Theory of excitonic high-order sideband generation in semiconductors under a strong terahertz field

Abstract

We present the theory of excitonic high-order sideband generation (HSG) in semiconductors by an intense terahertz (THz) field. When the Coulomb interaction is neglected, we give an analytical solution to the HSG with the help of Floquet theory. Besides, the HSG is also studied by the quantum trajectory theory (saddle-point method) by which the HSG is explained by the interference of different quantum trajectories of excitons when accelerating in the external THz field. Both the exact analytic solution and the saddle-point method obtain consistent results: the spectrum of sidebands has an extended plateau where all the sidebands have almost the same intensity, which is similar to the high-order harmonic generation in atomic system. Moreover the HSG provides more flexibility in studying the quantum trajectory theory. When Coulomb interaction is considered, we find considerable Coulomb enhancement of HSG, which is absent in atomic system. The mechanism is discussed based on numerical calculations.