Nanoscale plasmonic particles represent a crucial transformation on optical and electronic properties exhibited by advanced materials. Herein are reported remarkable interferometric optical effects with dependence on polarization for filtering or modulating electronic signals in multilayer nanostructures. Metallic nanoparticles were incorporated in randomly distributed networks of reduced graphene oxide by an in-situ vapor-phase deposition method. The polarization-selectable nonlinear optical absorption contribution on the photoconductivity of reduced graphene oxide decorated with gold nanoparticles was analyzed. Nanosecond pulses at 532 nm wavelength were employed in a two-wave mixing experiment to study photoconduction and nonlinear optical absorption in this nanohybrid material. The ablation threshold of the sample was measured in 0.4 J/cm2. Electrochemical impedance spectroscopy measurements revealed a capacitive response that can be enhanced by gold decoration in carbon nanostructures. A strong two-photon absorption process characterized by 5 × 10−7 m/W was identified as a physical mechanism responsible for the nonlinear photoconductive behavior of the nanostructures. Experimental shift of 1 MHz for the cutoff frequency associated with an electrical filter function performed by the sample in film form was demonstrated. Moreover, amplitude modulation of electronic signals controlled by the polarization of a two-wave mixing experiment was proposed. All-optical and optoelectronic nanosystems controlled by multi-photonic interactions in carbon-based materials were discussed. The key role of the vectorial nature of light in two-wave mixing experiments is a fascinating tool for the exploration of low-dimensional systems.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Interparticle and cohesive zone conditions in 2D bimaterials seem to dramatically influence their mechanical behavior as much as their electrical and optical properties. Modifications in either perfectly bonded or frictionless contact for two dissimilar elements integrating nanostructures may originate remarkable changes in surface-to-volume ratio and energy transfer functions . Nanoparticles (NP) incorporation in nanostructured platforms gives rise to an enhancement or an inhibition of particular effects usually associated with nonlinear phenomena . Metallic decoration of carbon nanostructures has demonstrated important advantages for simultaneously displaying sharp selective ultraviolet and optical resonances .
Carbon nanomaterials with large optical nonlinearity and ultrafast response have been considered as promising candidates for future quantum and nanophotonic applications . Regarding their electrical characteristics and high specific surface area (as high as 2630 m2g−1), the energy density in graphene represents ten times the magnitude that can be obtained with traditional lithium-ion batteries .
When graphene oxide is reduced (rGO), its typical hexagonal sp2 bonds hybridization can partially become sp3 bonds hybridization . These sp2 carbon domains play a key role in the nonlinear optical response of rGO due to their conduction band filling and the depletion of their valence band . It is worth noting that the porous morphology exhibited by rGO governs the ion-buffering reservoir related to the nanostructures, their high-speed ion transport, and their ability to store electrolyte ions . However, the presence of oxygen groups in graphene also participates in the quenching of the electrical capacitance of rGO , and the inclusion of additional materials has been proposed to address this issue . In this direction, metal NP supported in rGO have been contemplated for the development of novel nanohybrids with advantages derived from local Surface Plasmon Resonance (SPR) effects . Particularly, Au NP have been distinguished between noble metal NP because of the superlative electromagnetic field enhancement conducted by their SPR oscillations in their optical radiative properties that emerge in the visible region .
Constant progress in nonlinear optics has been facing extraordinary technological challenges mainly related to all-optical applications, but also with the development of optical contributions to photoconductive [7,13] and low-dimensional photonic fields .
The two-wave mixing (TWM) configuration is a powerful tool for manipulating nonlinear optical processes by absorptive and refractive nonlinearities . Multi-photonic absorption interactions together to an induced birefringence related to the optical Kerr effect enable all-optical functions by TWM experiments. The nonlinear refraction and absorption resulting from the superposition of high irradiance pulses can modulate phase, polarization, intensity and even wavelength of optical signals. Moreover, photoconductive and photo-thermal properties exhibited by nanomaterials can be also driven by optical nonlinearities .
Several fascinating factors arising from collective optical response of both rGO and Au NP have attracted the attention of numerous scientific researchers involved in the nonlinear optical topic . A π–conjugated structure in rGO provides characteristic electronic motion that has been indicated to be useful for designing ultraviolet single-photon devices . On the other hand, size scale of Au structures has been shrunk into few nanometers by advanced nanofabrication processes for improving nanomedicine , spectroscopic , imaging , or sensing systems  based on their nonlinear optical properties.
Outstandingly, Au NP incorporated onto rGO (Au/rGO) present a unique combination that promotes a solution for modulating nonlinear properties with advantages going from the UV to the optical spectrum . Electronic and optical properties exhibited by Au/rGO pointed out a good option to consider the exploration of multifunctional materials that may be assisted by nonlinear optical effects .
In view of these considerations, in this work an attempt has been made for further explaining the influence of nonlinear optical interactions in photoelectrical mechanisms involved in laser induced electrical signals travelling through Au/rGO samples. The tuning of the capacitive parameters of the nanostructures has been evaluated under nanosecond pulses. Within this report is analyzed and discussed how plasmonic nanoparticles enhance the nanoscale nonlinear optical response exhibited by graphene-based materials. A non-degenerated TWM was presented as an attractive alternative for controlling electrical capacitive effects in nanohybrids for filtering functions. The vectorial nature of light resulting from the superposition of two polarized laser waves assisted our measurements to explore electrical and third-order nonlinear optical phenomena in Au/rGO samples in film form. Potential applications for developing interferometrically-controlled signal processing devices can be considered.
2. Experimental section
2.1 Sample preparation
Au particles were supported on rGO (Graphenea) by a vapor-phase dissociative process of Au dimethyl(acetylacetonate) gold (III) precursor (C7H13AuO2) in a horizontal quartz tube reactor at Ptot = 7-9 torr. In a first step, a mixture of Au precursor and rGO was heat treated at 180 °C for 10 min. Then, the resultant products were moved to different reactor zone at 400 °C for additional 10 min under Ar gas flow (100 cm3/min) . A spin coating process was employed for the preparation of the films designed for exploring electrical and nonlinear optical experiments.
2.2 Structure and morphology
In order to analyze the Au NP size and homogeneity of dispersed particles in rGO, Scanning Electron Microscopy (SEM; JEOL JSM-6701F) coupled with a microanalysis detector for Energy-Dispersive X-ray Spectroscopy (EDS) was employed. Transmission Electron Microscopy (TEM; JEOL JEM-2200FS) measurements were also undertaken. The Au incorporation in rGO was analyzed by X-Ray Diffraction (XRD; Bruker D8 Focus).
2.3 Optical measurements
The optical absorbance studies of Au/rGO were performed by UV-VIS spectroscopy. For the experiments, 2 mg of Au/rGO was contained in 5 mL of an ethanol suspension. Then, the suspension was deposited on a quartz substrate and it was allowed to dry for 1 hour in a room temperature. Similar samples were analyzed by using a Perkin Elmer XLS spectrometer.
2.4 Nanosecond transmittance explorations
The second harmonic of a Nd:YAG laser system (Continuum Model SL II-10) was employed for monitoring the transmittance of a 532 nm wavelength single-beam irradiation with 4 ns pulses through the samples. The transmitted irradiance, I, as a function on the thickness L of a nonlinear optical media can be described by using the expression :
2.5 Electrochemical impedance spectroscopy studies
Electrical measurements of the samples in film form were undertaken by using an Autolab/PGSTAT302N high power potentiostat/galvanostat. A sinusoidal voltage signal with 10 mV was employed to acquire the characteristic impedance spectrum and electrical capacitance in an integration time of 1 s. A metallic diaphragm and quartz lens were employed to focusing the beam in the sample. Carbon electrodes were in direct contact with the film to be electrically connected with the system for recording conductivity. A 1 mm distance between electrodes was defined to examine the samples.
2.6 Modulation of electrical impedance by a TWM configuration
In this work, a TWM configuration was proposed to induce spatially-resolved optical interactions associated with an interference fringe pattern controlled by the polarization of the beams. Particularly, modifications in the capacitance induced by light in the samples were explored to contemplate potential applications for electrical filtering functions. The film thickness of the studied samples was estimated in about 21.2 µm ± 0.02% with 2 mg of rGO. This thickness of the film was determined by a systematic study carried out in this work for tailoring the electrical filtering performance of the system with a modification of cutoff frequency by light close to 1 MHz. The thickness parameter of the proposed sample must be related to the beam size employed for a particular TWM configuration. Figure 1 illustrates the experimental setup with the pump and probe beam in a quasi-phase matching condition. Our Nd:YAG laser system provided nanosecond pulses at 532 nm that were divided by a non-polarized beam splitter in a 1:1 irradiance relation. The maximum total irradiance in the sample was adjusted to be 80 MW/cm2. The two beams were focused in a 1 mm beam diameter drawing a geometrical angle of approximately 5°. The linear polarization of the probe beam was fixed during the experiment while the plane of the polarization of the pump beam was rotated by a quartz half-wave plate. The rotation of the polarization of the pump beam can modulate the contrast in the irradiance fringes associated with the interference pattern in the TWM. This irradiance was able to modify the electrical impedance in the sample, and eventually, the electrical capacitance can be controlled by this method for using the sample in a basic analog filter design.
3. Results and discussions
Figure 2 shows a typical XRD pattern of Au/rGO. XRD pattern exhibited one distinctive reflection at 2𝜃 = 25.9° corresponding to Joint Committee on Powder Diffraction Standards (JCPDS) 012-0212 card for graphite-type structure. Additional broad reflections were observed at 2𝜃 = 38.1°, 44.39°, 64.57°, 77.54 and 81.72°, which correspond to the (111), (200), (220), (311) and (222) planes of the Au fcc structure according to the JCPDS 4-0784 card. Broad reflections suggest small crystal size of Au. The Au crystal size was estimated to be about 6.5 nm using the Scherrer equation  and (111) reflection.
Figure 3 presents a SEM image of a typical Au/rGO sample. The bright points in Fig. 3(a) represent Au particles while the rough and black surfaces correspond to the rGO. This image reveals that Au particles are uniformly dispersed on the surface of rGO. Figure 3(b) shows a SEM image of the cross section of Au/rGO film. The average thickness of the film was about 21.24 μm.
Figure 4(a) shows a TEM image of Au particles on rGO. Au particles exhibited narrow size distribution with a mean size of 5.4 nm. The uniform distribution of Au particles is probably due to the surface oxygen groups that serve as active sites to promote Au nucleation during the vapor-phase dissociative process [27-29]. It is worth noting that the distribution of metal nanoparticles plays an important role in the electrical properties exhibited by carbon-based nanohybrids . Figure 4(b) exhibits an Au nanoparticle on rGO. The interplanar spacing was about 0.23 nm (inset of Fig. 4(b)) corresponding to the interplanar distance of (111) planes of the fcc Au crystal structure reported in the JCPDS 4-0784 card.
The calculated size of Au NP from XRD using Scherrer equation does not coincide with the Au NP size observed in TEM images because the Scherrer equation is only an approximation of the crystal size and it is important to complement this estimation with other techniques as TEM. Due the differences in crystal sizes between XRD and TEM, we added a histogram in Fig. 4(b). A representative zone of the sample was taken and 100 Au NP were measured from Fig. 4(a). Au particles exhibited narrow size distribution with a mean size close to 5.4 nm (inset b).
In Fig. 5 are depicted the UV-VIS spectra of Au/rGO and rGO in comparative ethanol suspensions heuristically selected to clearly observe the resonances exhibited by the samples. It can be seen the characteristic absorption band of rGO, close to 270 nm, resulting from π- π transitions of C = C bonds in sp2 structure. Au NP mainly absorbs visible wavelengths, near the green region of the electromagnetic spectrum (peak close to 550 nm).
Nanosecond pulses at a 532 nm wavelength were employed to explore any potential nonlinear optical response that characterized the samples. The rGO nanostructures did not show evident modifications in the linearity exhibited by the transmitted 532 nm beam in low irradiance conditions. However, the signature of a clear two-photon absorption behavior was obtained for Au/rGO. The experimental error bar in our measurements was about ± 15%. The experimental and numerical single-beam transmittance data with the best fit obtained by Eq. (1) and β = 5 × 10−7 m/W are plotted in Fig. 6. This nonlinear optical coefficient is two orders of magnitude stronger than the reported value for aqueous graphene oxide colloids , and three orders of magnitude stronger than comparative results reported in Au decorated graphene samples in water for a 532 nm during z-scan measurements . The responsibility for this enhancement is related to the preparation of the multilayer samples in film form instead of using dispersions in a liquid.
Figure 7 shows the electrical impedance exhibited by rGO and Au/rGO in film form as a function on the electrical frequency. Both plots revealed a monotonically decrease of electrical impedance for higher electrical frequencies. This decrease in electrical impedance indicates that a capacitive behavior is present for both materials. The increase in electrical impedance for the Au/rGO sample can be assumed to be responsibility of the strong capacitive effect resulting from the Au NP incorporation in the carbon nanostructures; this behavior is in good agreement with previous results for Au NP-embedded graphene  and Au decorated rGO . It has been demonstrated that the different nanoparticle size and density have effects on the electrochemical impedance exhibited by Au NP ; commonly, the conductivity of Au NP samples decreases as a function on the NP size and increases for particular 2D distributions . However, the incorporation of Au NP onto rGO gives origin to a strong capacitive effect that decreases the conductivity; and this characteristic importantly improves the performance of the Au/rGO hybrid structure for developing electronic filtering functions and memory devices.
From the imaginary part of the electrical impedance of the sample the capacitive behavior was computed and the results are presented in Fig. 8. The electrical capacitance, C, plotted in Fig. 8 was estimated from direct measurement of the electrical impedance of the sample and the expression:36], it was expected that the capacitive behavior of our sample does change across the film surface. The capacitance of rGO was estimated in 10 nF; while Au NP decorating the rGO give rise to a capacitance increase up to approximately 50 nF. These results are in good agreement with previous reports that describe the rGO electrical capacitance .
It is well known that thermal or photonic mechanisms can be responsible for photoconductivity and they can be identified taking into account that a thermal process usually generates an important decrease in photoconductivity when the sample is under light illumination. A decrement in conductivity exhibited by the Au/rGO sample under illumination provided by nanosecond pulses pointed out a thermal mechanism responsible for the modification of its electrical response. The photo-thermally activated conductivity in the sample can be regulated by the laser pulse energy up to the optical damage. The ablation threshold of the studied sample is 0.4 J/cm2 at a 532 nm wavelength with 4 ns pulses in single-shot mode. This ablation threshold is in good agreement with comparative irradiation conditions evaluated in Au-graphene nanocomposites . In this work, the nanosecond pulses were able to induce a modification in electrical characteristics of the sample; and in addition, we have identified that the nonlinear optical properties take part in this effect. In preliminary experiments, we have evaluated the sample with picosecond pulses but there is a strong decrease in the modulation of the electrical parameters in comparison to nanosecond pulses before ablation takes place.
In electrical filtering processes, the controlled modification of the electrical parameters of a system represents an important tool for defining signal attenuation. Conventional electronic devices meet the cutoff frequency when the half magnitude of the incident energy power is attenuated either by resistive, inductive or capacitive characteristics. Nanoscale plasmonic effects in nanohybrids seem to be able to simultaneously modulate resistive and capacitive properties exhibited by carbon/metal samples.
A low-pass electrical filtering function controlled by light was designed by taking into account the capacitive impedance exhibited by the Au/rGO samples and a 1.5 kΩ resistor. The electrical substrate losses were considered as it has been previously described for electromagnetic transmission . The scheme of the analog system is illustrated in Fig. 9. In the illustration, R represents the resistor and C is the capacitor of the system. Vin and Vout correspond to the input and output voltages of the electronic signals in the circuit. The light provided by our TWM experiment irradiates the sample in order to modulate the electrical capacitance that makes an impact in the electrical behavior of the filter.
The evolution of the capacitive response exhibited by the Au/rGO sample in the TWM model described by Fig. 1 was measured. The process for decorating the surface of rGO by Au NP promotes the modification of the electrical and optical properties as a consequence of the participation of the SPR of the NP with a wavelength close to the nanosecond pulses selected for irradiation. The cutoff frequency of the Au/rGO-based electrical filter was varied by a photoconductive behavior induced by nanosecond pulses at a 532 nm wavelength in a TWM configuration with a total irradiance of about 80 MW/cm2. The electrical impedance of the circuit proposed can be varied by the resulting vectorial superposition of the interacting laser optical beams. The shift in the cutoff frequency of the system was measured taking into account the electrical properties of the sample performing an electrical filtering function, the data are depicted in Fig. 10. The angle of polarization related to this plot is associated to the angle between the planes of linear polarization exhibited by the incident beams.
Electronic digital signals with electrical frequency above the different cutoff frequencies shown in Fig. 10 were verified to be filtered by at least half of incident energy. The irradiance fringe pattern in the sample was controlled by changing from parallel to mutually orthogonal polarizations of the beams for the measurements. Changes close to 50 nF can be able to produce a shift of more than 1 MHz associated with the cutoff frequency. However, GHz electronic signals can be potentially modulated by tailoring the resistance and capacitance of Au/rGO low-dimensional samples. High optical irradiances can be considered to control electrical signals by optical gating actions based on third-order nonlinear optical effects. Since the modification of the capacitive parameters in rGO can be derived from the Au NP incorporation, then the nanoparticle size and density in the sample could be responsible for the tuning of their electrical filtering functions. Moreover, the selective filtering effects exhibited by Au/rGO can be useful for automatic identification of electrical and optoelectronic signals in a bandwidth that could be also defined by the morphology and structure of the film.
It is possible to visualize the potential for amplitude modulation of electronic signals controlled by the polarization of the TWM irradiating the sample. Furthermore, numerical simulations plotted in Fig. 10 exemplified the Bode diagrams that correspond to electronic signals in our low-pass filter based on Au/rGO and irradiated by a TWM. It was considered a 532 nm wavelength and 80 MW/cm2 of total incident irradiance in our sample. Figure 10 demonstrates a contrasting relation of output amplitudes according to the polarization of the optical beams in the TWM. From Fig. 11 can be clearly observed a shift in the cutoff frequency of the system controlled by the polarization of the incident beams. The Gain in the plot represents the relation between the output voltage divided by the input voltage schematized in Fig. 9. This polarization-selectable modulation of electronic signals could be employed for developing digital logic gate inverters or time-division multiplexors.
Graphene-based materials are advantageous because of their two-dimensional nature and high carrier mobility, supporting plasmons with an extreme confinement and a wavelength that can be strongly reduced relative to photons of the same frequency . Also, graphene can be used to electrically control the damping of plasmonic resonances since it provides about 4% working wavelength modulation in the mid-infrared region . There have been previously reported hierarchical carbon-based systems that can take advantages of their distribution for performing electrical filtering functions . The design of different aperiodic platforms capable for sensing very low-power signal processes has gained considerable relevance regarding current nano-fabrication technologies. Attractive applications for plasmonic, spintronic and quantum operations have also emerged from the development of hybrid nanostructures . For instance, opto-electrical oscillators for improving optical timing or encrypting devices have been also envisioned . It can be contemplated that the study of nonlinear optical effects and electrical functions exhibited by carbon nanostructures may play a crucial role for future research related to engineering science. In particular, we demonstrated that dynamic electronic phenomena in Au/rGO can generate a nonlinear modulation associated with energy transference regulated by light. In regards to the remarkable modification in electrical capacitance that may be induced by light in Au/rGO, in this work is highlighted the potential of these nanostructures for designing filtering functions and optoelectronic low-dimensional devices.
The rotation of the polarization of the incident beams in a TWM configuration was proposed for modulating the irradiance that tunes the electrical capacitance exhibited by the carbon-based samples studied. The modification in the peak irradiance of an optical interference pattern enables strong changes in the electrical impedance effects related to carbon/metal nanohybrids. Third-order nonlinear optical properties of rGO can be importantly improved by the Au NP incorporation. It is worth noting the remarkable participation of two-photon absorption in the electrical characteristics associated with graphene-based materials. Synergy from plasmonic effects of metallic NP and rGO platforms can be considered good candidates for developing filtering and modulating functions in 2D devices.
Instituto Politécnico Nacional, Comisión de Operación y Fomento de Actividades Académicas del Instituto Politécnico Nacional, and Consejo Nacional de Ciencia y Tecnología (CB-251201).
The authors kindly acknowledge the financial support from Instituto Politécnico Nacional, COFAA-IPN and Consejo Nacional de Ciencia y Tecnología. The authors are also thankful to the Central Microscopy facilities of the Centro de Nanociencias y Micro y Nanotecnologías del Instituto Politécnico Nacional.
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