Emission of Terahertz Radiation

Abstract

As described by the theory set out in Chap. 1 and proven by the experiments discussed in Chap. 4, Bloch oscillations are associated with a harmonic spatial motion of carriers. It is obvious that these carriers should thus generate radiation owing to their continuous acceleration. In a semiclassical picture, the carriers perform a harmonic spatial motion, leading to dipole radiation; in a quantum electronics picture, this radiation is caused by the downward transition of carriers between states of the Wannier-Stark ladder. The radiation emitted from the carrier ensemble is coherent if the electronic coherence of the different wave packets is preserved, i.e. if the carrier ensemble performs Bloch oscillations with a common phase. Also, the radiation could be coherent if it was due to stimulated emission, i.e. in a Bloch laser.

In this chapter, we discuss experiments which have directly measured the radiation emitted from a carrier ensemble performing Bloch oscillations. As in the experiments discussed in the previous two chapters, the ensemble is coherently generated by a short laser pulse.We first briefly discuss the experimental principle and the setups used. Then, we discuss the basic observations and further experiments which have investigated the THz radiation in more detail.