Ultrafast electronic relaxation processes in porous silicon

Abstract

We have studied ultrafast relaxation processes in porous silicon using a femtosecond pump and probe pulse-correlation technique at 440-nm wavelength. We have observed photoinduced absorption with a response time of the order of 5 ps. In addition to this fast relaxation process, we have also observed a strong induced absorption change on millisecond timescales. From the observation of the picosecond response, the relevant carrier dynamics is clarified as follows: carriers are excited in Si microcrystals and then rapidly thermalize to the surface states within 5 ps. The observation of the strong absorption change on millisecond timescales suggests the possibility of fabricating optical logic gates or all-optical modulators using porous silicon.