Mechanism of THz Emission from Asymmetric Double Quantum Wells

Impulsive interband optical excitation of the lowest two conduction subbands of a suitably engineered $\mathrm{GaAs}/\mathrm{AlGaAs}$ double quantum well can lead to coherent 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 the continuum beats persists for several picoseconds even for excitation an optical phonon energy above the band edge. We attribute this to the small energy difference between the component eigenstates, which substantially reduces the number of relevant scattering events.