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

Description

Figure: A simplified view of how strong sound waves affect electrons in an artificial semiconductor superlattice. At low sound wave amplitudes (U), electrons follow the wave, but as U increases, they begin to nonlinearly oscillate and eventually move opposite to the wave. This surprising behavior, involving the exchange of vibrational energy (phonons), could pave the way for new methods to control and amplify electronic signals. 

The manipulation of phonons as coherent waves in solids is of fundamental interest and is also exploited in established applications including acoustic microscopy, sonar devices or emerging technological trends such as phonon lasers, also called “sasers”. These technologies have known counterparts and underlying concepts in optics domain. It is thus intriguing to wonder whether super-luminal (faster than light) phenomena such as Cherenkov and Doppler effects [1] can be manifested in condensed matter systems via interactions between electrons and phonons. Here we reveal super-sound phenomena, acting as super-luminal, which are involved in a series of novel acousto-electric effects. These effects are arising in a semiconductor superlattice (SL) irradiated by a sound wave.

For a sufficient large wave amplitude, normal backward and anomalous Doppler effects emerge resulting in the absorption and emission of phonons by fast moving electrons which begin to decelerate and eventually counter-propagate with respect to the propagation of the acoustic wave. Our calculations evidently show that the anomalous Doppler effect can be exploited for the amplification of THz radiation similar to Bloch gain. The anomalous Doppler effect can be exploited for Bloch lasing due to the unusual action of the standard Bohr-Einstein rule of quantum transitions in this situation: transition from lower to the upper quantum levels results in emission of quantum (instead of its absorption). Hereafter, utilization of this effect results first to back drift of electrons manifested by absolute negative mobility and as consequence negative differential conductivity which provides typical broadband Bloch gain properties.

Overall, the SLs could serve as a new playground for investigations of various superluminal effects whereas the use of superluminal lasing in the acoustic realm opens new opportunities for a development of novel phononic devices and fine manipulation of electromagnetic waves.

[1] Tamm, I. E. "General Characteristics of Vavilov-Cherenkov Radiation: The theory of radiation from systems moving with superlight velocities has uses in plasma physics." Science 131.3395 (1960): 206-210.