Population inversion field, to use quantum coherence effects to reduce the likelihood of an excited-state to ground-state transition. This technique, known as lasing without inversion, allows optical amplification to take place without producing a population inversion between the two states. Stimulated emission was first observed in the microwave region of the electromagnetic spectrum, giving rise to the acronym MASER for Microwave Amplification by Stimulated Emission of Radiation. In the microwave region, the Boltzmann distribution of molecules among energy states is such that, at room temperature all states are populated almost equally. To create a population inversion under these conditions, it is necessary
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