Observation of picosecond nonlinear optical response from porous silicon

Abstract Optical quality, free standing and highly photoluminescent PSi was prepared from both, p- and n-type Si wafers using electrochemical anodization of c-Si. A broad and intense absorption band is in the UV–Visible region with a cut-off edge at ∼400 nm. A strong emission was found in the red region. At 8 K the broadband luminescence is peaked at ∼670 nm. The band maximum shifts toward low-energy side while excitation wavelength increases. The size distribution of the nanoparticles was obtained from the optical transmission data and the theoretical relation between the size of the particles and the energy gap. Nonlinear optical response measurement was also performed using degenerate four-wave-mixing (DFWM) in a backward configuration with all waves in s-polarization. The response signal consists of an instantaneous component followed by a long lived, slowly decaying component. The former is associated with the third-order susceptibility of the material whereas the latter originates from the contribution of surface states created by laser excitation. The response due to the surface state is significant, which has potential in practical applications.