Inverse Bloch oscillator: Strong terahertz photocurrent resonances at the Bloch frequency

We have observed the inverse Bloch oscillator effect: resonant changes in the current-voltage characteristics of miniband semiconductor superlattices when the Bloch frequency is resonant with a terahertz field and its harmonics. The resonances consist of a peak in the current accompanied by a decrease of the current at the low bias side. At the highest intensities we observe up to a four-photon resonance. This is an analog of Shapiro steps in a S-I-S junction caused by the ac Josephson effect. The increase of the current is caused by stimulated emission of photons and we can estimate the THz gain of the superlattice from the induced current at the resonance.