Ultrafast carrier relaxation in hydrogenated amorphous silicon

The techniques of femtosecond laser spectroscopy have been applied to study the dynamics of carriers in the neighborhood of the mobility edge in hydrogenated amorphous silicon. Just after injection of a dense electron–hole plasma with pump photons above the mobility edge, the index of refraction decreases by as much as 2%, and the induced absorption is as large as 104 cm−1. Ten picoseconds later, intraband absorption becomes negligible and the temperature rise dominates the optical properties through the band-gap shrinkage, which produces an increase in the index of refraction and an increase in the interband absorption. The recovery takes place with two characteristic times, ~1 psec and ~10 psec. Possible interpretations of these times are presented.