Dispersive terahertz gain of non-classical oscillator: Bloch oscillation in semiconductor superlattices

This study demonstrates that Bloch oscillators have an essentially different nature from that of semi-classical charged harmonic oscillators and that it is indeed possible to extract a net gain from Bloch oscillations in semiconductor superlattices. By free-space THz electro-optic sampling method, this work has directly determined the spectral shape of the complex conductivities of Bloch oscillating electrons and has confirmed that the real part of the dynamical conductivity goes negative below the Bloch frequency