 nm. The Young´s modulus from these is in a range from 0.81 TPa to 1.28 TPa; thus, we assumed that our predictions are within reasonable results is in a range from 0.81 TPa to 1.28 TPa; thus, we assumed that our predictions are within agreement with previous results in carbon nanostructures [52]. On the other hand, our results from the reasonable agreement with previous results in carbon nanostructures [52]. On the other hand, our computational technique are also comparable to those provided by di erent models reported with results from the computational technique are also comparable to those provided by different models magnitudes from 1.0 to 1.24 TPa [53]. One advantage of this analysis is the simplicity of the working reported with magnitudes from 1.0 to 1.24 TPa [53]. One advantage of this analysis is the simplicity principle based on the classical structural mechanics and experimental observations. of the working principle based on the classical structural mechanics and experimental observations. Standard two-wave mixing measurements were conducted by using the setup illustrated in (3) Standard two-wave mixing measurements were conducted by using the setup illustrated in Figure 1 in order to evaluate the magnitude of  in the nanostructures integrated in film form. (3) Fi Figur gure e1 5in shows order tto he eva exper luate imental the ma results gnitude asof a function  in th of e the nanostr variation uctures ofintegr the volume ated in film fraction form. of nanoparticles of Pt decorating MWCNT in the sample studied in film form. The numerical fitting was Figure 5 shows the experimental results as a function of the variation of the volume fraction of (3) made by using Equation (1). The estimated value for the MWCNT was  = 2.2 10 esu, and for nanoparticles of Pt decorating MWCNT in the sample studied in film form. The numerical fitting was Photonics 2020, 10, x FOR PEER REVIEW 9 of 16 Photonics 2020, 7, 54 9 of 15 (3)  −9 made by using Equation (1). The estimated value for the MWCNT was m = 2.2 × 10 esu, and for (3) (3) 8 −8 the Pt nanoparticles, = 2.76 × 10 esu according to a decoupling approximation for the the Pt nanoparticles,  = 2.76 10 esu according to a decoupling approximation for the nonlinear non optical liner ar esponse optical re of composite sponse of comp media osite expr media essed expres by Equation sed by(6). Equation (6). ( (3 3)) Figure 5. Magnitude of  as a function of a change in the volume fraction of nanoparticles of Pt Figure 5. Magnitude of as a function of a change in the volume fraction of nanoparticles of Pt decorating MWCNT in a sample in film form. decorating MWCNT in a sample in film form. From Figure 5, it can be clearly observed that the incorporation of 30% of the volume fraction of From Figure 5, it can be clearly observed that the incorporation of 30% of the volume fraction of Pt Pt nanoparticles in the MWCNT can improve the nonlinearity by a factor of almost four. However, nanoparticles in the MWCNT can improve the nonlinearity by a factor of almost four. However, it is (3) it is remarkable that the estimated optical nonlinearity  of the metal nanoparticles seems to be one (3) (3) remarkable that the estimated optical nonlinearity s of the metal nanoparticles seems to be one order of magnitude higher than the same parameter exhibited by the MWCNT denoted as  . (3) (3) In order to resolve the imaginary part related to  , we explored an input–output experiment as a order of magnitude higher than the same parameter exhibited by the MWCNT denoted as . function of optical irradiance and volume fraction of the Pt nanoparticles incorporated in the MWCNT (3) In order to resolve the imaginary part related to , we explored an input–output experiment samples. The experimental results are depicted in Figure 6. The numerical fitting of obtained by as a function of optical irradiance and volume fraction of the Pt nanoparticles incorporated in the Equation (4) is shown in Table 1 together to the n coecients estimated by Equation (3) and the results (3) MWCNT samples. The experimental results are depicted in Figure 6. The numerical fitting of β of  measured in this research. Photonics 2020, 10, x FOR PEER REVIEW 10 of 16 obtained by Equation (4) is shown in Table 1 together to the n2 coefficients estimated by Equation (3) (3) and the results of  measured in this research. (3) (3) Figure 6. Magnitude of as a function of a change in the volume fraction of nanoparticles of Pt Figure 6. Magnitude of  as a function of a change in the volume fraction of nanoparticles of Pt decorating MWCNT in a sample in film form. decorating MWCNT in a sample in film form. Table 1. Nonlinear optical parameters evaluated in nanohybrids studied at a 532 nm and 4 ns pulses. ρ Related to Pt n2 β (3) Nanoparticles Incorporated  [esu] [m /W] [m/W] to [m /W] −15 −9 0 –1.46 × 10 - 2.21 × 10 −15 −9 −9 0.1 –4.03 × 10 3.1 × 10 6.10 × 10 −15 −9 −9 0.2 –5.62 × 10 5.9 × 10 8.53 × 10 −15 −9 −9 0.3 –6.08 × 10 10.9 × 10 9.25 × 10 It is interesting that the strong n2 exhibited by the pure MWCNT can be considered for envisioning all-optical modulation applications; however, the incorporation of the metal nanoparticles causes an increase in n2, but also, a detriment derived by β automatically emerges. With this implication, we designed a bilayer sample with Pt-MWCNT and pure MWCNT in order to obtain the advantage of a dissimilar bidirectional response for switching functions. Vectorial two-wave mixing experiments were carried out by using the setup illustrated in Figure 1 in order to identify the modification of the probe transmittance by the pump beam action. The experimental setup was calibrated by using a standard nonlinear CS2 sample with a nonlinear −14 2 refractive index n2 = 6 × 10 cm /W [38]. Figure 7a shows the evolution of a single-beam transmitted irradiance as a function of its incident irradiance; while Figure 7b plots the transmitted probe irradiance as a function of the angle between the planes of linear polarization of the incident beams. The best fitting of the experimental data were calculated by Equation (1) and the beam propagation method. Statistical measurements were conducted in order to estimate the third-order nonlinear optical parameters shown in Table 2 for the bilayer film in the forward and backward direction of irradiation for the bilayer film with an error bar of about ±10%. These results are in good agreement with previous publications [30,54]. Photonics 2020, 7, 54 10 of 15 Photonics 2020, 10, x FOR PEER REVIEW 11 of 16 Table 1. Nonlinear optical parameters evaluated in nanohybrids studied at a 532 nm and 4 ns pulses. Table 2. Nonlinear optical parameters evaluated in the samples studied at 532 nm and 4 ns pulses. Related to Pt Nanoparticles n 2 (3) Sample Incidence n2 (cm /W) β (cm/W) [esu] 2 2 Incorporated to [m /W] [m /W] [m/W] Forward −11 15 9 –1.12 × 10 - 0 1.46 10 - 2.21 10 MWCNT/Pt-MWCNT 15 9 9 0.1 4.03 10 3.1 10 6.10 10 Backward 15 9 9 0.2 5.62 10 5.9 10 −7 8.53 10 - 4.9 × 10 15 9 9 Pt-MWCNT/MWCNT 0.3 6.08 10 10.9 10 9.25 10 The data shown in Figure 7a represent a clear signature of a two-photon absorption process in It is interesting that the strong n exhibited by the pure MWCNT can be considered for envisioning the backward direction of the experiment as a decrease in the transmittance as a function on all-optical modulation applications; however, the incorporation of the metal nanoparticles causes irradiance [38], while the inhibition of the nonlinear optical absorption seems to occur if the sample an increase in n , but also, a detriment derived by automatically emerges. With this implication, is explored in the forward direction. Conversely, Figure 7b shows a clear change in the transmitted we designed a bilayer sample with Pt-MWCNT and pure MWCNT in order to obtain the advantage of probe beam by the pump beam action just in the forward direction of the two-wave mixing. It is a dissimilar bidirectional response for switching functions. worth mentioning that an unbalanced distribution in the composition or structure of the film could Vectorial two-wave mixing experiments were carried out by using the setup illustrated in be responsible for the dispersion and asymmetry in the nonlinear data plotted in Figure 7b [55]. From Figure 1 in order to identify the modification of the probe transmittance by the pump beam action. Figure 7b, it can be seen that the maximum transmittance of the probe beam can be obtained for an The experimental setup was calibrated by using a standard nonlinear CS sample with a nonlinear angle of polarization of 45°, which is in good agreement with the fact that parallel or orthogonal 14 2 refractive index n = 6 10 cm /W [38]. Figure 7a shows the evolution of a single-beam transmitted polarizations between the incident beams do not generate any induced birefringence. We observed irradiance as a function of its incident irradiance; while Figure 7b plots the transmitted probe irradiance in our screen a good contrast in an irradiance fringe pattern by using a microscope objective located as a function of the angle between the planes of linear polarization of the incident beams. The best fitting in the interaction region of the beams in absence of the sample in the experimental setup. The of the experimental data were calculated by Equation (1) and the beam propagation method. Statistical generation of a diffracting grating was expected; however, the absence of a self-diffraction signal was measurements were conducted in order to estimate the third-order nonlinear optical parameters verified in our experiment with nanosecond pulses. The main physical mechanism responsible of the shown in Table 2 for the bilayer film in the forward and backward direction of irradiation for the nonlinearity of index seems to be a dynamic thermal effect with instability in a modulation of an bilayer film with an error bar of about 10%. These results are in good agreement with previous induced birefringence and diffraction grating. publications [30,54]. (a) (b) Figure 7. (a) Single-beam transmittance as a function of the incident irradiance; (b) transmitted probe Figure 7. (a) Single-beam transmittance as a function of the incident irradiance; (b) transmitted probe irradiance as a function of the angle between planes of polarization of the incident beams. irradiance as a function of the angle between planes of polarization of the incident beams. Table 2. Nonlinear optical parameters evaluated in the samples studied at 532 nm and 4 ns pulses. It is clear from Figure 1 that different interferometric effects can be induced by the two-wave Sample Incidence n (cm /W) (cm/W) mixing in the forward or backward configuration associated to the superposition of the pump and probe beams. Then, we experimen Forward tally confirmed the observed nonlinear optical behavior in the 1.12 10 - MWCNT/Pt-MWCNT transmitted Kerr measurements just by switching the position of the bilayer sample in the Backward experiments in a standard two-wave mixing experiment - . 4.9 10 Pt-MWCNT/MWCNT The reproducibility of the data plotted in Figures 5–7 was guaranteed for a pulse repetition rate of 1 Hz. Comparative experiments in carbon MWCNT pointed out that the participation of dynamic The data shown in Figure 7a represent a clear signature of a two-photon absorption process in the thermal transport in photothermal effects induced by nanosecond pulses at a 532 nm wavelength backward direction of the experiment as a decrease in the transmittance as a function on irradiance [38], corresponds to a decay time of about 500 ms [56,57]. For further investigation of the contrast in the nonlinear Kerr transmittance with potential modulation of the nanosecond third-order nonlinear optical behavior of the bilayer sample, we Photonics 2020, 7, 54 11 of 15 while the inhibition of the nonlinear optical absorption seems to occur if the sample is explored in the forward direction. Conversely, Figure 7b shows a clear change in the transmitted probe beam by the pump beam action just in the forward direction of the two-wave mixing. It is worth mentioning that an unbalanced distribution in the composition or structure of the film could be responsible for the dispersion and asymmetry in the nonlinear data plotted in Figure 7b [55]. From Figure 7b, it can be seen that the maximum transmittance of the probe beam can be obtained for an angle of polarization of 45 , which is in good agreement with the fact that parallel or orthogonal polarizations between the incident beams do not generate any induced birefringence. We observed in our screen a good contrast in an irradiance fringe pattern by using a microscope objective located in the interaction region of the beams in absence of the sample in the experimental setup. The generation of a di racting grating was expected; however, the absence of a self-di raction signal was verified in our experiment with nanosecond pulses. The main physical mechanism responsible of the nonlinearity of index seems to be a dynamic thermal e ect with instability in a modulation of an induced birefringence and di raction grating. It is clear from Figure 1 that di erent interferometric e ects can be induced by the two-wave mixing in the forward or backward configuration associated to the superposition of the pump and probe beams. Then, we experimentally confirmed the observed nonlinear optical behavior in the transmitted Kerr measurements just by switching the position of the bilayer sample in the experiments in a standard two-wave mixing experiment. The reproducibility of the data plotted in Figures 5–7 was guaranteed for a pulse repetition rate of 1 Hz. Comparative experiments in carbon MWCNT pointed out that the participation of dynamic thermal transport in photothermal e ects induced by nanosecond pulses at a 532 nm wavelength corresponds to a decay time of about 500 ms [56,57]. For further investigation of the contrast in the nonlinear Kerr transmittance with potential Photonics 2020, 10, x FOR PEER REVIEW 12 of 16 modulation of the nanosecond third-order nonlinear optical behavior of the bilayer sample, we analyzed a standard two-wave mixing interaction by Equation (1). Figure 8 shows numerical results describing analyzed a standard two-wave mixing interaction by Equation (1). Figure 8 shows numerical results the Kerr transmittance in the bidirectional propagation of the beams in the bilayer film interacting in a describing the Kerr transmittance in the bidirectional propagation of the beams in the bilayer film two-wave mixing. interacting in a two-wave mixing. (a) (b) Figure 8. Numerical results of the propagation of a degenerated two-wave mixing in the studied bilayer Figure 8. Numerical results of the propagation of a degenerated two-wave mixing in the studied sample at 532 nm wavelength: (a) forward direction; the beams initially interacts in the MWCNT film bilayer sample at 532 nm wavelength: (a) forward direction; the beams initially interacts in the and then in the Pt-MWCNT; (b) backward direction; the beams initially interacts in the Pt-MWCNT MWCNT film and then in the Pt-MWCNT; (b) backward direction; the beams initially interacts in the film and then in the MWCNT. Pt-MWCNT film and then in the MWCNT. It can be stated that the maximum peak of irradiance in the sample can be obtained for parallel It can be stated that the maximum peak of irradiance in the sample can be obtained for parallel polarizations between the incident beams, because this condition promotes the maximum contrast polarizations between the incident beams, because this condition promotes the maximum contrast in in the irradiance pattern generated in the sample by constructive interference. From Figure 8a,b, the irradiance pattern generated in the sample by constructive interference. From Figure 8a,b, remarkable di erences in the Kerr transmittance obtained by the propagation of the probe beam in the remarkable differences in the Kerr transmittance obtained by the propagation of the probe beam in forward and backward direction can be clearly observed in the two-wave mixing interaction. These the forward and backward direction can be clearly observed in the two-wave mixing interaction. conditions are associated to the participation of the two-photon absorption in the Pt-MWCNT that These conditions are associated to the participation of the two-photon absorption in the Pt-MWCNT results in a depletion of the incident beams before an optical Kerr e ect can be excited in the MWCNT. that results in a depletion of the incident beams before an optical Kerr effect can be excited in the MWCNT. A noticeable self-focusing effect occurs in the MWCNT when this layer initially interacts with the two-wave mixing, and it promotes a modulation of the polarization that derives in a Kerr transmittance that can be observed in an optical Kerr gate. Regarding the results demonstrated in Figures 6–8, nonlinear optical Kerr functions induced by ultrafast optical irradiance or polarization effects can be considered by hierarchical nanostructures. Researchers have previously reported important changes in the mechanical, electronic and optical properties of hybrid nanostructures because of the inclusion of CNT [58,59]. In addition, CNT can produce flexural strength in advanced nanocomposites as a function of their concentration of the tubes [60] that could be employed for the instrumentation of optomechanical signals. The density of CNT-based samples receives an important contribution from their thermo-mechanic and photo- physical characteristics [61]. The electromagnetic properties in the hierarchical composites can be correlated to hierarchical structure in good agreement with estimations related to CNT samples [62]. We highlight in this work that the polarization and phase-change in nonlinear optical signals in propagation through CNT can be modulated by orientation-selectable optical effects. Immediate applications of the influence of third-order nonlinear optical properties on hierarchical nanostructures can be considered for developing multifunctional sensors and actuators that are able to control photonic signals. The possibility of low-dimensional interconnections modulated by the vectorial nature of light in quantum systems can be contemplated. An advantage of these findings is the simplicity of the working principle based on the vectorial nature of light and the selectivity of hierarchical nanostructures for controlling nonlinear optical phenomena. 4. Conclusions The impact of platinum decoration in the orientation-selectable nonlinear optical response of hierarchical carbon nanostructures was analyzed. A unidirectional modulation of optical polarization in a bilayer sample was demonstrated by an optical circuit with a bidirectional optical Kerr gate. A strong dependence on the polarization of the induced third-order phenomena in the nanostructures Photonics 2020, 7, 54 12 of 15 A noticeable self-focusing e ect occurs in the MWCNT when this layer initially interacts with the two-wave mixing, and it promotes a modulation of the polarization that derives in a Kerr transmittance that can be observed in an optical Kerr gate. Regarding the results demonstrated in Figures 6–8, nonlinear optical Kerr functions induced by ultrafast optical irradiance or polarization e ects can be considered by hierarchical nanostructures. Researchers have previously reported important changes in the mechanical, electronic and optical properties of hybrid nanostructures because of the inclusion of CNT [58,59]. In addition, CNT can produce flexural strength in advanced nanocomposites as a function of their concentration of the tubes [60] that could be employed for the instrumentation of optomechanical signals. The density of CNT-based samples receives an important contribution from their thermo-mechanic and photo-physical characteristics [61]. The electromagnetic properties in the hierarchical composites can be correlated to hierarchical structure in good agreement with estimations related to CNT samples [62]. We highlight in this work that the polarization and phase-change in nonlinear optical signals in propagation through CNT can be modulated by orientation-selectable optical e ects. Immediate applications of the influence of third-order nonlinear optical properties on hierarchical nanostructures can be considered for developing multifunctional sensors and actuators that are able to control photonic signals. The possibility of low-dimensional interconnections modulated by the vectorial nature of light in quantum systems can be contemplated. An advantage of these findings is the simplicity of the working principle based on the vectorial nature of light and the selectivity of hierarchical nanostructures for controlling nonlinear optical phenomena. 4. Conclusions The impact of platinum decoration in the orientation-selectable nonlinear optical response of hierarchical carbon nanostructures was analyzed. A unidirectional modulation of optical polarization in a bilayer sample was demonstrated by an optical circuit with a bidirectional optical Kerr gate. A strong dependence on the polarization of the induced third-order phenomena in the nanostructures allowed us to systematically control electronic signals in the nanotube networks integrating an electronic circuit. A two-wave mixing method was used to modulate interferometrically controlled laser pulses by third-order optical nonlinearities in the nanostructures. The switching of the optical signals was assisted by the rotation of the angle between the planes of linear polarization of the incident beams in the sample. The experiments were carried out far from the nanosecond ablation threshold of the samples in single-pulse mode. This work highlights the possibility of tailoring polarization-selectable e ects in nanosystems by nonlinear optical phenomena. Potential applications for developing signal processing functions driven by nonlinearities induced by light in low-dimensional systems can be considered. The collective behavior exhibited by carbon-based nanostructures with applications for designing nanophotonic platforms driven by nonlinear optical signals can be envisioned. Author Contributions: Analysis of optical experiments, S.M.-B.; preparation, decoration and morphology characterization of the samples, C.M.-Z.; mechanical studies, J.P.C.-L.; evolution of the nonlinear optical signals during experiments, C.C.-D.; design of hierarchical vectorial functions, C.L.M.-G.; elucidated and evaluated the nonlinear optical properties and designed the paper, C.T.-T. The manuscript was written through contribution of all authors. All authors have read and agreed to the published version of the manuscript. Funding: The authors kindly acknowledge the financial support from the Instituto Politécnico Nacional, COFAA-IPN, Universidad Politécnica del Valle de México, Universidad Politécnica del Bicentenario, Tecnológico de Estudios Superiores de Coacalco and from the Consejo Nacional de Ciencia y Tecnología (CB-2015-251201). Acknowledgments: The authors kindly acknowledge to Instituto Politécnico Nacional, Comisión de Operación y Fomento de Actividades Académicas del Instituto Politécnico Nacional, Universidad Politécnica del Valle de México, Universidad Politécnica del Bicentenario, Tecnológico de Estudios Superiores de Coacalco and CONACyT. The authors are also thankful to the Central Microscopy facilities of the CNMN-IPN. Conflicts of Interest: The authors declare no conflict of interest. Photonics 2020, 7, 54 13 of 15 References 1. Di Ventra, M.; Evoy, S.; Heflin, J. Carbon Nanotubes. In Introduction to Nanoscale Science and Technology; Kluwer Academic Publishers: Ottawa, ON, Canada, 2004; Volume 1, pp. 137–181. 2. Zhao, Q.; Wood, J.R.; Wagner, H.D. Stress fields around defects and fibers in a polymer using carbon nanotubes as sensors. Appl. Phys. Lett. 2001, 78, 1748–1750. [CrossRef] 3. Ma, L.; Wang, J.; Yip, J.; Ding, F. Mechanism of Transition-Metal Nanoparticle Catalytic Graphene Cutting. J. Phys. 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PhotonicsMultidisciplinary Digital Publishing Institute

Published: Jul 30, 2020

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