NONLINEAR-OPTICAL EFFECTS IN POROUS SILICON - PHOTOLUMINESCENCE SATURATION AND OPTICALLY INDUCED POLARIZATION ANISOTROPY

We present an analysis of strong nonlinear optical effects observed in the photoluminescence of porous Si. Two groups of effects are discuss ed. The first includes photoluminescence saturation, suppression of th e polarization memory, and pump coincident optically induced polarizat ion anisotropy all observed at room temperature. These effects are wel l described by nonradiative Auger quenching of the photoluminescence i n nanocrystals containing more than one electron-hole pair and which a re selectively excited by linearly polarized light. The second group i s connected with photoluminescence degradation and a persistent optica lly induced polarization anisotropy at helium temperature. These effec ts arise from and are very well described by Auger autoionization of c rystals selectively excited by polarized light, and subsequent Auger q uenching of all radiative recombination in them since they contain lon g-lived charged carriers. Upon heating the samples to room temperature the electron returns back to the nanocrystal. This restores the initi al photoluminescence intensity and washes out the long-lived optically induced polarization anisotropy. The high efficiency of all these eff ects is provided by the large ratio of the rate of Auger processes to the radiative recombination rate in the nanosize Si crystals.