Stimulated emission energy of an absorbed photon causes an identical but opposite atomic transition: from the lower level to a higher energy level. In normal media at thermal equilibrium, absorption exceeds stimulated emission because there are more electrons in the lower energy states than in the higher energy states. However, when a population inversion is present, the rate of stimulated emission exceeds that of absorption, and a net optical amplification can be achieved. Such a gain medium, along with an optical resonator, is at the heart of a laser or maser. Lacking a feedback mechanism, laser amplifiers and superluminescent sources also function
Sound amplification by stimulated emission of radiation a p-n transition. It has been mentioned that a quantum well is basically a potential well that confines particles to move in two dimensions instead of three, forcing them to occupy a planar region. In coupled quantum wells there are two possible ways for electrons and holes to be bound into an exciton: indirect exciton and direct exciton. In indirect exciton, electrons and holes are in different quantum wells, in contrast with direct exciton where electrons and holes are located in the same well. In a case where the quantum wells are identical, both levels have a two-fold degeneracy. Direct