Density-dependent terahertz emission in biased semiconductor superlattices: From Bloch oscillations to plasma oscillations

We present theoretical and experimental results for the density dependence of terahertz emission from undoped biased semiconductor superlattices excited via an ultrashort optical pulse. We show that exciton-exciton interactions lead to a density-dependent terahertz frequency. At low to moderate densities, the carriers undergo Bloch oscillations, with the oscillation frequency initially redshifting with increasing density. At higher densities, the Bloch oscillations transform into plasma oscillations and the oscillation frequency blueshifts with increasing density. We model the system using our excitonic Bloch equations, solved nonperturbatively in the optical field by employing a factorization scheme that allows for the inclusion of dynamic screening. We show that the observed density dependence on the terahertz frequency cannot be obtained using a method that is perturbative in the optical field.