Degenerate four wave mixing spectroscopy of oxygen vibrations in amorphous silicon

We report on DFWM spectroscopy on the asymmetric stretching mode in a-SiOx with x from 0.1 to 1.11. For a-SiO0.1, a 10 K lifetime of 2.8 ps is obtained using laser-induced transient grating spectroscopy. Two-pulse photon echo measurements reveal phase dynamics having a temperature dependent component due to elastic phonon scattering and an excitation dependent component suggesting the presence of non-equilibrium Si-Si modes. At high excitation densities, the DFWM signal is dominated by a quasi-thermal grating whose population decay is extremely long lived (much greater than 1 ns). This effect is enhanced as the oxygen concentration is increased. (C) 2004 Elsevier B.V. All rights reserved.