Wavelength dependence of nonlinear optical susceptibility of ZnSe nanocrystalline film

The present work reports the nonlinear optical susceptibility of nanocrystalline ZnSe thin films with different thicknesses, measured using third-order harmonic technology at irradiation wavelengths lambda of 1200-1900 nm. The results show that the third-order nonlinear susceptibility chi(3) of ZnSe thin films is a function of the pump wavelength and film thickness. The chi(3) values for the thinner 202-nm film generally decreased with the redshift of the wavelength and were approximately 10-20 m2/V2 at lambda <= 1500 nm and 10-21 m2/V2 at lambda > 1500 nm. The chi(3) values for the thicker 1021-nm film did not follow a decreasing trend, but instead exhibited local peaks at 1350, 1500 and 1750 nm, with a maximum value of approximately 2.4 x 10-19 m2/V2 at lambda = 1750 nm. Within 1200-1900 nm, the chi(3) of the thicker film was always greater than that of the thinner film. In addition, a weak second-order harmonic at wavelengths within 1100-1300 nm was observed for the 1021 nm ZnSe thin film. The large difference in the nonlinear optical properties between the thinner and thicker film samples is conjectured to be caused by the increased density of geometric defects and the gradual formation of local anisotropy with increasing film thickness. Nonlinear optical coefficient measurements of ZnSe thin films with different thick-nesses over a broad spectrum are important for the application of these films in nonlinear thin-film optoelec-tronic devices.