Strong Optical Nonlinearities and Ultrafast Carrier Dynamics in Luminescent and Nonluminescent Porous Silicon

Abstract

Since the first experimental observation of efficient visible photoluminescence (PL) from porous silicon (PS) [1] this material has attracted much interest for its possible applications in all-silicon-based optoelectronics. Until now experimental studies of PS have mainly concerned PL, yielding information on the energy structure and population dynamics of the low-energy relaxed states in the material. Time-resolved nonlinear-transmission measurements can provide complementary information to conventional PL spectroscopy, revealing the structure of the higher-lying states and fast carrier dynamics in the initial stage after photoexcitation. In the present paper we report studies of the optical nonlinearities and ultrafast carrier dynamics in free-standing porous silicon films performed by femtosecond pump-probe experiment. The large photoinduced absorption observed in the spectral range 1.2–2.65 eV is attributed to molecular like complexes (clusters) which are most likely intrinsic to PS layers. Comparison of the transient absorption in samples of luminescent and nonluminescent PS shows that PL and photoinduced absorption originate from different species inside PS layers.