Enhancement of two-photon absorption in anisotropic mesoporous silicon

In experiments on nonlinear-optical transmission of picosecond laser pulses at the wavelength of 1.064 mu m three-order-of-magnitude enhancement of the photoinduced absorption in optically anisotropic mesoporous silicon films compared to crystalline silicon (c-Si) was found. The effect is not sensitive to the polarization of the laser radiation and it saturates at laser peak intensities about 5 MW/cm(2). Higher laser intensity results in the polarization-sensitive photoinduced absorption, which is merely one-order-of-magnitude more effective than in c-Si. These efficient nonlinear-optical responses can be attributed to the resonant excitation of the defect states in the direct gap of silicon and local-field enhancement in the mesoporous films. [GRAPHICS] Total transmittance of the birefringent mesoporous silicon film versus the laser intensity for the sample of 49% porosity. The crystallographic direction [001] corresponds to the optical axis