Observation of shapiro steps and direct evidence of bloch oscillations in semiconductor suer;attoces

摘要

We have observed resonant changes in the currentpvoltage characteristics of miniband semcionductor superlattices when the Bloch frequency is resonant with a terahertz fild and its harmonics: the inverse Block osillator effect. The resonant feature consists of a peak in the current wich grown with increasing laser intensity acompanined by a decrease of the current at the low bias side. When the intensity is increased further the first peak starts to decrease and a second peak at about twice the voltae of the first peak is observed due to a two-photon resonance. At the highest intensities we observe up to a four-photon resonance. This is an analogy of Shapiro steps in a S-I-S juction subject to the AC Josephson effect. The increase of the current is casued by stimulated emission of photons and from the value of the induced current we can estimate the THz gain of the superlattice.