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J. Phys. Chem. C 2010, 114, 10108–10113
Contribution of the Two-Photon Absorption to the Third Order Nonlinearity of Au Nanoparticles Embedded in TiO2 Films and in Ethanol Suspension M. Trejo-Valdez,† R. Torres-Martı´nez,‡ N. Pere´a-Lo´pez,§ P. Santiago-Jacinto,| and C. Torres-Torres*,⊥ ESIQIE-Instituto Polite´cnico Nacional, Zacatenco, Me´xico, D.F., 07738 Me´xico, Centro de InVestigacio´n en Ciencia Aplicada y Tecnologı´a AVanzada Unidad Quere´taro, Instituto Polite´cnico Nacional, Santiago de Quere´taro, Quere´taro, 76090 Me´xico, Laboratory for Nanoscience and Nanotechnology Research (LINAN) and AdVanced Materials Department, IPICYT, Camino a la Presa San Jose´ 2055, Lomas 4a. Seccio´n, San Luis Potosı´, 78216 Me´xico, and Instituto de Fı´sica, UniVersidad Nacional Auto´noma de Me´xico, Me´xico, D. F., 04510, Seccio´n de Estudios de Posgrado e InVestigacio´n-ESIME-IPN, Zacatenco, Me´xico, D.F., 07738 Me´xico ReceiVed: February 3, 2010; ReVised Manuscript ReceiVed: April 30, 2010
We measured the absorptive and refractive nonlinearities in Au nanoparticles when they are embedded in a TiO2 film and when they are suspended in ethanol. The morphology of the nanoparticles was estimated by using HRTEM microscopy. Different contributions related to electronic polarization and two-photon absorption were observed in the samples using a self-diffraction technique with pulses of 26 ps at 532 nm. Transmittance experiments were performed in order to confirm the mechanisms of optical absorption. The Au nanoparticles were grown by the photoreduction of TiO2 sol-gel solutions which contain Au3+ ions. The thin films were prepared by using the dip coating technique with fresh UV exposed sol-gel solutions. We observed that when the sample does not present any important nonlinear absorption it is possible to enhance the participation of the nanoparticles in the optical Kerr response of the media. Introduction Potential applications of different nonlinear optical properties of Au nanoparticles (Nps) have attracted the attention of several scientific researchers in the last few years due to their overwhelming magnitude and fast response.1 All-optical switches, filters, modulators, or multiplexers for photonic signals can be achieved by taking advantage of the nonlinear properties of thin film materials containing nanostructures.2 Interparticle interaction plays a significant role in the optical response of a nanocomposite;3 therefore, the physical properties of their surrounding can produce important changes for the nonlinear optical response.4 Apparently, if the metallic clusters dominate the nonlinearity, then the third order susceptibility χ(3) of the medium depends slightly on the particle size,5 but the metal volume-fraction concentrated into the film6,7 and the distribution density of Nps are strongly important.8 It seems that an enhancement of the nonlinear refraction in metallic Nps can be given by the electromagnetic field near and out of the surface of the Nps.9 Particularly, Au Nps can form small complexes of different geometries, and each complex has a unique thermal and optical response.10 For instance, a significant increment in χ(3) has been reported for Au Nps due to their core-shell structure when they are accompanied by different elements.11 On the other hand, apparently a matrix with high refractive index results in a high optical nonlinearity exhibited by the nanocompos* Corresponding author: Carlos Torres-Torres, Seccio´n de Estudios de Posgrado e Investigacio´n, ESIME-IPN, Zacatenco, Me´xico, D.F., 07738, Me´xico, Phone +52(55)57-29-60-00, ext. 54686, Fax +52(55)57-29-6000, ext. 54587. *Author to whom any correspondence should be addressed. † ESIQIE-Instituto Polite´cnico Nacional. ‡ Instituto Polite´cnico Nacional. § IPICYT. | Universidad Nacional Auto´noma de Me´xico. ⊥ Seccio´n de Estudios de Posgrado e Investigacio´n-ESIME-IPN.
ite.12 In this direction, the study of Au Nps embedded in TiO2 thin films becomes very attractive because TiO2 presents a high refractive index, besides pure TiO2 thin films exhibit a fast and reasonably large nonlinear optical response.13 Furthermore, an ultrafast nonlinear optical response of TiO2 films doped with Au Nps has been observed by using femtosecond optical Kerr effect (OKE), zscan, and pumpprobe methods,14-18 and it has been suggested that χ(3) can be enhanced using additional treatments.19,20 However, nonlinear optical studies in Au/TiO2 nanocomposites can be difficult to preparation. For instance, the surface plasmon resonance (SPR) absorption band associated with Au Nps in TiO2 films can be modified according to the processing route, and a decrease in the nonlinear absorption coefficient β can take place, while the nonlinear refractive index can change sign with temperature.21 In order to further investigate how the absorptive and refractive nonlinearity is associated with the participation of Nps, in this work we study the absorptive and refractive nonlinearities of Au Nps when they are suspended in an ethanol solution and when they are embedded in a TiO2 film. We found that the modification of the surrounding of the Nps can be one of the most important properties that generate the changes in the different contributions related to electronic polarization and two-photon absorption for pulses of 26 ps at 532 nm. We show nonlinear measurements for optical absorption using a self-diffraction technique and input-output transmittance experiments. Our measurements of nonlinear optical properties of the samples demonstrate that it is possible to inhibit the two-photon absorption mechanism and to enhance the value of nonlinear refractive index, highlighting the potential applications of the nanocomposite film for developing all-optical devices operating in the visible region.
10.1021/jp101050p 2010 American Chemical Society Published on Web 05/13/2010
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Figure 1. Experimental set up for nonlinear optical response measurements.
Experimental Section Sample Synthesis. Au Nps were synthesized as follows. As the TiO2 sol-gel precursor solution, a titanium i-propoxyde [Ti(OC3H7)4] solution with a C ) 0.05 Mol/L, pH ) 1.25, and a water/alkoxyde molar ratio (rw) of 0.8 was used. This solution, which was called SG1, was stored in the dark for at least 1 week before their use in the synthesis step. The Au Nps 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 by using 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 Au 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). This light source 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 volume of 10 mL of the SGG1 solution was exposed to the light source for a time range that was comprised between 15 and 20 min. After UV exposure, the light source was turned off and the irradiated sol-gel solution was recuperated and used to coat glass substrates by means of the dip coating technique. Furthermore, pure Au Nps were extracted from 30 mL of fresh UV-irradiated SGG1 by centrifugation at 4500 rpm. This sample was rinsed with absolute ethanol and centrifuged again at 4500 rpm. The material which presented a reddish color was redispersed in some volume of absolute ethanol, centrifuged one more time at 4500 rpm, rinsed with absolute ethanol, and finally redispersed in absolute ethanol. This sample was stored the dark and used for nonlinear optical response measurements and HRTEM observations. Nonlinear Optical Response. We used a Variac UV-visible spectrophotometer in order to measure the optical absorption of the samples. A multiwave mixing experiment was setup for measuring and identifying the absorptive and refractive nonlinearities associated with the self-diffraction irradiances generated into the samples.22,23 Figure 1 shows the scheme of our experimental setup. We used a Nd:YAG laser with 26 ps pulse duration and 532 nm wavelength. L represents the lenses of the system, BM is a beam splitter, M1-3 are mirrors, S is the sample, and PD1-4 are high speed silicon detectors (DET210, Thorlabs). For avoiding optical damage and nonlinearity related to the response of these detectors, PD1-2 are protected using a 5OD filter, whereas PD3-4 are protected using a 1.2OD filter. To calibrate the experimental apparatus, we measured the thirdorder susceptibility tensor of a CS2 sample which is a well-
Figure 2. (a) TEM micrograph of Au Nps. (b) HRTEM of round nanoparticle showing individual gold atoms and atomic planes, the inset is the FFT transform of the image where the {111} family of planes can be identified. (c) TEM micrograph of Au Nps and (d) HRTEM of the bigger rod of the inset in (c) showing that the growth axis of the rod is along the [111] direction.
known nonlinear media.24 At this point we mention some recommendations for obtaining an intense self-diffraction signal originated by the multiwave mixing experiment in a CS2 sample. First, some fractions of mJ of energy per pulse must be used in the experiment. Second, in order to hold a good phase matching condition, it is important to generate the interference pattern into the sample with a small incident angle between beams (1-5°). For measuring the samples, we used 0.35 mJ of energy pulse with linear polarization; the incident angle between beams was 2°. The irradiance rate I1:I2 was 1:1, and the radius of the beam waist at the focus in the sample was measured to be 0.2 mm. We measured transmitted and self-diffracted irradiances when the polarization of the incident wave of one of the beams is fixed and the polarization of the other beam is rotated by means of a λ/2 phase retarder. To further investigate the nonlinear optical absorption at 532 nm, we also measured the direct transmission of a single beam from the laser in propagation through the samples. Results Au Nps were observed in a Tecnai G2 microscope operated at 300 KeV with a spatial resolution of 0.9 Å point to point, equipped with an EDAX energy dispersive X-ray spectroscopy (EDS) microanalysis system. The sample was prepared by pouring a few drops of the ethanol suspended Au Nps in a commercial lacey carbon film. A statistical cumulative analysis of several micrographs of our samples was undertaken and we estimate that less than the 20% of the Nps can be considered nanorods. The morphology of a fraction of the Nps is elongated and fundamentally single crystal; however, most of the population corresponds to round Nps which show a {111} family of planes (Figure 2a,c). From the HRTEM micrograph in Figure 2d, it is possible to identify the {111} family of planes with an interplanar distance of 0.235 nm, which means that the Nps grew along the [111] direction.
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Trejo-Valdez et al.
Figure 3. (a) Electron micrograph of Au Nps acquired in STEM mode. (b) Map of Au-M X-ray line, and (c) EDX spectrum showing the red cross position in panel (b); the spectrum shows mainly Au lines, but some Cu lines can be perceived and are caused by the TEM grid used to support the sample.
The experimental and numerical results for the multiwave mixing experiment are shown in Figure 5. 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 obtained the nonlinear optical coefficients for the samples according to the equations described by25
Figure 4. Linear absorption spectra of the samples.
From all of the analyzed Nps, there was not crystallographic evidence of a TiO2 precursor surrounding the gold nanostructures or isolated titania Nps. A density of 7.5 × 109 ( 20% particles/cm2 was obtained for the TiO2 film with Au Nps, and a quite similar value of density corresponds to the ethanol with Au Nps. In order to determine if the Nps present any contamination, an EDS elemental mapping was performed using the scanning transmission electron microscopy (STEM) mode (Figure 3a) of the Tecnai microscope. The elemental mapping scans show that the Nps are pure gold (Figure 3b), and the corresponding EDS spectrum does not show contamination. For the nonlinear optical measurements, the samples had different thickness, i.e., optical path length. The thickness of the sample with Au Nps suspended in ethanol was 1 mm, and the thickness of the TiO2 film with embedded Au Nps was close to 200 nm. Figure 4 shows the linear optical absorption spectra associated with the samples. One can clearly observe the surface plasmon resonance (SPR) band related to the Au Nps suspended in ethanol; on the other hand, the Au Nps embedded in the TiO2 film exhibit very low absorbance caused by the thickness of the thin film and the small number of embedded Nps.
0 (1) 0 0 E1((z) ) [E1( J0(Ψ( ) + (iE2( - iE3( )J1(Ψ(1) () R(I)z 0 (1) (0) E4(J2(Ψ( )] exp -iΨ( 2
)
(1)
0 (1) 0 0 E2((z) ) [E2( J0(Ψ( ) + (iE4( - iE1( )J1(Ψ(1) () R(I)z 0 (1) (0) E3(J2(Ψ( )] exp -iΨ( 2
)
(2)
( (
0 (1) 0 (1) 0 (1) E3((z) ) [E3( J0(Ψ( ) + iE1( J1(Ψ( ) - E2( J2(Ψ( )R(I)z 0 (1) (0) (3) iE4(J3(Ψ( )] exp -iΨ( 2
(
)
0 (1) 0 (1) 0 (1) E4((z) ) [E4( J0(Ψ( ) - iE2( J1(Ψ( ) - E1( J2(Ψ( )+ R(I)z 0 (1) (0) (4) iE3(J3(Ψ( )] exp -iΨ( 2
(
)
where E1((z) and E2((z) are the complex amplitudes of the circular components of the transmitted waves beams; E3((z) and E4((z) are the amplitudes of the self-diffracted waves; E01(, E02(, 0 0 , and E4( are the amplitudes of the incident and selfE3( diffracted waves at the surface of the sample; R(I) is the
Contribution of the Two-Photon Absorption
J. Phys. Chem. C, Vol. 114, No. 22, 2010 10111 TABLE 1: Optical Nonlinearities Exhibited by the Samples experiment
R [cm-1]
Au Nps embedded 5.2 × 10 in a TiO2 film Au Nps suspended 5 in ethanol
Figure 5. Self-diffraction efficiency exhibited by the samples. The continuous lines represent numerical results, and the marks represent experimental data.
irradiance dependent absorption coefficient; I is the total irradiance of the incident beams; Jm(Ψ((1)) stands for the Bessel function of order m; z is the thickness of the nonlinear media, and (0) Ψ( )
[(
n0β 4π2z A+ n0λ 2π
)∑ 4
j)1
|Ej( | 2 +
(
A+B+
(1) Ψ( )
[(
) (
n0β 4π2z A+ n0λ 2π
3
n0β 2π
)∑
]
(5)
]
(6)
4
|Ej- | 2
j)1
j)1 k)2
A+B+
n0β 2π
)∑ ∑ 3
4
Ej-E*k-
j)1 k)2
(3) (3) (3) are the phase increments. Here A ) 6χ1122 ) 3χ1122 + 3χ1212 (3) and B ) 6χ1221 , where the components of the third-order susceptibility tensor χ(3) for an isotropic material are related by24
(3) (3) (3) (3) (3) χ(3) 1111 ) χ1122 + χ1212 + χ1221 ) 2χ1122 + χ1221
(7)
Given the isotropy of the material, the nonlinear refractive index, n2, and the nonlinear absorption coefficient, β, are usually related to χ(3) (esu) by24
χ(3) ) 2n02ε0cn2 + i
n02ε0c2 β ω
(8)
here ε0 represents the permittivity of the vacuum and ω the optical frequency. The magnitude of the third-order nonlinear optical susceptibility, χ(3), can be expressed as
|χ(3) | ) √(Reχ(3))2 + (Im χ(3))2
n2 [m2/W] 9.5 × 10
6 × 10-11
-16
|χ(3)| [esu] 2.1 × 10-9
5 × 10-18 3.8 × 10-12
the nonlinear refraction. Table 1 summarizes the resulting parameters, which have an error bar of approximately (10%. Figure 6 shows the numerical and experimental transmittance in the samples. The typical TPA curve was obtained for the case of the Au Nps suspended in ethanol; while apparently there is no contribution related to nonlinear absorption for the case of the Au Nps embedded in the TiO2 film. The fit for the data was made using the same parameters β ( 10% obtained with the self-diffraction calculations. A transmittance decreasing with increasing input irradiance was identified for the Au Nps suspended in ethanol, which is exactly the case of a two-photon absorption behavior; on the other hand, results for the Au Nps embedded in the TiO2 film show that this sample has practically no two-photon absorption. Discussion
4
∑ ∑ Ej(E*k( +
β [m/W]
3
There is no evidence of a crystalline or amorphous layer of TiO2 surrounding the nanostructures, this is corroborated by the EDS elemental mapping scans. From the TEM micrographs shown in Figures 2 and 3, it can be observed that the Nps are composed of pure gold and our statistical observations indicate that the most of the Nps can be consider spheroids. It has been mentioned that a reduction of the Nps size can be strongly important for the modification of the third order optical nonlinearities in a nanocomposite.8 Moreover, it has been noted that anisotropic Nps can exhibit important differences in their nonlinear optical response as a function of its alignment.26 For instance, a different physical mechanism of absorptive nonlinearities can be observed in anisotropy-controlled Au nanoellipsoids.27,28 Nevertheless, when we rotated our film sample with Au Nps in order to study the magnitude of the nonlinear response, we obtained similar measurements which are well described with our error bar of 10% of variation. We state that, even though our Au Nps do no present a preferential alignment, our samples can be considered optically isotropic. Certainly, there must be a crystallite size dependence on the nonlinear optical properties. As an example, it has been probed that the relaxation of the nonlinear optical response of Au Nps is dependent on the crystallinity when thermal conductivity contributes to the mechanism of nonlinearity;29 and furthermore, stability of Au Nps is different for an amorphous matrix and a crystalline matrix.30 It has been observed that linear and nonlinear optical responses increase with the enhancement of
(9)
The best fit for the sign and magnitude of the nonlinear parameters in our numerical simulations indicates that similar mechanisms of optical third order nonlinearity are exhibited by the samples. Two-photon absorption (TPA) and self-focusing seem to be present in the Au Nps embedded in the TiO2 film and in the ethanol suspension; nevertheless, the TPA can be considered negligible for the film. The self-diffraction signal obtained for orthogonal polarizations of the incident beams allows us to indentify an electronic polarization responsible for
Figure 6. Transmittance vs incident irradiance.
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TiO2 crystallinity in some nanohybrids.31 Although the highest TiO2 crystallinity has been related to a highest two-photon absorption coefficient and nonlinear refractive index, on the contrary, in our experiments we observed that the inclusion of Au Nps in an amorphous TiO2 matrix can contribute to the enhancement of the pure electronic optical Kerr response without a contribution to the two-photon absorption. With regard to the manifestation of nonlinear optical absorption as optical damage, we did not observe the ablation threshold of the samples even with the highest irradiances available in our experiments (45 GW/cm2 in the picoseconds regime at 532 nm). We guarantee that there was not optical damage in either sample during the self-diffraction experiments by monitoring the SPR band of the Au Nps embedded in the film. Besides, it has been observed that is possible to modify the Nps in the sample using UV irradiation,32 but we have not obtained significant changes with 532 nm laser radiation. On the other hand, from Figure 4 it is possible to see that the irradiation wavelength used for our nonlinear experiments excites the SPR of the Au Nps; however, Table 1 shows that there is not a strong linear absorption excitation for the Au Nps suspended in ethanol. This last circumstance could be an important reason to explain that this sample exhibits TPA instead of optical saturation at high optical irradiances. Another explanation could be that this high TPA could be originated from a large residual absorption at this wavelength or by a bandedge local enhancement due to surface trapped states. A deep investigation related with the mechanism of optical absorption of these Nps must be made. The nonlinear optical response of TiO2 films seems to be strongly dependent on the processing route.33,34 Our pure sol-gel TiO2 film exhibits a refractive nonlinear optical response at least 2 orders of magnitude below the value that exhibits our sol-gel TiO2 film with Au Nps, so we can consider that the metallic clusters dominate the nonlinearity in our film sample with Nps. Quite similar self-diffracted signals are shown in Figure 5, but a considerable difference in the thickness was present for each sample containing Nps. According our numerical simulations and the experimental data, the self-diffracted signals for both samples are mainly generated by induced birefringence associated with the optical Kerr effect. As a result we obtained a considerably large nonlinear refractive index exhibited by the film with Nps in comparison to the value for the ethanol with Nps; the density of Nps for the ethanol sample is almost the same corresponding value for the thin film, whereas the width is about 5000 times thicker, which means that more Nps interact with the beams in the ethanol media. Even so, we believe that the participation of the matrix in the optical nonlinearity plays an important role for the stimulation of the optical Kerr effect and the inhibition of the nonlinear optical absorption. In addition to the results presented in Table 1, it is important to mention that we could not observe an increment of the optical Kerr effect with a change of the reported concentration of the Au Nps in our ethanol sample; however, the TPA always was present for the ethanol sample even when the nonlinear refraction disappeared from our measurements when decreasing the concentration of Au Nps by a factor of about ten times. On the other hand, the linear absorption coefficients shown in Table 1 also indicate that the film presents a large linear optical absorption near to its SPR band, we propose that this characteristic also contributes to modify the effect of the matrix of TiO2 in the nonlinear response of the Au Nps. As it can be observed from Figure 5, there exists an enhancement of the selfdiffraction efficiency when the Au Nps are embedded in the
Trejo-Valdez et al. TiO2 film. The self-diffraction signal is strongly related with the nonlinear refraction grating resulting from the interaction of the high irradiance incident beams; but also, nonlinear absorption can give a contribution to this modulation of index and detriment of the energy of the transmitted and self-diffracted beams. From the numerical and experimental results shown in Figure 6 it is possible to see that these results are consistent with the fact that the self-diffraction experiment was made in a sample where nonlinear absorption takes places and when a sample is free of nonlinear optical absorption. There is a good agreement with the results obtained by self-diffraction and by transmittance experiments. The large nonlinear refraction coefficient, together with a negligible nonlinear optical absorption, makes the Au Nps embedded in the TiO2 film a promising material for all-optical switching applications. Conclusions We modified the nonlinear optical response for Au Nps by surrounding them with TiO2 in a thin film. A pure electronic response for the optical Kerr effect in Au Nps embedded in a TiO2 film was detected using a self-diffraction technique. The same mechanism of nonlinearity of index was measured in Au Nps suspended in ethanol, but it was found that a contribution of the two-photon absorption strongly weakens the third order nonlinear response. The optical response of the Au Nps changes dramatically depending on the media where they are contained. A TPA mechanism of nonlinearity was observed near of the SPR of the Nps resulting from a low linear optical absorption exhibited by the sample at irradiations below 45 GW/cm2 at the picosecond regime The inhibition of this nonlinear optical absorption reached when the Au Nps are embedded in a TiO2 film seems to have an important role to enhance the nonlinearity of index associated to the pure electronic response. Numerical and experimental transmittance results are in agreement with our self-diffraction experiments. Acknowledgment. We acknowledge the financial support from IPN, through Grants SIP20100836, SIP 20100564, and SIP-20100800 and also from CONACyT-Mexico, through Grant Nos. 80024 and 82708. The authors are also thankful to the Nonlinear Optics Lab facilities at IFUNAM, and also the technical support from the Lab of Catalysis and Materials at ESIQIE, IPN. References and Notes (1) Gonella, F.; Mazzoldi Handbook of nanostructured materials and nanotechnology; Academic Press: San Diego, CA, 2000; Vol. 4 (Optical properties), Chapter 2 (Metal Nanoclusters Composite Glasses). (2) Rigneault, H.; Lourtiouz, J.-M.; Delalande, C.; Leven, A. Nanophotonics ISTE Ltd.: Newport Beach, CA, 2006; Chapter 5 (Nonlinear Optics in Nano- and Microstructures). (3) Ung, T.; Mulvaney, P. Optical Properties of Thin Films of Au@SiO2 Particles. J. Phys. Chem. B 2001, 105, 3441–3452. (4) Nasu, H.; Tanaka, A.; Kamada, K.; Hashimoto, T. Influence of matrix on third order nonlinearity for semiconductor nancrystals embedded in glass thin films prepared by Rf-sputtering. J. Non-Cryst. Solids 2005, 351, 893–899. (5) Hache, F.; Ricard, D.; Flytzanis, C.; Kreibig, U. The optical Kerr effect in small metal particles and metal colloids: the case of gold. Appl. Phys. A: Mater. Sci. Process. 1988, 47, 347–357. (6) Rangel-Rojo, R.; Yamada, S.; Matsuda, H.; Kasai, H.; Nakanishi, H.; Kar, A. K.; Wherrett, B. S. Spectrally resolved third-order nonlinearities in polydiacetylene microcrystals: influence of particle size. J. Opt. Soc. Am. B 1998, 15 (12), 2937–2945. (7) Lamarre, J.-M.; Billard, F.; Martinu, L. Local field calculations of the anisotropic nonlinear absorption coefficient of aligned gold nanorods embedded in silica. J. Opt. Soc. Am. B 2008, 25 (6), 961–971. (8) Lo´pez-Sua´rez, A.; Torres-Torres, C.; Rangel-Rojo, R.; ReyesEsqueda, J. A.; Santana, G.; Alonso, J. C.; Ortı´z, A.; Oliver, A. Modification
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