Abstract

Thin films of polycrystalline PZT ceramics are studied by optical second harmonic generation (SHG) and linear optical reflectance (LR) techniques using the output of a Ti-sapphire femtosecond laser. Essentially diffuse SHG (hyper-Rayleigh scattering) as well as diffuse linear (Rayleigh) scattering from the samples is observed. The pronounced azimuthal anisotropy of the intensity in specular direction for SHG and nonspecular directions for LR is detected. We describe these effects in terms of correlation functions of fluctuating sources of incoherent light scattering: for SHG these sources are fluctuations of both nonlinear susceptibility and local fields, whereas the linear scattering is produced only by the local-field fluctuations. The comparison of SHG and LR data allows one to estimate the relative contributions to the incoherent SHG from fluctuations of the nonlinear susceptibility and local fields. At the same time, small azimuthal anisotropy of the SHG intensity is observed in nonspecular directions with the contrast increasing with decreasing laser spot size. The reduction of the laser spot size down to 25 microns leads to an increase of relative magnitudes of high-order (usually attributed to as the noise) Fourier components of the SHG anisotropic dependence for the specular (Fig. 1) and diffuse SHG response. It also leads to dramatic changes in the structure of both SHG and LR scattering indicatrices: a large number of very sharp disordered peaks appear on these curves (Fig. 2). This fine structure is absolutely reproducible and individual for each laser spot position: the small (several microns) shift of the laser spot leads to significant changes in the picture. These effects can be interpreted as the evidence of mesoscopic (on the micron scale) fluctuations of the film optical parameters within the finite-size laser spot area.

© 1998 IEEE