Far-infrared picosecond time-resolved measurement of the free-induction decay in GaAs:Si

Abstract
By measuring changes in the photoconductivity induced by picosecond far-infrared pulse pairs from a free-electron laser, we have time resolved the free-induction decay of the 1s-210 and 1s-2p+ Si-shallow-donor transitions in bulk GaAs. The method frees us from the problem of measuring the optical emission of the transitions and allows us to obtain their dephasing times. We expect to be able to use the same method in the future to measure other coherent phenomena in these systems, such as photon echoes.