Nonlinear optical characteristics of nanocomposites of ZnO–TiO2–SiO2
Introduction
The field of nanocomposite materials has been widely recognized as one of the most promising and rapidly emerging research areas [1]. There are some material designs to strengthen and toughen ceramics by using composite techniques to incorporate particulate, whisker or platelet reinforcement. Recent investigations have shown that ceramic composites having nano-sized metal particulate dispersions show excellent optical, electrical and mechanical properties [2]. Promising applications are expected or have already been realized in many fields of technology such as optical and electronic materials, solid electrolytes, coating technology and catalysis. Significant investigations have been done in the photophysical and photochemical behavior of single and multicomponent metal and semiconductor nanoclusters [1]. Such composite materials are especially of interest in developing efficient light-energy conversion systems and optical devices.
With many advantages such as low cost, nontoxicity and stability, ZnO is becoming a very promising n-type oxide semiconductor. Most of the work has been devoted to the electrical and fluorescent properties of ion-doped zinc oxide materials, while only a few reports can be found using ZnO as the matrix for nanoparticle composite films [3]. These nanocomposites may lead to optically functional properties.
In recent years, interest in the synthesis, characterization, and application of colloidal “quantum dot” semiconductor materials has grown markedly [4]. Nanoscale composite materials containing titanium oxides are interesting because of their potential applications in optoelectronic devices and the bulk TiO2 has a direct band gap of 3.2 eV. A great deal of research effort has been focused on both synthesis of TiO2 nanocomposites, and on their linear optical properties. Recently the nonlinear optical properties of such materials have also received attention. A large, reverse saturable type of nonlinear absorption is observed in polystyrene maleic anhydride–TiO2 nanocomposites with a continuous wave He–Ne laser beam [5].
Extensive investigations of the photoluminescence and the third-order optical nonlinearities of nanometer-sized semiconductor materials have demonstrated interesting physical properties and potential applications. The absorption and luminescent properties of TiO2, CdS and PbS particles can be easily tuned by selecting appropriate matrix materials. Recently, a microemulsion technique has been developed to prepare semiconductor nanocomposites such as ZnS/CdSe, ZnSe/CdSe, ZnS/CdS or TiO2–SiO2 in a core-shell structure [6]. Chemically synthesized semiconductor nanocomposites offer necessary and basic materials promising color-tunable, flexible, all-purpose chromophore systems, in which the strong quantum confinement effect of the carriers leads to unique, size dependent linear and nonlinear optical properties.
The synthesis of new nonlinear optical materials based on transparent semiconductor and insulator that contain metal nanoparticles is nowadays of great interest for applications in nonlinear optics [1]. For optical limiting applications it is necessary that the sample possess low linear losses and high nonlinear losses. Nonlinear losses can be due to multiphoton absorption, reverse saturable absorption, nonlinear scattering, and self-action of laser radiation (Kerr and thermal self-focusing and self-defocusing) [7]. The nonlinearity of the silica colloid is low and its optical limiting response can be improved by making composites with ZnO and ZnO–TiO2 which gives rise to wide applications in optoelectronic devices.
Different metal particles, organic nanocrystals and fullerenes doped in sol–gel glasses and silica composites are well studied for optical limiting applications [1], [2]. It is also known that doping significantly improve the limiting performance of ZnO. In this paper we present the nonlinear optical properties of ZnO–TiO2–SiO2 nanocomposites.
Section snippets
Experimental
Colloids of ZnO are synthesized by a modified polyol precipitation method [7]. The ZnO colloidal spheres are produced by a two-stage reaction process. The method of preparation involves the hydrolysis of zinc acetate dihydrate (ZnAc) in diethylene glycol medium (DEG). Among the different polyols, diethylene glycol (DEG) is chosen because it is reported to give particles with uniform shape and size distribution. The size of the particles and hence the stability of this colloidal suspension
Results and discussion
Optical absorption measurement is an initial step to observe the single colloid and metal-semiconductor nanocomposite behaviour. Fig. 1 gives the room temperature absorption spectra of the ZnO–TiO2–SiO2 nanocomposites. There is a change in absorption with the ZnO content and as the volume fraction of ZnO increases, the excitonic peak exhibits its signature. It is seen that the absorption edge corresponding to the nanocomposites gets red shifted and the exciton oscillator strength increases as a
Conclusion
The nonlinear optical properties of ZnO–TiO2–SiO2 nanocomposites prepared by a colloidal chemical synthesis are investigated. These nanocomposites show self-defocusing nonlinearity and good nonlinear absorption behaviour. The nonlinear refractive index and the nonlinear absorption increases with increasing ZnO volume fraction. The observed nonlinear absorption is explained by two photon absorption followed by weak free carrier absorption and nonlinear scattering. These materials can be used as
Acknowledgments
LI acknowledges UGC for research fellowship and CELOS for experimental facility.
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