We use two different synthesis approaches for the preparation of TiO2 films in order to study their resulting third order optical nonlinearity, and its modification by the inclusion of Au nanoparticles in one of the samples. An ultrasonic spray pyrolysis method was used for preparing a TiO2 film in which we found two-photon absorption as a dominant nonlinear effect for 532 nm and 26 ps pulses; and a purely electronic nonlinearity at 830 nm for 80 fs pulses. A strong optical Kerr effect and the inhibition of the nonlinear optical absorption in 532 nm can be obtained for the first sample if Au nanoparticles embedded in a second TiO2 film prepared by a sol-gel technique are added to it. We used an optical Kerr gate, z-scan, a multi-wave mixing experiment and an input-output transmittance experiment for measuring the optical nonlinearities.
©2010 Optical Society of America
Titanium dioxide has three different crystalline phases: brookite, anatase and rutile. Among them, the brookite phase has an orthorhombic crystalline structure. However, this is an unstable phase and because of this, it is of very little interest. Both anatase and rutile phases have a tetragonal crystalline structure. The anatase and rutile are stable phases with mass densities of 3.84 and 4.26 g cm−3, respectively . In general, the rutile phase is formed at high temperatures, while the anatase phase is formed at low temperatures. Due to their electrical properties: high dielectric constant and high resistivity; their optical properties: high refractive index and high optical transparency over a wide spectral range, and their mechanical properties: relatively high hardness; titanium oxide thin films have several possible applications like electric insulator and protective layers in electronic devices, antireflective and protective layers for optical coatings, wear resistant coatings, etc [2–5]. Furthermore, for all optical switching applications, TiO2 thin films are very attractive because they present a fast and reasonably large nonlinear optical response . It has been noted that the physical properties, and thus the nonlinear optical response of TiO2 films, are strongly dependent on the processing route , as well as on the inclusion of metallic nanoparticles (NPs) . For instance, the optical absorption edge of TiO2 thin films and composite films exhibits a blue shift with decreasing annealing temperature. Because the preparation of dense Au NPs in TiO2 films requires an increase of the annealing temperature, the Surface Plasmon Resonance (SPR) absorption band is shifted and its strength is modified according to the preparation procedure. Consequently, with high annealing temperatures, a decrease of the nonlinear absorption coefficient β can take place, while the nonlinear refractive index can change sign . The absorption peak of Au/TiO2 core–shell NPs can be shifted towards the red end of the spectrum by means of the formation of a TiO2 shell on the surface of the Au particles . On the other hand, the large second- and third-order optical nonlinearity of the Au/TiO2 composite films has been attributed to the high density of Au particles in nonconductive films, and the strong local field enhancement caused by the SPR absorption . An ultrafast nonlinear optical response of TiO2 films doped with Au NPs has been observed by using femtosecond optical Kerr effect (OKE) and pump-probe methods, and it has been suggested that the third-order nonlinear susceptibility can be enhanced using additional heat treatments . Additional time-resolved OKE and pump-probe results also suggest that the main physical mechanism involved in the large nonlinear birefringence and dichroism of the Au:TiO2 composite films stem mainly from the hot electron contributions . Although at high solution concentration, Au nanocrystals seem to be preferentially aligned by adsorption along certain TiO2 crystallite boundaries , measurements of the anisotropy of the absorption indicate that Au nanocrystals can be also dispersed within the vertically aligned mesopores and distributed throughout the TiO2 films . For experiments performed at 532 nm, a the self-defocusing (n2 <0) nonlinear refractive index and induced absorption for TiO2 nanocomposites seem to increase with increasing TiO2 volume fraction , and the nonlinear absorption coefficient for Au/TiO2 thin films can be enhanced using multi-layers of Au/TiO2 . Au/TiO2 composites can exhibit a self-defocusing and saturating nonlinearity at optical intensities of a few GW/cm2 , but measurements of the nonlinear optical properties of TiO2 polymer nanocomposites demonstrate that negligible two-photon absorption and a negative n2 value can be also obtained . Stable Au and Ag NPs protected with TiO2 shells show saturable absorption when excited with moderately energetic nanosecond pulses at 532 nm, but exhibit strong optical limiting at higher intensities. This behavior is explained in terms of the induced optical nonlinearity and nonlinear light scattering. The inherent stability of the core–shell structure renders a high laser damage threshold for these materials, making them promising candidates for high energy optical limiting . The nonlinear optical effects increase with increasing Au concentration in multilayer Au/TiO2 composites . So, it has been noted that Au NPs-doped multilayer thin film Au/SiO2 and Au/TiO2 can be used as optical filters, due to their high damage threshold value, and also to their larger third-order nonlinear susceptibilities . In view of all this, the nonlinear optical contribution of metallic NPs seems to be different depending on the preparation procedure for the TiO2 film that interacts with them, and also on the wavelength, peak irradiance, and pulse duration of the light pulses employed to study it. In this work, we study the modification of the absorptive and refractive nonlinearities exhibited by a TiO2 film prepared by an ultrasonic spray pyrolysis method when it interacts with Au NPs that are embedded in a different TiO2 film prepared by a sol-gel technique. Our measurements of the nonlinear optical properties of the samples demonstrate that it is possible to use Au NPs in order to enhance the optical Kerr effect while avoiding any enhancement of the nonlinear absorption of the TiO2 host. We present different nonlinear optical measurements for picosecond and femtosecond third order optical nonlinearities exhibited by our samples.
2. Experimental details
2.1 Spray pyrolysis TiO2 Sample synthesis
Ultrasonic spray pyrolysis is a versatile technique able to produce nanoscale sized powders and thin films. By varying the concentration of the source solution and the atomization parameters, the particle size could be easily controlled within these powders. TiO2 films were prepared by using an ultrasonic spray pyrolysis device (see Fig. 1 ). The deposition system consisted of a piezoelectric transducer which was set to 1.2 MHz. The precursor solution was prepared by dissolving titanium (IV) oxide acetylacetonate [TiO[CH3COH = C(O-)CH3]2] (Aldrich) in pure methyl alcohol [CH3OH] (Baker) at a 0.05 Mol/L concentration. Fused quartz plates were used as substrates and were ultrasonically cleaned with trichloroethylene [ClCH = CCl2] (Baker), acetone [CH3COCH3] (Baker), methyl alcohol and finally dried under nitrogen flow (N2) (PRAXAIR). The substrate temperature (TS) during the spray deposition was set to 550 °C, within an accuracy of ± 1 °C. Filtered air was used as the carrier and director gas, whose flow rates were set to 3.5 and 0.5 l min−1, respectively. The deposition time was 7.5 min. The thickness of the resulting sample was close to 500 nm.
2.2 Au/TiO2 sample synthesis
Titanium oxide films doped with gold NPs were synthesized as follows. As the TiO2 sol-gel precursor solution, titanium i-propoxyde [Ti(OC3H7)4] solution with C = 0.05 Mol/L, pH = 1.25, together with water/alkoxyde with molar ratio (rw) of 0.8 was used. This solution, which was called SG1, was stored in the dark for at least one week before using it in the synthesis step. The gold nanorods precursor solution was an Aldrich standard solution for AAS analysis with a gold nominal concentration of 1000 mg/L. This solution was used as received and some volume was added drop to drop to a bottle which contained the SG1 solution under vigorous stirring with a magnetic stirrer plate. The molar ratio of the Au/Ti(OC3H7)4 mixture was 0.76% (mol/mol) and this solution was called SGG1. The photocatalytic reduction of the gold ions was carried out in a homemade UV-reactor with twelve UV light sources. Each light source was a black light blue UVA lamp (8 W, Hitachi). These light sources provided a broad range of UVA light from 320 to 390 nm with λmax (emission) = 355 nm and a light intensity of 732 µW/cm2. Then, a 10 ml volume of the SGG1 solution was exposed to the light source for a time range that was comprised between 15 and 20 minutes. After UV exposure, the light source was turned off and the irradiated sol-gel solution was recovered and used to coat glass substrates by means of the dip coating technique. The thickness of the resulting samples was close to 250 nm. An Atomic Force Microscope (Dimension 3100, Nanoscope IV) was used to measure the size and density of the Au NPs.
2.3 Bilayer TiO2 film with embedded Au NPs
The Au doped TiO2 film prepared by the sol-gel technique was just been put on the top of the un-doped TiO2 layer prepared by the spray pyrolysis technique.
2.4 Picosecond nonlinear optical response
The vectorial self-diffraction experiments were performed as described in  using a Nd-YAG laser with a 26 ps FWHM pulse duration and wavelength λ = 532 nm, the pulses had linear polarization. In Fig. 2 we show the scheme of our experimental setup, where L represents the focusing lenses, BM is a beam splitter, λ/2 is a half-wave plate, M1-5 are mirrors, S is the sample and PD1-4 are photodetectors with integrated filters. The irradiances ratio of the incident beams was 1:1. The radius of the beam waist at the focus in the sample was measured to be 0.1 mm. We measured the transmitted and self-diffracted irradiances in two configurations, when the incident waves have parallel, and when they have orthogonal polarizations.
In order to confirm the presence and sign of nonlinear optical absorption at 532 nm, we conducted standard input-output single beam experiments. Additionally we measured a closed-aperture z-scan experiment , with 28 μJ pulse energy in order to corroborate the presence and sign of nonlinear refraction.
2.4 Femtosecond nonlinear optical response
Optical Kerr effect measurements were performed using the standard configuration for the time resolved Kerr gate technique . We used a Ti:sapphire laser with λ = 830 nm, 80 fs pulses, 3 nJ maximum pulse energy and a repetition rate of 94 MHz. Figure 3 shows the experimental setup for our Kerr gate experiments. BS1 is a beam splitter, M1-6 are mirrors. A half wave plate, λ/2, with a polarizer, Pol1, are used for controlling the plane of polarization of the probe beam. L represents the focusing system. Pump and probe beams, with an irradiance relation of 15:1 and their linear polarizations making an angle of 45°, are focused on the sample with a spot size of 80 μm. An analyzer, Pol2, with its transmission axis crossed respect to the initial polarization of the probe beam, is placed before the photodetector Pd. The probe beam energy is captured using a lock-in amplifier. By delaying the probe beam with respect to the pump beam, we can observe a change in the transmittance of the system and measure the decay of the induced birefringence in the sample.
The linear absorption spectra of the samples are presented in Fig. 4 . The modulation observed for the TiO2 sample prepared by spray pyrolysis is due to multi reflection interference effect in the thin film.
From these results it is possible to observe that when the two samples are joined, a bilayer sample is obtained and the absorption peaks change uniformly their position in the spectrum. In order to measure the Au NPs size and density, a statistical cumulative analysis of several AFM micrographs of the samples with Au NPs, recorded in standard tapping mode, was undertaken. Figure 5 shows a representative Atomic Force Microscopy (AFM) image performed in the resulting sample of TiO2 with embedded Au NPs.
The results of the statistical analysis showed that the average size of the Au NPs was approximately 72 nm with a density of about 6 × 109 cm−2. Results shown in Fig. 6 indicate that 10% of the NPs have sizes smaller than 55 nm, while 10% are larger than 93 nm (d10 = 55 and d90 = 93 nm).
The sequence of the self-diffraction experiments was as follows: separate measurements on the TiO2 film without Au NPs prepared by spray pyrolysis and on the sol-gel TiO2 film with embedded Au NPs were performed. Finally, both films were measured together as a bilayer film, but considering that the interaction of the beams started with the sol-gel TiO2 film and then it occurred in the spray pyrolysis TiO2 film when they are joined as a bilayer sample; this was done in order to give the maximum irradiance of the incident pulses to the NPs.
The evolution of the amplitudes of the different fields involved in the self-diffraction experiment has been described in detail elsewhere  and the result are:26],27],
Given the isotropy of the material, the nonlinear refractive index, n2, and the nonlinear absorption coefficient, β, are usually related to χ (3) (esu) by :
The experimental and numerical results for the self-diffraction experiments are shown in Fig. 7 . The self-diffraction efficiency, η, represents the rate between self-diffracted and transmitted irradiances obtained; φ represents the angle between the planes of polarization of the incident beams. We consider the Fresnel losses for the beams in each layer and we obtained the nonlinear optical coefficients for the samples according to Eqs. (1-8).The ratio between transmitted and self-diffracted beams allows us to calculate the absorptive and refractive nonlinearities of the samples. The continuous lines represent numerical results and the marks represent experimental data.
Numerical results are shown in Table 1 . Two-photon absorption (TPA) and self-focusing seem to be present in the TiO2 film prepared by spray pyrolysis; nevertheless, the TPA can be considered negligible for the Au NPs embedded in the TiO2 film. The physical mechanism responsible for the nonlinearity is the same for all the samples. The self-diffracted signal obtained for the case of incident beams with orthogonal polarizations indicates that the nonlinear response originates from a purely electronic polarization. At this point, it is important to mention that we did not observe any third order optical nonlinearity in a sample of TiO2 prepared by sol-gel without Au NPs.
In order to confirm the nonlinear absorption present in the samples, input-output experiments were performed using the same ps pulse source previously described. Figure 8 shows the numerical and experimental transmittance differences in the spray pyrolysis TiO2 sample and the bilayer sample. The fit for the data was made taking into account the different reflectance and absorbance of each film. We consider that the irradiance dependent absorption coefficient is given by α(I) = α0 + βI, where α0 and β represent the linear and two-photon absorption coefficients, respectively. We used the same parameters obtained with the self-diffraction calculations. The results from Fig. 8 clearly show that the spray pyrolysis TiO2 sample presents noticeable two-photon absorption, i.e. a transmittance decreasing with increasing input irradiance, and that the bilayer Np-containing sample has practically no nonlinear absorption, thus β = 0 was also the resulting value for fitting the experimental transmittance measured for our bilayer sample.
Picosecond z-scan studies were performed at 532 nm in our TiO2 films in order to resolve the refractive and absorptive contributions to the nonlinearity and to determine its sign. Figures 9a , 9b and 9c show the signature of a positive n2, i.e. a pre-focal minimum followed by a post-focal maximum. These results corroborate the self-focusing effects in the three samples and a two photon absorption behavior only in the spray pyrolysis TiO2 film.
In order to achieve high peak irradiance values while minimizing the thermal load to the sample, we used fs pulses at 830 nm to determine the electronic nonlinearity in this regime, without the concurrent effect of a thermal process. Figure 10 shows the data obtained from the transmittance Kerr gate experiments. Since the Kerr gate signal arises from |χ(3)|, we conducted standard pump-probe experiments to quantify the possible contribution from nonlinear absorption to the response. From the results, we did not observe TPA even with the highest energies (3nJ) available from our laser system. Table 1 summarizes the resulting parameters obtained, which have an error bar of approximately ± 10%.
The statistical cumulative distribution analysis of the AFM micrographs on the Au NPs shows important differences in the size and density distributions of particles in different parts of the nanostructured film. However, the nonlinear parameters extracted from the picosecond experimental results remain approximately constant within the experimental error level, which is far below the observed effect consisting of a reduction of approximately ten times the value of n2 of the Au NPs, when the distribution density is reduced to 1.1 × 109 cm−2. Apparently, if the metallic clusters dominate the nonlinearity, then the third order susceptibility χ(3) of the medium depends slightly on the particle size , but the environment and the distribution density of NPs are strongly important [29,30]. For the picosecond regime, the spray pyrolysis TiO2 film shows a significant refractive nonlinearity, but two-photon absorption seems to be the dominant nonlinear effect. On the other hand, the sol-gel TiO2 film with Au NPs exhibits a higher refractive nonlinearity while nonlinear absorption was not observed at 532 nm. Z-scan and input-output results confirm what is observed in the self-diffraction experiments. For instance, Fig. 9a shows an asymmetric closed aperture Z-scan curve with a deeper, and consequently narrower, valley indicating the presence of nonlinear absorption related to multiphoton processes. On the other hand, Z-scan curves shown in Fig. 9b and 9c are much more symmetric, indicating the suppression of nonlinear absorption processes. Furthermore, this is consistent with the results in Fig. 8b, which shows that the bilayer sample, does not exhibit a measurable nonlinear absorption coefficient β, but the nonlinear refractive index n2 is still evidently strong, as it is shown in Table 1. The irradiation wavelength used for our picosecond nonlinear experiments, 532nm, excites the SPR of the Au NPs; we estimate that this excitation allows a high rate of electron-hole pair production together with an increase of the intraband transitions and hot electrons. Apparently, for the bilayer sample with NPs, a compensation of the two-photon absorption effect is given by the excitation of the SPR of the Au NPs. These mechanisms seem to be the most reliable to explain the origin of the inhibition of the optical nonlinearity of the nanocomposite. However, another explanation to the inhibition of the TPA could be a reduction of residual absorption from the SPR, and the band-edge local field enhancement due to the elimination of surface trapped states. We claim that the excitation of the SPR is the main responsible agent for the inhibition of the nonlinear absorption mechanism in the TiO2 film prepared by spray pyrolysis. In this sense, these results can be easily extended to other optical wavelengths by tuning the position of the SPR during the synthesis of the Au NPs. On the other hand, according with our time-resolved Kerr effect experiments we showed that the response time of the studied samples is shorter than the 80 fs pulse duration employed in the experiments. Our femtosecond results indicate that an important contribution to the pure electronic response of the TiO2 sample is obtained when the Au NPs are added, even for the case of irradiation wavelength far from resonance. Observing the data presented in Table 1, the femtosecond nonlinear response of the spray pyrolysis TiO2 film is quite similar to that from the other samples; conversely to the case for the picoseconds regime, where the nonlinear absorption is noticeable, there seems to be a reduction of the refractive nonlinearity associated. The femtosecond results are consistent with the picoseconds experiments.
A clear modification of the optical third order nonlinear response for a TiO2 film prepared by a spray pyrolysis method was obtained using Au NPs embedded in a TiO2 film prepared by a sol-gel technique. In the picosecond regime, two-photon absorption was observed in the TiO2 film prepared by the spray pyrolysis method as a dominant nonlinear effect; but this effect was inhibited when a TiO2 film with embedded Au NPs coated the sample. A strong self-focusing effect was observed in the studied samples. The mechanisms of picosecond nonlinear absorption and nonlinear refraction were verified by different experimental techniques. A pure electronic response for the OKE in the films was detected using a Kerr gate femtosecond experiment. These resulting nonlinearities for the Au NPs seem to depend slightly on the particle size over the sample but mainly on the density of NPs. Apparently, the excitation of the SPR of the NPs plays the most important role to participate in the nonlinearity of index associated to the pure electronic response of the nanocomposite and it allows inhibiting the two-photon absorption of the pure TiO2 film.
We acknowledge the financial support from IPN, through grants SIP-20100836, SIP-20100800; from UNAM, through grants IN108510, IN103609-3; from DF through grant PICCT08-80 and from CONACyT, through grants 82708, 80024, 80019, 102937. L. Castañeda acknowledges financial support from IFUAP and PROMEP-SEP.
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