We present detailed studies of the initial relaxation processes of photoexcited carriers in hydrogenated amorphous silicon. We have carried out time-resolved measurements of the photoexcited carrier response in HWCVD a-Si:H thin films using a wavelength-resolved femtosecond pump-probe technique, in which an intense 35-fs pump pulse excites carriers in the sample and a time-delayed probe pulse measures the resulting change in optical properties as a function of time delay following the pump pulse. Measurements of the transient optical absorbance were carried out as a function of the density of excited carriers, sample temperature, and probe wavelength. These studies indicate fast carrier thermalization via phonon emission on a ∼ 150 fs time scale and rapid phonon equilibration on a ∼ 230 fs time scale.

The high-order harmonic generation from the tritium molecular ions in the coherent state has been numerically simulated by solving the three-state time-dependent Schrödinger equation based on the non-Born-Oppenheimer approximation. Two resonant pump pulses prepare the system in the coherent superposition of the 2pσu and the 3dσg states. A long-wavelength probe pulse is selected to generate the intense harmonics. By increasing the population of the 3dσg state, the dependence of the harmonic spectra on pump-probe delay time tends to exhibit obvious redshift. In this work, not only is the physical image of harmonic emission from coherent wave packets established, but also a new physical mechanism of the redshift in long-wavelength region is revealed. Moreover, we present a scheme of manipulating the redshift to further verify the underlying mechanism.