Laser diffraction limit. All such devices are classified as "lasers" based on their method of producing light, i.e., stimulated emission. Lasers are employed where light of the required spatial or temporal coherence can not be produced using simpler technologies. The word "laser" started as an acronym for "light amplification by stimulated emission of radiation". In this usage, the term "light" includes electromagnetic radiation of any frequency, not only visible light, hence the terms "infrared laser", "ultraviolet laser", "X-ray laser" and "gamma-ray laser". Because the microwave predecessor of the laser, the maser, was developed first, devices of this sort operating at microwave
Sound amplification by stimulated emission of radiation eigenstates of the Hamiltonian as well. These states are equally spaced both in energy and real space and form the so-called Wannier-Stark ladder. In the proposed scheme, an application of an electrical bias to a semiconductor superlattice is increasing the amplitude of coherent folded phonons generated by an optical pulse. This increase of the amplitude is observed for those biases in which the energy drop per period of the superlattice is greater than the phonon energy. If the superlattice is biased such that the energy drop per period of the superlattice exceeds the width of electronic minibands (Wannier-Stark regime), the