Sound amplification by stimulated emission of radiation this scheme with a rather moderate pumping rate can hold against the fact that excitons in coupled quantum wells have low dimensionality in comparison to phonons. Hence, phonons not confined in the coupled quantum well are considered. An example is longitudinal optical (LO) phonons that are in AlGaAs/GaAs heterostructure and thus, phonons presented in this proposed system are three-dimensional. Differences in dimensionalities of phonons and excitons cause upper level to transform into many states of phonon field. By applying this information to specific equations we can conclude to a desired result. There is no additional requirement for the laser pumping
Stimulated emission frequency, phase, polarization, and direction of propagation as the incident photons. The photons involved are thus mutually coherent. When a population inversion (formula_6) is present, therefore, optical amplification of incident radiation will take place. Although energy generated by stimulated emission is always at the exact frequency of the field which has stimulated it, the above rate equation refers only to excitation at the particular optical frequency formula_9 corresponding to the energy of the transition. At frequencies offset from formula_9 the strength of stimulated (or spontaneous) emission will be decreased according to the so-called line shape. Considering only homogeneous broadening affecting