Negative high-harmonic dynamic conductivity in a superlattice

In this paper, we discuss high-frequency generation in a semiconductor superlattice (SL). In particular, we have studied the diffusive regime where Bloch oscillations and electrical domains do not develop and generation may occur as a result of the negative high-harmonic non-linear conductivity. We underline the difference between this regime and frequency conversion in a stable non-linear medium: for high-harmonic instability to occur, thresholds in ac and dc driving forces must be reached. We point to AlGaN/GaN as a possible material system for high-power SL terahertz sources.