Observation of picosecond nonlinear optical response from porous silicon

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 similar to 400 nm. A strong emission was found in the red r egion. At 8 K the broadband luminescence is peaked at similar to 670 nm. Th e band maximum shifts toward low-energy side while excitation wavelength in creases. The size distribution of the nanoparticles was obtained from the o ptical transmission data and the theoretical relation between the size of t he particles and the energy gap. Nonlinear optical response measurement was also performed using degenerate four-wave-mixing (DFWM) in a backward conf iguration with all waves in s-polarization. The response signal consists of an instantaneous component followed by a long lived, slowly decaying compo nent. The former is associated with the third-order susceptibility of the m aterial whereas the latter originates from the contribution of surface stat es created by laser excitation. The response due to the surface state is si gnificant, which has potential in practical applications. (C) 1999 Elsevier Science B.V. All rights reserved.