Bidirectional optical Kerr transmittance in a bilayer nanocomposite with Au nanoparticles and carbon nanotubes

The third-order nonlinear optical response exhibited by nanoparticles (NPs) has attracted substantial attention from scientific research due to their extraordinary potential in all-optical applications [1]. There have been increasing demands on developing nonlinear optical materials, principally, because nonlinear optics is facing new technical challenges such as for imaging, microscopic and spectroscopic devices [2]. Constant progress in the development of nonlinear optical systems based on nanostructures has been possible thanks to the enhancement of advanced materials by the integration of different composites [3]. Nowadays, it is well known that the size scale of optical structures has been shrunk into few nanometers by state-of-the-art nanofabrication processing routes. Then, it seems to be essential to study some of the most important mechanisms of optical nonlinearities that can be improved by surface plasmon resonance (SPR) excitations of metallic NPs [4, 5]. The main idea behind this process is associated with a fascinating collective optical response that takes advantage of a high area/volume relation [6]. Among the diverse characteristics of metallic NPs, it can be stated that their nonlinear optical properties are remarkably dependent on their SPR [7]. Taking these factors into account, it could be expected that the nonlinear optical behavior of a nanosystem based on Au NPs is notably dependent on size, shape and wavelength of interaction [8]. Thus, the combination of different nanocomposites as a solution to tune the nonlinear optical properties can be pointed out [9]. However, there are some concerns that involve the contribution of the media surrounding the NPs [10], and some methods have been assisted by this peculiar characteristic in order to promote third-order nonlinear optical effects [11].

On the other hand, the two-wave mixing (TWM) is a powerful technique for all-optical modulation of optical signals [12]. In a typical all-optical modulation function, the optical Kerr effect (OKE) usually is the key for the modification of the output irradiance. In order to enable the probe beam transmittance in a TWM configuration, a modulation of polarization use to be performed by a pump beam capable of inducing a birefringence. But a major drawback of approaches to obtain a strong OKE is given by the contribution of absorptive nonlinearities and potential ablation involved in high optical interactions.

Interestingly, numerous architectures based on carbon nanostructures exhibiting exceptional absorptive nonlinearities, can promise unique advantages in the field of nonlinear optical effects [13–18]. Thus, carbon nanotubes (CNTs) seem to be useful for producing versatile nonlinear optical materials that can be easily tailored by their morphology [19]. This attribute about shape and size can be also employed in such approaches that have to be based on the design of hybrid materials [20–23].

With this motivation, in this work we investigate the optical Kerr transmittance of a bilayer system integrated by Au NPs embedded in a TiO₂ thin solid film and CNTs in a thin film form. It is worth noting that Au NPs and CNTs present optical absorption bands centered in a different wavelength. TWM experiments were conducted to observe resonant and off-resonant nonlinearities. All-optical modulation differences result from bidirectional propagation of optical beams exposed to absorptive and refractive characteristics associated with the integrated bilayer sample.

The synthesis of Au NPs was performed by following a sol–gel approach that has been previously described [24]. Titanium i-propoxyde [Ti(OC₃H₇)₄] and TiO₂ were employed in the sol–gel process as a solution and precursor, respectively. This Ti(OC₃H₇)₄ was characterized by a C = 0.05 Mol L⁻¹, pH = 1.25, and a water/alkoxyde molar ratio (r_w) of 0.8. The synthesis of the Au NPs was accomplished by using an Aldrich standard solution for AAS analysis with an Au nominal concentration of 1000 mg L⁻¹ as a precursor. The resulting molar ratio of the Au/Ti(OC₃H₇)₄ mixture was 0.76% (mol/mol); it was combined by vigorous stirring carried out with a magnetic stirrer plate. For the photocatalytic reduction of the Au ions in our experiment corresponds 20 min of exposition in a UV reactor emitting 8.8 mW cm⁻² average irradiance through 12 Hitachi UV lamps, with 8 W power. The extraction of the Au NPs was made from 30 ml of the resulting sample by a centrifugation process at 4500 rpm. The resulting sample was dispersed in 5 ml of absolute ethanol and it was centrifuged again at 4500 rpm. A quartz cuvette with 1 mm width was employed as a container for exploring the optical absorption spectrum of this nanogold sample with a Perkin Elmer XLS UV-visible spectrophotometer. Moreover, a thin film sample prepared with a spin coating processing route was obtained with these Au NPs embedded in TiO₂ on a SiO₂ substrate in order to perform microscopic and nonlinear optical experiments. atomic force microscope (AFM) measurements were undertaken by a Nanoscope IV system, Dimension 3100, with a lateral resolution of 1 nm and a vertical resolution of 0.1 nm.

Alternatively, CNTs were prepared by using a ferrocene/benzylamine thermal decomposition as has been previously reported [25]. A solution of hydrocarbon and an organometallic precursor were employed as solvent and solute, respectively, in the ultrasonic preparation of the aerosol. For the process we selected 0.5 wt% ferrocene, 2.5 wt% ethanol, and an argon flow (2.5 l min⁻¹) for placing the aerosol inside a furnace system at about 800 °C for 30 min. For the observation of the optical spectrum of the resulting CNTs, about 1 mg of the CNTs was suspended in an ethanol solution contained in a quartz cuvette with a volume of 5 ml. Moreover, the characterization of the tubes was also carried out by scanning electronic microscopy (SEM) analysis in a SEM ULTRA 55 FEG System from ZEISS with secondary electron and backscattering detector. A Renishaw inVia Raman Microscope was employed to evaluate the multi-wall nature of the sample in air at room temperature through a 50x lens and a 532 nm wavelength delivered by a Nd:YAG laser.

In order to carry out the nonlinear optical measurements, a thin film with an approximate average of 2 μm composed by CNTs was put on top of the TiO₂ film with embedded Au NPs. The thickness of the TiO₂ film was about 500 nm. The third-order nonlinear optical measurements were conducted in this bilayer sample by using a vectorial TWM technique [26]. Figure 1 schematizes the experimental setup where BS1-2 are beam splitters, and A1-4 are polarizers. A Nd:YAG system at 532 nm wavelength was used as a laser source. Single-shots with 4 nanoseconds pulse duration and 45 mJ of maximum energy with an incident spot size of 6 mm were employed as a pump beam. We separately tested two different probe beams supplied by continuous-wave (CW) solid-state lasers emitting at 532 nm and 405 nm wavelengths, respectively. Both probe beams provide 100 mW average power each and an incident spot size of 1 mm. The incident probe and pump beams have linear polarizations before reaching the sample. During the nonlinear optical experiment the plane of polarization of the pump beam was fixed while the polarization plane of the probe beam was rotated by a half-wave plate λ/2. The measurements of the transmittance of the beams were acquired through the PIN photodetectors PD1-3. The axes of transmission of the analyzers placed in front of the photodetectors PD1-2 were aligned in order to capture the orthogonal component of the transmitted polarization of the correspondent probe beam for each detected data. The geometrical angle between the probe and pump beam was around 35°. In addition, the TWM experiments were also performed in both backward and forward direction for each wavelength, with respect to the sample, by switching the position of the probe beams. This was made in order to see if a change in the transmittance of the different wavelengths of the probe beams depends on the zone of the sample that interacts first with the beam, Au NPs or CNTs.

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From the optical absorbance evaluations of the sample were notable two absorption bands related to the SPR excitations of the Au NPs and CNTs, close to 590 nm and 270 nm, respectively. Figure 2 shows the optical absorption spectra. The concentration of the Au NPs and CNTs were chosen in order to better observe the SPR optical absorption phenomena.

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A representative Raman spectroscopy measurement of the CNTs studied is plotted in figure 3. The typical radial breathing vibration mode (RBM) that appears at frequencies below 350 cm⁻¹ and corresponds to the presence of single-wall CNTs (SWCNTs) was revealed. The predominant peaks in the spectrum named D, G, G' can be clearly distinguished. It has been previously described that the D mode represents a measure of disorder in the carbon structures, while the G mode is related to the tangential vibrations of the carbon atoms and G' is an overtone of the D mode [27, 28]. From the Raman band intensities can be quantified the approximate content of SWCNTs and multi-wall CNTs (MWCNTs) in the studied sample [29]. In our case, a content of about 80 wt% for SWCNTs and 20 wt% for MWCNTs is estimated in the sample.

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Microscopic observations were carried out for evaluating the morphology of the CNTs and the Au nanocomposites. Statistical results from morphology explorations indicated that the mean size of the Au NPs is close to 100 nm, while diameters of about 100 nm were found in the MWCNTs detected. Figure 4(a) depicts the TiO₂ thin film with embedded Au NPs in a typical AFM micrograph, whereas figure 4(b) illustrates the studied CNTs by a SEM image.

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In order to describe the fields in the TWM interaction, we consider the expressions for representing the amplitude of the transmitted waves [24]:

$$\begin{eqnarray}&&{E}_{1\pm }(z)=\left[{E}_{1\pm }^{0}{J}_{0}\left({\Psi }_{\pm }^{(}\right)+\left({\rm{i}}{E}_{2\pm }^{0}-i{E}_{3\pm }^{0}\right){J}_{1}\left({\Psi }_{\pm }^{(}\right)\right.\\ &&\quad \quad \;\;\;\;\;\;\;\;\;-\left.{E}_{4\pm }^{0}{J}_{2}\left({\Psi }_{\pm }^{(}\right)\right]\exp\left(-{\rm{i}}{\Psi }_{\pm }^{(}-\displaystyle \frac{\alpha (I)z}{2}\right),\end{eqnarray} \tag{ 1 }$$

$$\begin{eqnarray}&&{E}_{2\pm }(z)=\left[{E}_{2\pm }^{0}{J}_{0}\left({\Psi }_{\pm }^{(}\right)+\left({\rm{i}}{E}_{4\pm }^{0}-i{E}_{1\pm }^{0}\right){J}_{1}\left({\Psi }_{\pm }^{(}\right)\right.\\ &&\quad \;\;\;\;\;\;\;\;\;\;\;\;\;\;\;-\left.{E}_{3\pm }^{0}{J}_{2}\left({\Psi }_{\pm }^{(}\right)\right]\exp\left(-{\rm{i}}{\Psi }_{\pm }^{(}-\displaystyle \frac{\alpha (I)z}{2}\right),\end{eqnarray} \tag{ 2 }$$

where E_1±(z) and E_2±(z) are the complex amplitudes of the circular components of the transmitted waves beams; E_3±(z) and E_4±(z) are the amplitudes of the self-diffracted waves, while ${E}_{1\pm }^{0},$ ${E}_{2\pm }^{0},$ ${E}_{3\pm }^{0}$ and ${E}_{4\pm }^{0}$ are the amplitudes of the incident and self-diffracted waves at the surface of the sample; α(I) = ${\alpha }_{o}$+βI is the irradiance dependent absorption coefficient, where ${\alpha }_{o}$ and β are the linear and nonlinear optical coefficients, respectively; $I$ is the total irradiance of the incident beams; J_m(ψ_±⁽¹⁾) stands for the Bessel function of order m, $z$ is the thickness of the nonlinear media, and,

$$\begin{eqnarray}&&{\Psi }_{\pm }^{(0)}=\displaystyle \frac{4{\pi }^{2}z}{{n}_{0}\lambda }\left[\left(A+\displaystyle \frac{{n}_{0}\beta }{2\pi }\right)\displaystyle \sum _{j=1}^{4}{\left|{E}_{j\pm }\right|}^{2}+\left(A+B+\displaystyle \frac{{n}_{0}\beta }{2\pi }\right)\right.\\ &&\quad \;\;\;\;\;\;\;\;\times \left.\displaystyle \sum _{j=1}^{4}{\left|{E}_{j\pm }\right|}^{2}\right],\end{eqnarray} \tag{ 3 }$$

$$\begin{eqnarray}&&{\Psi }_{\pm }^{(1)}=\displaystyle \frac{4{\pi }^{2}z}{{n}_{0}\lambda }\left[\left(A+\displaystyle \frac{{n}_{0}\beta }{2\pi }\right)\displaystyle \sum _{j=1}^{3}\displaystyle \sum _{k=2}^{4}{E}_{j\pm }{E}_{k\pm }^{*}\right.\\ &&\quad \quad \;+\left(A+B+\displaystyle \frac{{n}_{0}\beta }{2\pi }\right)\times \left.\displaystyle \sum _{j=1}^{3}\displaystyle \sum _{k=2}^{4}{E}_{j\pm }{E}_{k\mp }^{*}\right],\end{eqnarray} \tag{ 4 }$$

are the nonlinear phase changes and the optical wavelength is represented by λ. The independent components of the third-order susceptibility tensor χ⁽³⁾ for an isotropic material are denoted as $A=6{\chi }_{1122}^{(3)}$ and $B=6{\chi }_{1221}^{(3)}$ [30]. The calibration of the TWM setup was based on carbon disulfide (CS₂) as a nonlinear optical media contained in a quartz cuvette with 1 mm width; the magnitude of the third-order nonlinear susceptibility of CS₂ is $\left|{\chi }^{(3)}\right|$ = 1.9 × 10^-12 esu [30]. For investigating the induced absorption in the interactions, a pump-probe experiment was performed in the bilayer sample with parallel polarization of the interacting beams while the sample was rotated. Pump-probe results are shown in figure 5. From the experimental data it can be confirmed that the optical absorption of the bilayer film is inhomogeneous. Since pump-probe results carried out exclusively in the Au NPs embedded in TiO₂ do not show any considerable nonlinear optical absorption, it can be said that the CNTs are mainly responsible for the nonlinear optical absorption effect exhibited by the bilayer sample.

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In figure 6 are plotted the vectorial probe transmitted irradiances through the sample in fixed location with the corresponding best fitting from numerical simulations of equations (1)–(4). The adjusted nonlinear optical parameters of the bilayer nanocomposites constituted by CNTs and Au NPs are presented in table 1.

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Surprisingly, forward and backward transmission through the bilayer presents different results for equivalent input irradiances; it can be appreciated from data plotted in figures 6(a) and (b). Moreover, it is quite interesting that opposite behaviors in Kerr transmittance were accomplished for the different wavelengths employed, i.e., the maximum Kerr transmittance is obtained when the 532 nm probe beam firstly impinges into the Au nanocomposites, while in contrast, the 405 nm probe beam must propagate first into the CNTs film in order to achieve the maximum Kerr transmittance.

For presenting a graphical description of the Kerr transmittance effects, we plotted in figure 7(a) the calculations associated to the focusing of the pump beam in the bilayer sample before ablation with 1 GW cm^-2 of incident irradiance. In addition, in figure 7(b) is sketched a degenerated TWM interaction from where it is possible to see an evident inhomogeneous propagation of the pump beam through the bilayer sample and the formation interference fringes of irradiance.

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From figure 2 it can be deduced that the 532 nm wavelength corresponds to a near resonance excitation for the Au NPs; in this case the optical nonlinearities should be stronger than in the off-resonance case. Considering the Kerr transmittance of the bilayer sample, we assume that the contribution of the nonlinear optical absorption of the CNTs is strongly important in the reduction of the Kerr transmittance at 532 nm if the pump firstly interacts with the CNTs film. The physical mechanism responsible for absorptive nonlinearity of CNTs at 532 nm can be expected to be two-photon absorption (TPA) [31], and it is in good agreement with the results shown in figure 5. This TPA process should be in detriment of the birefringence induced by a pump beam with a diminished irradiance propagates in the Au NPs region. On the other hand, for the case of the Kerr transmittance of the 405 nm probe beam, it can be estimated that there is a reduction of the nonlinear optical refraction at the Au NPs zone regarding that this is an off-resonant excitation [32].

In order to elucidate about the bidirectional behavior of the sample, we consider as a first approximation that the Au nanocomposites present a dominant OKE while the CNTs absorb in a selective direction. So, under this assumption the system behaves like a phase retarder and a linear polarizer and from this consideration it is evident that in the proposed case is not optically isotropic.

The selective optical transmittance of the proposed system seems to be useful for automatic identification of the direction of a probe beam in propagation through an optical system. It can be argued that by changing the resonance of the samples it can be also expected to tune the Kerr response. Moreover, it can be contemplated to design different inhomogeneous nanocomposites in order to modify the configuration to make it capable of switching or gating the signals for potential applications in the development of all-optical diodes based in third-order optical effects.

The nonlinear optical absorption exhibited by a bilayer nanocomposite with Au nanoparticles and CNTs allows us to observe a remarkable difference in bidirectional Kerr transmittance. A vectorial TWM together with a pump-probe technique were employed for elucidating the participation of an inhomogeneous TPA in the modification of optical transmittance. A contrast in the transmittance behavior of the studied nanostructures was observed at 532 nm and 405 nm in regards to their different resonant mechanism responsible for third-order optical nonlinearity. Furthermore, the potential combination of different composites conforming to nanosystems seems to be a good option for developing all-optical gate functions and identification of optical vectors of propagation.

The authors kindly acknowledge the financial support from Instituto Politécnico Nacional, from Universidad Politécnica del Bicentenario, from Centro de Investigación en Materiales Avanzados Nuevo León, from Universidad Nacional Autónoma de México, and from Consejo Nacional de Ciencia y Tecnología. The authors are also grateful to the Weizmann Institute of Science and to the Centro de Nanociencias y MicroNanotecnologías-IPN.