Nature of Coherent Four-Wave Mixing Beats in Semiconductors

Coherent quantum beat spectroscopy has recently gained importance for the investigation of semiconducting materials and structures. The variety of excitonic resonances in bulk and low-dimensional semiconductors, and the advent of ultrafast lasers with pulse lengths shorter than typical dephasing times of these excitons, have made it possible to observe quantum beats between close-lying excitonic transitions.1,2,3 In particular, the beat phenomena have been observed in four-wave mixing (FWM) experiments on exciton complexes in GaAs multiple quantum wells,4,5,6,7,8 and in bulk semiconductors.3