Two-photon absorption in Si-nanocrystals deposited by plasma-enhanced chemical-vapor deposition

We present a systematic z-scan study of the nonlinear absorption coefficient β of Si-nanocrystals embedded in SiO2 (Si-nc/SiO2) excited in the nanosecond regime. Two different wavelengths of a Nd:YAG laser have been used, λ=1064 and 532 nm. The samples under study were deposited on silica substrates by plasma-enhanced chemical-vapor deposition technique and subsequently annealed up to 1250∘C. We have observed an increase in β as the Si content in the Si-nc/SiO2 films rises, indicating that the nonlinear behavior strongly depends on the amount of Si. Typical values of β around 1 cm/MW were obtained at 1064 nm, while they are one order of magnitude higher exciting at 532 nm, following the same trend as bulk Si with respect to the excitation energy. Large Si-nc present an energy of high-symmetry transitions (Eog, E2) that approaches the system to a resonant condition with the excitation energy. On the other hand, we observed an increase of β with the annealing temperature that can be associated to an additional absorption from free carriers, as the crystalline degree scales with the thermal budget.