1. Introduction

All-optical devices are extensively used for photonic applications such as processing and storage of large capacity information [1]. The most investigated nonlinear optical effect which is necessary for developing such devices is the process for control the radiation power of one wavelength by radiation power of another wavelength (so-called optical switching) [2, 3]. In this case, the radiation with shorter wavelength is used as a pump, and other radiation is a probe. Pump radiation modifies the optical properties of a sample, namely refractive index and/or absorption coefficient. Commonly, probe radiation does not alter the optical properties of the sample, and used only as a scanning radiation [1, 3].

The development of optical switching devices is basically associated with the use of photochromic materials as an active medium [2, 3]. Such approach has a significant drawback due to degradation of the active medium under prolonged exposure of pump radiation [2]. To date the scientific attention is given to nonlinear optical nanocomposite materials based on quantum dots (QD) incorporated into organic shell. These materials are unique in that they manifest different properties (such as photoconductivity, optical bistability, photorefraction) simultaneously as well as they have significant photo and chemical stability compared to photochromic materials [4].

Well studied luminescent properties [5] and electronic structure of CdS QD [6] as well as the mentioned above opto-electronical properties allow us to conclude that these QDs are promising for photonic application, particularly optical switcher. Therefore, in this paper we study the laser-induced optical switching effect in the nanocomposite based on CdS QD (hereinafter - NcQD) stabilized by thiomalic acid shell and incorporated into a porous silica matrix THEOS (Tetrakis (2- hydroxyethyl) orthosilicate) [7].

2. Materials and methods

Synthesis of QD was performed by methods proposed in [8]. THEOS was synthesized by the method described in [9]. QD stabilized by thiomalic acid were dispersed in deionized water with concentration of 0.3 wt. %. After that THEOS was added in stabilized solution of QD under vigorous stirring. The obtained nanocomposite was poured into a flat cuvette with thickness of 1 mm.

The structural studies of nanocomposite were carried out using the small angle X-ray diffractometer SAXS («HECUS», Austria). The experimental scattering curves were obtained at wavelength λ = 0.1542 nm in the range of wave vectors 0.07<s<6.0 nm⁻¹ (s = 4π sinθ/λ, 2θ - angle scattering). To study the dynamic changing of the structural properties of the nanocomposite under UV radiation we used solid state laser at λ = 405 nm connected with 600 μm optical fiber. Optical fiber was arranged in SAXS chamber, so that the X-ray and UV beams were in the same plane. The experimental SAXS data were normalized to the intensity of the incident beam after that the collimation distortions correction was performed in accordance with the standard procedure [10]. Initial processing of the experimental SAXS data was performed using the PRIMUS software [11]. For analysis of size distribution of structural inhomogeneities of the samples we used GNOM software [12]. Determination of the maximum size of the scattering object (L) and restoration of the spatial structure of the nanocomposite was performed by DAMMIN software [13]. Optical absorption spectra were studied by spectrophotometer Varian Cary 5000i in transmission mode and with an integrating sphere. The luminescence spectra were obtained using a high-speed spectrometer Andor iStar.

3. Results and discussion

We have found that NcQD’ optical absorption of radiation within the spectral range (300;700) nm changes when exposed to radiation at wavelength below 470 nm. It was observed that this effect depends on the value of exposure dose (J/cm²) and has no significant dependence of the radiation wavelength. The effect of changing the optical properties was not found when NcQD was exposed to radiation at wavelength above 470 nm with the exposure dose up to 500 J/cm².

As the pump source the solid-state laser emitting at wavelength λ = 405 nm was used. Maximum emission power of this laser was 45 mW, diameter of the beam in contact with the sample was 1 mm. Figure 1(a) shows NcQD and pure silica matrix (THEOS) optical absorption before, in process and after radiation exposure. Exposure of pure silica matrix with dose up to E = 500 J/cm² does not changes its absorption (solid line in Fig. 1(a)). The change in NcQD optical properties was observed with exposure doses up to E = 150 J/cm² (dash-double dotted curve in Fig. 1(a)). Exposure of NcQD with doses above E = 150 J/cm² causes the saturation without any subsequent changes in NcQD optical properties. It should be noted that absorption in spectra obtained in transmission mode somewhat more that absorption obtained with integrating sphere (Fig. 1(a)). This could indicate a predominate influence of optical absorption to photoinduced changing of NcQD optical properties. Moreover, changing of NcQD optical properties is determined only by the value of exposure dose and does not dependent on radiation power. Obtained results demonstrate that observed effect has a photochemical nature [14], in contrast to the optical bistability and photoconductivity effects described for cadmium sulphide nanoparticles [15, 16]. When NcQD are exposed to pump radiation, one can observe the region of modified material (inserts in Fig. 1(b)).

 

Fig. 1 Optical properties of NcQD before and after pump radiation exposure: (a) absorbance spectrums; (b) luminescence spectrums (1 – unexposed area; 2 – exposed area; 3 – bare THEOS). Inserts shows exposed area of NcQD immediately after expose ((a) upper), since 12 hours after exposure ((a) lower). At a 12 hours after exposure image ((a) lower insert) there is almost unseen modified area, but under UV lighting (insert at Fig. 1(b)) this area is observable.

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Turning off the pump laser causes the partial relaxation of optical properties which expressed in the returning of absorption to the original value. But the value of absorption is fixed at a higher level (Fig. 1(a) dash-doted curve) and does not reached the original value (Fig. 1(a) dashed curve) and do not changed in subsequent cycles.

Modified area has a substantial change of the luminescence spectrum. When NcQD are exposed to laser radiation the maximum of the luminescence spectrum is red-shifted (Fig. 1(b)). This fact allows us to consider hypothesis that the observable optical changing can appear due to increasing in the size of the emitting centers [6].

To study optical properties of modified area we were performed pump and probe technique [1, 3] wherein premodified area of NcQD (S) was lighting by two laser beams (Fig. 2). Laser beam at the wavelength λ = 405 nm was used as a pump beam and laser beam at the wavelength λ = 633 nm was used as a probe beam. Probe beam was directed collinear to pump beam. As was noted above, light at the wavelength λ = 633 nm does not modify the NcQD structure and so has no effect on the NcQD’ optical properties. To reduce the possible thermal influence of the probe beam it intensity was set at I = 100 μW/cm². Pump beam power was set by attenuator (Att), which allowed adjusting the exposure dose. Synchronization of photodetectors PD₁ and PD₂ allowed associate the optical response of the sample with turning on of a pump beam. Colored glass filters F₁ and F₂ (Newport FSQ-BG3 and FSQ-OG590, correspondingly) were used to block probe radiation on PD₁ and pump radiation on PD₂.

 

Fig. 2 Experimental pump-probe setup.

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As a result, turning on the pump laser leads to significant increase of the photo-induced absorption coefficient and correspondingly to decrease of output power of probe beam (Fig. 3). The value of photoinduced absorption is dependent of the pump beam power. But the dynamic properties of the system have not significant dependence of pump beam power.

 

Fig. 3 Photodynamic control of probe beam (λ = 633 nm) by pump beam (λ = 405 nm) and its dependence of pump beam’s power.

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When the pump laser is turned on the time t₉₀ (time to reach 90% of the maximum absorbance) does not exceed 5 seconds. In the case of turned off the pump laser the value of t₉₀ is somewhat more than 15 seconds. The maximum achieved value of photoinduced absorption coefficient was α ≈13.86 ± 0.003 cm⁻¹ at a laser power P = 45 mW, corresponding to an intensity of I = 1.5 W/cm². For the minimum intensity of I = 0.05 W/cm² (P = 1.25 mW) this value was α ≈0.4 ± 0.001 cm⁻¹. It should be noted that the value of photoinduced absorption coefficient is related by nanoparticles concentration in matrix. Concentration of 0.3 wt. % used at present study is an upper threshold limit because further increasing of concentration lead to opalescence and decreasing of NcQD optical transparency.

For a more clearly understanding of the processes occurring under exposure was used data of the structural parameters of the NcQD obtained by SAXS. The study of these structural parameters was performed at three stages. At the first stage the structure of the pure THEOS gel matrix was investigated. At the second stage the structure of the NcQD was investigated. At the third stage the dynamics of change of the NcQD structural characteristics under exposure to UV radiation was investigated.

SAXS profiles for all this samples (not shown) are characterized by central scattering, i.e., scattering in the range of the smallest angles, which points to the inhomogeneous electron density of the samples at which scattering takes place. For a gel matrix, this is scattering by the pores of internal matrix cavities. For the composite with the NcQD the scattering curve is determined by the superposition of the influence of matrix and nanoparticle [10].

The first stage of structural analysis for these samples lies in determining the size distribution of scattering inhomogeneities. To successively study the processes of NcQD formation in THEOS matrices using small-angle X-ray scattering, we studied the structure of the gel matrix (50% THEOS) itself, and then system with that impregnated into NcQD. The difference spectrum of scattering for determining the properties of NcQD was used. The volumetric size distribution functions D_V(R) and the distribution of radii of gyration P(R) calculated by the GNOM and DAMMIN software revealed that QD consist of a crystalline core of CdS with D = 1.6-2.5 nm surrounded by a shell of thiomalic acid which are increasing the diameter up to D = 4.3-4.5 nm. The estimated maximum size of such nanoparticles is in the range of L = 5.5-6.0 nm and form-factor of such particle is closed to the elliptical at axis Z (Fig. 4 (a)). The calculations for THEOS gel matrix revealed that the most appropriate spatial model for such pores is an elongated cylinder which has a length of L = 11.6 - 12.6 nm and diameter D = 2R_g = 4.3-4.4 nm, where R_g - electron radius of gyration.

 

Fig. 4 The resulting spatial model of a silica nanocomposite based on cadmium sulphide quantum dots under exposure to UV radiation. (a) Reconstructed shapes of NcQD clusters (I) before exposure, (II) exposure dose 40 J/cm2, (III) relaxation: SAXS experimental curve, scattering curve from the ab initio model and spatial model of NcQD calculated by the DAMMIN software (lower insert). The top rihgt insert shows 3-D model of the NcQD calculated by CRYSOL program; (b) schematic representation of NcQD clusters formation

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Analysis of the scattering intensity values at the zero diffraction angle shown that THEOS matrix has five times smaller scattering intensity than the NcQD. This fact allows suggesting that during synthesis of NcQD nanoparticles became incorporated into the pores of the matrix. Thus comparison of the radii of gyration of CdS nanoparticles and pores of THEOS matrix shows that in most cases one pore may not contain more than two nanoparticles. The resulting spatial model of NcQD is shown in Fig. 4(b).

Analysis of NcQD SAXS profiles under UV light exposure shown that in this case dominant size of scattering centers is increased. This fact has good agreement with the proposed model of NcQD, according to which the pores of the matrix are closely spaced pairs of QD. Under pump radiation QD can interact so that the distance between them becomes smaller than D and the new state is energetically advantageous. NcQD goes into a new state with the size of the main scattering centers increased about twice. The spatial model of the QD agglomerate represents two contiguous ellipse which can equiprobably stand as in the same matrix pore as well as in the two closely-spaced pores (Fig. 4(b)). This well explains the increasing of the absorption coefficient value of the NcQD after the first exposure (Fig. 1(a)).

The variations of D and L of NcQD under the pump light exposure obtained by SAXS are presented in Table 1. The curve of scattering and the spatial structural model obtained using DAMMIN software are shown in Fig. 4 (a). In this experiment were calculated volumetric size distribution function D_V(R) and the distribution of radii of gyration P(R) in cases continued growth the exposure, relaxation and repeated growth the exposure (see Table 1 Comment).

Table 1. Variations of D and L of NcQD Scattering Centers under Exposure of Pump Radiation

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Obtained data revealed that increasing the exposure dose causes gradually increase the size of scattering centers. At exposure dose equal E = 60 J/cm² occurs the saturation and maximum values of scattering centers became D = 8.5-8.7 nm and L = 24.8 nm. This data allows to assume that under exposure of pump radiation electron shells of QD has transition moving to a higher energy level and those that are closely arranged into the matrix begin to interact with each other. This process is accompanied by forming the agglomerates of 2-4 particles. Saturation occurs when all potential agglomerations of QD are implemented. The sizes of the agglomerates are proportional to the exposure dose of pump radiation and determined the size of the scattering centers. This state is unstable and it possible only under the pump radiation.

After turning off the pump radiation the stage of relaxation of NcQD comes which exhibit the gradual deagglomeration and restoration of the form and size of the scattering centers to their original state. There is a steady repetition of results in subsequent cycles of NcQD exposure/relaxation.

Thus, the data obtained by SAXS fit well with the proposed model and explain the dynamics of change of the NcQD absorption coefficient during the cycles of exposure/relaxation (Fig. 3).

4. Conclusion

We have experimentally investigated all-optical switching of nanocomposite system based on QD cadmium sulphide which were stabilized by thiomalic acid solution and placed in a silica matrix using 405 nm laser as pump beam and 633 nm laser as probe beam. The maximum achieved absorption value was α ≈13.86 ± 0.003 cm⁻¹ at a laser intensity of I = 1.5 W/cm². Using Small-Angle X-ray Scattering technique it was found that exposure of nanocomposite to radiation at λ = 405 nm causes the formation of the agglomerates of 2-4 QDs. The number of particles in the agglomerates is proportional to the exposure dose. These agglomerates are unstable and after turning off the pump radiation agglomerates are returned to initial state.