Picosecond absorption imaging of photoexcited carriers in GaAs quantum wells

The lateral motion of photoexcited carriers has been studied in GaAs/Al0.3 Ga0.7 As quantum wells with 2-μm and 10-ps resolution at ambient temperatures ranging from 2 to 40 K. The experiments monitor the transmission of a weak probe beam tuned to the exciton absorption resonance. The results indicate that the motion of the photoexcited carriers is coupled to the transport of these excess acoustic phonons. The excess thermal energy diffuses slowly and does not influence the carrier transport, leading to a generally Gaussian carrier distribution. At lower temperatures, the phonons propagate ballistically, producing a wind that drives the carriers out of the excitation region. In this case, a shell of carriers is formed around a depleted central region. At sufficient pump powers, a central hot spot with a local temperature above 20 K is formed. The phonon motion is diffusive inside the hot spot, and the absence of the phonon wind leads to a high carrier density. Carriers outside the hot spot are still driven to a surrounding shell.