Nonlinear Optical Properties of CuS Nanocrystals with Modified Surface
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* International Laser Center, Belarus State Polytechnic Academy, F. Skaryna av., 65, bid. 17, Minsk 220027, Belarus, tel/fax: 375 (172) 326286 ** Physico-Chemical Research Institute of Belarus State University, Minsk 220080, Belarus
ABSTRACT Nonlinear bleaching and induced absorption effects in CuS microcrystals with modified (after oxidation) surface have been studied. Relaxation times of these processes demonstrate two components - fast (- 80 ps) and slow (>>500 ps). Energy level scheme for explanation of these phenomena was proposed. Appearance of bleaching effect in CuS microcrystals due to surface oxidation makes them as perspective materials for saturable absorbers for pico- and femtosecond lasers. INTRODUCTION In the field of semiconductor physics, a considerable activity is dedicated to the study of quantum confinement effects in systems of low spatial dimensionality. Especially interesting are the nonlinear optical properties of these structures which make them promising for different optical and electronic devices. Theoretical investigations [1] have shown that one has to distinguish between various confinement regimes which depend on the quantum dots' size compared to the Bohr radius of the elementary excitations of the semiconductor (excitons, electrons, holes, etc.). However, the role of the surface of such quantum confined microcrystals in producing optical nonlinearites has not been elucidated unambiguously both theoretically and by experiment. It has been found recently, that chemical modification (oxidation) of surface of copper sulphide (CuS) nanocrystals results in the appearance of additional absorption band in the NIR (that corresponds to the appearance of new level in the band gap of CuS) [2,3]. X-ray photoelectron spectroscopy and photoelectron chemical studies have shown the extra absorption band of oxidized CuS particles is related to formation of CuO surface shell [3]. In order to clarify the origin of this extra absorbance in the NIR and to get some information of the electronic structure of surface-oxidized CuS particles, we have carried out picosecond pump-probe measurements on CuS and oxidized CuS nanoparticles embedded in polyvinylalcohol (PVA) thin
films. EXPERIMENT CuS-particles were chemically synthesized in the form of aqueous colloidal solution by a reaction between CuCi2 and H 2 S. CuC!2 and H 2 S were dissolved in 0.5% aqueous PVA solution. The part of this solution was poured out onto 2 mm thick quartz glass substratum (whose sides were parallel to < 0.080) and then dried. The rest of CuS aqueous colloidal solution was carefully heated to 70 0 C: As the result of this heating surface oxidation of CuS particles by dissolved oxygen took place [2]. After cooling to room temperature colloidal solution was used to prepare the sample of PVA film containing oxidized CuS particles [CuS(ox) sample]. The mean 289 Mat. Res. Soc. Symp. Proc. Vol. 405 01996 Materials Research Society
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diameter of CuS and CuS(ox) nanoparticles determined from transmission electron microscopy photograp
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