Mechanism of THz emission from asymmetric double quantum wells

Impulsive interband optical excitation of the lowest two conduction subband s of a suitably engineered GaAs/AlGaAs double quantum well can lead to cohe rent THz emission. We demonstrate that, contrary to previous expectations, the dominant emission mechanism can involve the beating of either continuum or exciton states, depending on excitation conditions. The coherence of th e continuum beats persists for several picoseconds even for excitation an o ptical phonon energy above the band edge. We attribute this to the small en ergy difference between the component eigenstates, which substantially redu ces the number of relevant scattering events.