Transition from classical to quantum response in semiconductor superlattices at THz frequencies

The response of a sequential resonant tunneling superlattice to intense THz radiation shows a transition from classical rectification at frequencies below 600 GHz to quantum response above 1 THz. In the quantum regime, the dc current-voltage characteristics show distinct peaks due to absorption and stimulated emission of up to three THz photons. For sufficiently high ac field strengths the photon-assisted channels dominate the transport, leading to absolute negative conductance near zero bias, and gain just below the Stark splitting of the ground states in adjacent quantum wells. Quantitative agreement with these observations is obtained by invoking photon-assisted tunneling following Tucker, but with an instantaneous $I-V$ free of domain formation.