Infrared transient grating and photon echo spectroscopy of oxygen vibrational modes in amorphous silicon thin films

We report on picosecond coherent transient spectroscopy on the asymmetric stretching mode in a-SiO_0.1 thin films using the Dutch free-electron laser FELIX. A fast 10-K lifetime of 2.8 ps is obtained using laser-induced transient grating spectroscopy. Its thermal behavior suggests relaxation into two accepting modes possibly via the symmetric stretch mode of the Si-O-Si complex. Two-pulse photon echo measurements reveal phase dynamics with both an excitation density and temperature dependence, suggesting the presence of nonequilibrium Si-Si phonons. The temperature-dependent component of the pure dephasing may be attributable to two-phonon elastic scattering. From the probe pulse diffraction efficiency, a value of the nonlinear refractive index has been determined to be n₂ = 3.73 × 10^-3 cm ²/GW.