Coupling of electromagnetic waves and Bloch oscillations in quantum superlattice

In this report we analyze, for the first time to our knowledge, the linear coupling of the Bloch oscillations and transversal electromagnetic waves in a quantum semiconductor superlattice (QSSL) towards the problem of realization of the tunable THz source. The analysis is implemented by means of wave equation for the electromagnetic field and the material equations with quasi-classic description of the electron transport in a biased QSSL. In the case when the Bloch frequency is greater than plasma frequency at the bottom of the lowest miniband of QSSL, the coupling leads to the reconnection of the dispersion curves at the region of their crossing, forming a slit between always stable high-frequency branch and lower frequency branch which has the region of an instability due to electron bunching in the momentum space. The last circumstance opens the great possibility to generate THz radiation in QSSL superimposed with an inhomogeneous dc field that is provided by the presence of the turning points for the electromagnetic waves. Such turning points play the role of the mirrors making up a resonator for the unstable waves. For the typical GaAs/GaAlAs QSSL with miniband electron density 10¹⁷ cm^-3 and superlattice period 5 nm the critical strength of applied dc electric field which leads to spectrum splitting is about 9 kV/cm.