Infrared transient grating measurements on the Ge-H stretch mode at 1880 cm −1 in hydrogenated amorphous germanium show a long, multiexponential relaxation time having a mean decay constant of 306 ps. The thermal activation of the decay rate suggests a decay via two Ge-H bending modes with the energy mismatch bridged by three bulk Ge-Ge vibrations. From the probe pulse diffraction efficiency, we have determined a value for the nonlinear refractive index of n 2 = 5.9ϫ 10 −4 cm 2 / GW. Utilizing the photon echo technique, we determine that the dominant contribution to a vibrational dephasing near room temperature is the elastic scattering of TO modes near 270 cm −1 . At 10 K additional dephasing from nonequilibrium phonons are readily observable as an excitation dependent contribution.
We report on picosecond, time-resolved measurements of the vibrational relaxation and decay pathways of the Si-H and Ge-H stretching modes in hydrogenated amorphous silicon-germanium thin films ͑a-SiGe: H͒. It is demonstrated that the decay of both modes has a nonexponential shape, attributable to the local environment of the Si-H and Ge-H bonds. Temperature dependent measurements of the ensemble averaged population decay time ͗T 1 ͘ are used to demonstrate that the stretch modes relax to Si͑Ge͒-H bending modes and that the excess energy is dissipated into a combination of bulk vibrations. The influence of the mixed character Si-Ge bulk vibrations upon the relaxation dynamics is discussed.
We have used a free electron laser to measure the picosecond vibrational dynamics of the SiH, SiH 2 , and O 3 SiH stretching modes in porous silicon. A three beam pump-probe technique has been employed to make temperature dependent measurements of the population relaxation times. We demonstrate that both bending modes and scissors modes play important roles as does the vibrational bath provided by the pore walls themselves. Using a forward box, two beam photon echo technique we have measured the homogeneous dephasing times of all modes which have dynamic linewidths in the range 0.6-1.2 cm −1 . The inferred pure dephasing rates are dominated by the elastic scattering of acousticlike Si-Si vibrations.
Using the Dutch free electron laser FELIX, we have investigated vibrational relaxation in free standing porous silicon (p-Si) films. Pump-probe measurements resonant with the SiH, SiH 2 and O 3 SiH stretching modes yield temperature dependent measurements of the decay rates which demonstrate that all the modes decay via at least one internal defect mode with the excess vibrational energy distributed among the Si-Si bath phonons in a fourth order decay process.
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