A SOURCE OF TERAHERTZ COHERENT PHONONS

Resonant tunnelling in a double-barrier heterostructure (DBHS) designe d in order to produce an intense coherent emission of phonons is studi ed theoretically. This emission is achieved when the energy difference between the first excited level and the ground state in the well is c lose to the LO-ehonon energy. The LO phonons emitted during the electr on transition decay into a pair of ((LO) over tilde and TA) phonons th at leave the well in the [111] direction. These secondary TA phonons a re coherent and form a beam that we call a SASER (by analogy with 'las er'). The circulating current and the populations of phonons and elect rons in the well are calculated by solving appropriate kinetic equatio ns. Some possible applications of this system are discussed.