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 discussed. The first includes photoluminescence saturation, suppression of the polarization memory, and pump coincident optically induced polarization anisotropy all observed at room temperature. These effects are well described by nonradiative Auger quenching of the photoluminescence in nanocrystals containing more than one electron-hole pair and which are selectively excited by linearly polarized light. The second group is connected with photoluminescence degradation and a persistent optically induced polarization anisotropy at helium temperature. These effects arise from and are very well described by Auger autoionization of crystals selectively excited by polarized light, and subsequent Auger quenching of all radiative recombination in them since they contain long-lived charged carriers. Upon heating the samples to room temperature the electron returns back to the nanocrystal. This restores the initial photoluminescence intensity and washes out the long-lived optically induced polarization anisotropy. The high efficiency of all these effects is provided by the large ratio of the rate of Auger processes to the radiative recombination rate in the nanosize Si crystals.