FEMTOSECOND SPECTROSCOPY OF HOT CARRIER RELAXATION IN BULK SEMICONDUCTORS

Time resolved optical techniques have been proven as powerful tools to investigate hot carrier relaxation channels in semiconductors on a subpicosecond time scale. Their improvements in time resolution by the use of femtosecond laser pulses opened the way to quantitative details concerning the initial steps in the chain of relaxation processes. In this paper, the most recent experimental results obtained from time resolved spectroscopy with femtosecond laser pulses are reviewed. Special attention is paid to the internal thermalization of optically generated electron-hole pairs, their ultrafast separation in surface space charge fields and to specific intercarrier interactions during cooling via phonon emission. Ultrafast hot carrier transport in surface space charge fields results in the synchronous excitation of coherent LO-phonons. The nature of the driving force for these coherent vibrations is explored.