Sound amplification by stimulated emission of radiation (TA) phonons is better because TA phonons have lower energy and the greater life-time than LA phonons. Therefore, their interaction with the electronic subsystem is weak. In addition, simpler quantitative evaluations require a pumping of direct exciton level performed by a laser irradiation. A further analysis of the scheme can help us to establish differential equations for direct exciton, indirect exciton and phonon modes. The solution of these equations gives that separately phonon and indirect exciton modes have no definite phase and only the sum of their phases is defined. The aim here is to check if the operation of
Sound amplification by stimulated emission of radiation dispersion law away from the radiation zone. The importance of this, lies on the fact that electric and in-plane magnetic fields normal to coupled quantum wells, can control the dispersion of indirect exciton. Normal electric field is needed for tuning the transition: direct exciton --> indirect exciton + phonon into resonance and its magnitude can form a linear function with the magnitude of in-plane magnetic field. We note that the mathematical analysis of this scheme considers of longitudinal acoustic (LA) phonons instead of transverse acoustic (TA) phonons. This aims to more simple numerical estimations. Generally, the preference in transverse acoustic