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Optical quality porous silicon (PSi) and nanoparticle crystalline-Si (c-Si) enriched SiO2 materials were studied by absorption; luminescence; time-resolved emission measurements. Using tunable ultrafast laser as a light source, the excited state dynamics was also investigated. In a nonlinear optical (NLO) spectroscopic study the pulsed degenerate-four-wave-mixing (DFWM) technique was employed. In absorption measurement all samples show a broad and intense UV absorption with a cut-off edge at ~400 nm. In photoluminescence (PL) measurement, red emission can generally be observed in both Psi and c-Si samples. At 8 K the broadband PL of PSi is peaked at ~670 nm, with the PL lifetime in microseconds regime. The band maximum shifts toward low energy side while excitation wavelength increases. The particle size distribution was estimated using the optical transmission data according to the size- energy gap relationship. DFWM measurement reveals a long lived, slowly decaying signal which emerges from the coherent response. It indicates that the excitation was highly localized. For c-Si sample, the luminescence maximum was found at higher energy side around 590 nm, but the emission lifetime is much shortened to as ~10ns. In order to understand the nature of nanocomposite, Eu dopant was introduced into the sample. The investigation of luminescence and time-resolved emission of Eu3+ shows that the c-Si nanoparticles are distributed in SiO2 matrix with an average size of the nanoparticles being around 2.2 nm. The valence band (VB) to conduction band (CB) energy gap is about 2.7 ev. By ultrashort (femtoseconds and picoseconds) laser pulse excitation the charge carriers produced in CB of c-Si nanoparticles contributes to the observed optical responses. The excited state dynamical process associated with the movement of charge carriers is characterized by an instantaneous response signal, followed by buildup of the slowly decaying signal within 500ps. The analysis of the charge creation, trapping and reactivation is discussed.
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