Tunable Coherent THz Radiation Pulses From Optically Excited Bloch Oscillations

Abstract

Quantum—mechanical tunneling is attractive for the realization of THz radiation sources both in superconducting and semiconducting structures. This paper deals with the phenomenon of coherent tunneling of electrons in semiconductor superlattice structures. Charge carrier tunneling through any potential barrier is called “coherent” if three conditions are fulfilled: (i) states of the same energy are involved on both sides of the barrier, (ii) the wave functions of the charge carriers populating the states have a fixed phase relationship, and (iii) the dynamics of the tunneling process is governed by a time-dependent phase difference between the wave functions. The concept of coherent tunneling is better known in the field of superconductivity where it has long been applied in Josephson junctions consisting of two superconductors separated by a thin insulating layer [1]. Application of a dc bias voltage induces a phase oscillation with voltage-dependent frequency between the ensembles of coherent Cooper pairs on both sides of the barrier. The periodic phase change results in a periodic motion of charge through the barrier which is the source for tunable electromagnetic radiation.