Photon echoes from band-to-band transitions in semiconductors excited with 10-fs pulses

Summary form only given. Even one decade after the pioneering photon echo experiments by Shank et al. (1988,1991) in which GaAs was investigated at room temperature using /spl ap/10 fs optical pulses, not a single experiment has actually shown directly that the coherent real-time response of semiconductor band-to-band transitions under these conditions is a photon echo indeed. Recent experiments with /spl ap/10 fs pulses exciting GaAs at low temperatures have explicitly shown that the response is not a photon echo for low excitation densities (/spl ap/10/sup 15/ cm/sup -3/). From these experiments it has also become clear that the coherent response after excitation with 10 fs pulses is distinctly different from that after excitation with 100 fs pulses. We investigate the coherent real-time response at elevated carrier densities using transient four-wave mixing and an up-conversion scheme with a time resolution of 5 fs. We focus on approximately resonant excitation (50 meV above gap) of a 0.6 /spl mu/m thin film of bulk GaAs at T=300 K.