Tunable solid-state terahertz-wave sources: New ideas and prospects

An idea to build a solid-state analog of vacuum travelling and backward wave tubes operating in the THz frequency range has long been discussed in the literature. Experiments directed to the realization of this idea using the radiative decay of grating-coupled two-dimensional (2D) plasmons in semiconductor heterostructures have been made since 1980, however the intensity of emitted radiation remains too low so far. A general theory presented here describes the main physical phenomena underlying the operation principle of this kind of devices, and answers the two most important practical questions: why the devices have not worked properly, and what should be done to improve their characteristics. Particular attention is given to recently proposed ideas of using the quantum-wire (or quantum-dot) gratings, instead of commonly employed metal ones. The use of the new type of gratings is shown to lead to a substantial reduction of the threshold velocity of ampliication, and to a very large enhancement (by a few orders of magnitude) of the gain. Physically, this is a consequence of the resonant interaction of plasma waves in the 2D electron layer and in the quantum-wire (quantum-dot) grating. Speciic recommendations on how to build a tunable semiconductor travelling wave tube are formulated, unsolved theoretical problems are discussed.