Stimulated emission times this net transition rate. In order for this to be a positive number, indicating net stimulated emission, there must be more atoms in the excited state than in the lower level: formula_6. Otherwise there is net absorption and the power of the wave is reduced during passage through the medium. The special condition formula_7 is known as a population inversion, a rather unusual condition that must be effected in the gain medium of a laser. The notable characteristic of stimulated emission compared to everyday light sources (which depend on spontaneous emission) is that the emitted photons have the same
Sound amplification by stimulated emission of radiation semiconductor heterostructure where the tuning into resonance of one-phonon transition of electron - hole recombination can be carried out by external pressure, magnetic or electric fields. The second scheme uses one-phonon transition between direct and indirect exciton levels in coupled quantum wells. We note that an exciton is an electrically neutral quasiparticle that describes an elementary excitation of condensed matter. It can transport energy without transporting net electric charge. The tuning into the resonance of this transition can be accomplished by engineering of dispersion of indirect exciton by external in-plane magnetic and normal electric fields. The magnitude of phonon wave