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 of semiconductors that together form quantum wells. In these wells, electrons can be excited by parcels of ultrasound of millielectronvolts of energy. This amount of energy is equivalent to a frequency of 0.1 to 1 THz. Just as light is a wave motion that is considered as composed of particles called photons, we can think of the normal modes of vibration in a solid as being particle-like. The quantum of lattice vibration is called phonon. In lattice dynamics we want to find the normal modes of vibration of a crystal. In other words, we need to calculate the energies (or