COHERENT TRANSIENT CYCLOTRON EMISSION FROM PHOTOEXCITED GAAS

Femtosecond photoexcitation near, but above, the band gap in GaAs epit axial layers and doped heterostructures in magnetic fields is shown to produce transient coherent cyclotron emission, lasting for several pi coseconds at low temperatures. This emission reflects the relatively l ong phase relaxation of coherent electronic excitations associated wit h free electron inter-Landau-level transitions. In a doped, wide parab olic quantum well, the photoexcited carriers drive the magnetoplasmon mode of the equilibrium carriers, producing a dramatic resonance as th e cyclotron frequency crosses the magnetoplasmon frequency.