In this paper we study the laser-induced modification of optical properties of nanocomposite based on cadmium sulphide quantum dots encapsulated into thiomalic acid shell which were embedded into a porous silica matrix. It was found that exposure to laser radiation at λ = 405.9 nm leads to modification of optical properties of nanocomposite. For this exposed area there is a significant amount of photodynamic changes under subsequent exposure to laser radiation at λ = 405.9 nm, namely photoabsorption and photorefraction which were studied at λ = 633 nm. The value of these effects dependent on the concentration of quantum dots and modifying radiation parameters. Moreover, it has dependence from polarization of exposure radiation.
© 2015 Optical Society of America
Increased interest has recently been observed for the application of nanodimensional particles for creation of all-optical information processing devices . Possibility for the control of radiation of one wavelength by radiation of another wavelength (pump and probe method) is distinguished from the nonlinear optical effects used in these kind of devices [2–4]. Optical beam of smaller wavelength is used in this case as an operating beam (modifying the optical characteristics of material - pump), and that of a larger wavelength – as the operated (probe) beam. One of the perspective fields for the realization of these kinds of devices is application of nanocomposite materials based on metal or semiconductor nanoparticles enclosed in an organic cover and placed into an optically transparent matrix. These materials possess a number of unique properties including, in particular, photoconductivity, optical bistability, photorefraction, photo- and chemical stability [5,6]
As we reported previously  the effect of reversible change of the photo-induced absorption as a response to the high-energy radiation dose value is observed in a nanocomposite based on the cadmium sulfide quantum dots (QD) enclosed in a thiomalic acid shell and introduced into an optically transparent silicate matrix by means of a precursor of tetrakis (2-hydroxyethyl) orthosilicate (THEOS) (hereinafter - NQD). Herein, the first exposure of NQD leads to formation of exposed zone (EZ) in the spectral range of 300-700 nm. This EZ characterized the increase in the absorption and the red-shift of luminescence maximum. Obtained data revealed that impact of laser radiation with λ = 405.9 nm causes gradually increase the size of scattering centers. This process is accompanied by forming the agglomerates of 2-4 particles. 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.
This paper presents the results of complex research of the particular NQD targeting the determination of dependence of its nonlinear optical response magnitude on the concentration of quantum dots and modifying radiation parameters.
2. Materials and methods
Synthesis of cadmium sulfide QD stabilized with thiomalic acid solution was carried out based the technique suggested in . THEOS precursor was synthesized by the technique described in . THEOS was introduced with intensive hashing into the QD dispersed in deionized water in the appropriate concentration (0.01; 0.05; 0.1; 0.3) %wt. until formation of a homogeneous mix to receive NQD. The obtained gel was poured into flat cuvettes to form nanocomposite samples. It is worth mentioning that a concentration of 0.3%wt. is the maximum at which optical transparency of samples is preserved. It further increases leads to emergence of their opalescence. Precursor concentration in all cases made up 50%wt.
An experimental device was developed for the realization of the pump and probe method; its scheme and description have been provided in . A continuous solid-state laser (λ = 405.9 nm, with a maximum emergent radiation power of 45 mW, diameter of a beam at the site of contact with a sample - 1 mm) was used as a modifying one (pump), and a helium-neon laser (λ = 633 nm, radiation intensity 100 μW/cm2) – as a reading one (probe). Most parts of the studies (except dependence of NQD response on polarization) were carried out at circular polarization of the modifying radiation.
3. Results and discussion
The “pump and probe” method described above was applied to study dynamic characteristics of the NQD. EZ of the NQD where the pump (λ = 405.9 nm) and probe (λ = 633 nm) beams were directed in a collinear mode was used as the active environment. As mentioned above, radiation with the wavelength λ = 633 nm produces no impact on the optical properties of a sample. Intensity of the probe beam was chosen to be equal to 100 µW/cm2 for the purpose of reduction of its possible thermal effect. Power of the pump beam was regulated by an attenuator, which allowed the precise selection of an exposure dose.
A fall in the output power value of the probe subsystem is observed with switch-on of the pump beam, which evidences the increase in the photo-induced absorption (Fig. 1(a)). Its advanced dependence on the exposure dose of the modifying radiation and QD concentration is observed at the same time. We found that the QD concentration of 0.1%wt. is a critical point in a nonlinear dependence of the response of the investigated system at the dose of laser radiation. In particular, nonlinear optical response occurs at exposure dose up to 90 J/cm2 for QD concentration 0.1%wt. and demonstrates equivalence of optical responses values NQD with QD concentration of 0.3%wt. Results for the NQD with QD concentration of 0.1%wt. are presented at Fig. 1(b).
Time from the moment the pump laser was turned on, at which the probe signal value reaches 90% of its minimum value, did not exceed 5 s in this experiment. When switching the pump laser off, the system recovery time is slightly more, reaching 15 s. At the same time, the power of the modifying radiation and QD concentration produced minor impact on the dynamic characteristics of the system.
As it is obvious from the aforementioned, one of physical features of the studied NQD is the existence of the dynamic photo-induced optical absorption, depending on the exposure dose of the pump radiation. Its advanced dependence on the QD concentration is observed at the same time. Dependencies of the nonlinear optical response magnitude of the EZ of the NQD on the concentration of quantum dots and exposure dose are presented in Fig. 2(a). To represent dependence in the field of small concentrations, these results are presented in Fig. 2(b) in the logarithmic scale.
Dependence of the photo-induced absorption value on the exposure dose and QD concentration is presented at Fig. 3. The maximal value of the photo-induced absorption coefficient α achieved in our experiments, which was calculated following the Bouguer law  makes up α ≈13.86 ± 0.003 cm−1 at the exposure dose being E = 90 J/cm2 for concentrations of 0.3%wt.
The received results confirm once again that the observed effects are the most expressed for the QD concentration of 0.3%wt., therefore further studies were carried out for this particular concentration.
One more effect discovered in the studied NQD was dependence of the magnitude of optical response of the EZ (radiation power λ = 633 nm transmitted through the NQD) from polarization of the pump beam, presented for the QD concentration of 0.3%wt. at Fig. 4. A smooth increase of the radiation power transmitted the NQD with the change of the polarization angle of the pump beam from 0 to 50 degrees (being practically two-fold at 50 degrees) and a smooth decrease at change from 50 to 90 degrees are observed.
Some authors  describe the organic and inorganic hybrid nanocomposite consisting of a polymeric charge transferring matrix, with the introduced CdS nanoparticles acting as the sensitizer having a photorefractive effect. Therefore we investigated the possibility of the photo-induced change of the refraction index in the EZ of the NQD with QD concentration of 0.3%wt. For this purpose the sample of NQD was located in one of the arms of the Mach–Zehnder interferometer. The modifying and reading beams, similarly to the previous experiments, were directed collinearly, which allowed to detect the photo-induced phase progression in the EZ of the studied sample. It is known  that distribution of intensity in an interferential pattern is defined as , where I1 and I2 stand for intensity values of the interfering beams, and δ – for the shift of phases defined as where stands for the difference in the course of optical beams between the measuring and reference arms of the interferometer. In our case, the change in the phase shift by the π value corresponds to the change of the refraction index of a sample by the value Δn ≈3∙10−4.
Results of experiments presented in Fig. 5 demonstrate that at radiation of a sample with a modifying beam λ = 405.9 nm, the phase progression starts at an exposure of about 0.1 J/cm2 and makes up δ ≈ 5π, which corresponds to the change in the refraction index by Δn1≈1.5∙10−3. At the same time, the change in absorption of about 6 J/cm2.
This allows us to suggest that the initial phase progression corresponding to the change of the refraction index value by Δn1 ≈1.5∙10−3 results from the change in the dimensions of the QD caused by their agglomeration. Further phase progression and, respectively, change in the refraction index by the value of Δn2 ≈1∙10−2 is mostly effected by sample heating resulting from increase of its absorption value. Saturation mode occurs at an exposure dose of about 150 J/cm2.
It should be noted at the same time that the photo-induced change in the refraction index of the EZ of the NQD is also reversible and returns to the initial value upon termination of irradiation. Results of studying of the dynamics of photo-induced change of the refraction index in the EZ of the NQD with QD of 0.3%wt. along with changing in the output power of the reading subsystem are presented at Fig. 6. Power of the modifying laser was 45 mW. In this case, the change in the phase progression made up δ = 18π, which corresponds to the change of the refraction index of a sample by the value of Δn≈5.4∙10−3.
The experiments described above and their results allow us to suggest that, as a result of exposure to the laser UV radiation, a nanocomposite system based on cadmium sulfide QD stabilized with thiomalic acid shell and placed in a silicate matrix comes into some new state that differs from the initial state by the photo-induced absorption and refraction index, due to the agglomeration of CdS quantum dots into larger structures. Based on the obtained data, a conclusion may be made on the prevailing contribution of optical absorption in the observed effects. At the same time, the grade of the effect depends on the exposure dose and QD concentration. The possibilities to control the magnitude of the absorption of the NQD using UV radiation parameters, such as the dose of exposure and polarization, are shown. The obtained results of the study demonstrate the principal possibility to use them to create all-optical devices.
Financial support from Russian Foundation of Basic Research (project 13-02-12415) is gratefully acknowledged.
References and links
1. K. M. Dani, Z. Ku, P. C. Upadhya, R. P. Prasankumar, A. J. Taylor, and S. R. J. Brueck, “Ultrafast nonlinear optical spectroscopy of a dual-band negative index metamaterial all-optical switching device,” Opt. Express 19(5), 3973–3983 (2011). [CrossRef] [PubMed]
5. S. Ren, L.-Y. Chang, S.-K. Lim, J. Zhao, M. Smith, N. Zhao, V. Bulović, M. Bawendi, and S. Gradecak, “Inorganic-organic hybrid solar cell: bridging quantum dots to conjugated polymer nanowires,” Nano Lett. 11(9), 3998–4002 (2011). [CrossRef] [PubMed]
6. D. Bera, L. Qian, T.-K. Tseng, and P. H. Holloway, “Quantum dots and their multimodal applications: a review,” Materials. 3(4), 2260–2345 (2010). [CrossRef]
7. S. S. Voznesenskiy, A. A. Sergeev, A. N. Galkina, Y. N. Kulchin, Y. A. Shchipunov, and I. V. Postnova, “Laser-induced photodynamic effects at silica nanocomposite based on cadmium sulphide quantum dots,” Opt. Express 22(2), 2105–2110 (2014). [CrossRef] [PubMed]
8. Q. Xiao and C. Xiao, “Surface-defect-states photoluminescence in CdS nanocrystals prepared by one-step aqueous synthesis method,” Appl. Surf. Sci. 255(16), 7111–7114 (2009). [CrossRef]
9. E. Ruiz-Hitzky, K. Ariga, and Yu. M. Lvov, Bio-inorganic Hybrid Nanomaterials (Weinheim, 2007), Chap. 3.
10. J. D. J. Ingle and S. R. Crouch, Spectrochemical analysis (Prentice Hall, 1988).
11. L. Huang and Y. Zhong, “Photorefractivity in a bi-functional polymer nanocomposites sensitized by CdS nanoparticle,” J. Wuhan Univ. Technol. 25(4), 550–554 (2010). [CrossRef]
12. P. Hariharan, Optical Interferometry, II ed, (Elsevier, 2003).