Examining the terahertz signal from a photoexcited biased semiconductor superlattice for evidence of gain

Abstract

We present a careful analysis of the coherent terahertz emission from an undoped biased semiconductor superlattice excited via an ultrashort optical pulse. We use both a semiclassical model and a fully quantum-mechanical model that includes the excitation process and excitonic effects to analyze emission. We conclude that, in contrast to what has recently been claimed, it is not possible to deduce whether there is terahertz gain from the analysis of the emitted coherent terahertz pulse.