Beyond the ordinary acoustoelectric effect: superluminal phenomena in the acoustic realm and phonon-mediated Bloch gain

It has been shown that coherent phonons can be used as a potent tool for controlling and enhancing optoelectronic and transport properties of nanostructured materials. Recent studies revealed that interaction of acoustic phonons and fast-moving carriers in semiconductor heterostructures can be accompanied by electron-phonon instabilities that cause ordinary and induced Cherenkov effects. However, the development of such instabilities is still poorly understood. Our study shows that other supersonic phenomena, beyond the Cherenkov instability, are possible for nonequilibrium charge transport in the miniband semiconductor superlattices (SLs) driven by an acoustic plane wave. Using semiclassical nonperturbative methods and elements of the bifurcation theory, we find the conditions for the onset of dynamical instabilities (bifurcations) which are caused by the emission of specific SL phonons by supersonic electrons, and their back action on the electrons. Notably, the underlying radiation mechanism is connected either to normal or anomalous Doppler effects in full accordance with the Ginzburg-Frank-Tamm theory. The appearance of induced Doppler effects is also discussed in relation to the formation of electron bunches propagating through the spatially periodic structure of the SL. When the amplitude of the acoustic wave exceeds a certain threshold, the dynamical instabilities developed in the system are manifested as drift velocity reversals, resonances in sound attenuation, and absolute negative mobility. We demonstrate that the discovered superluminal Doppler phenomena can be utilized for tunable broadband amplification and generation of GHz-THz electromagnetic waves, which creates a ground for the development of novel phononic devices.