Optics include illumination engineering, photonics, and optoelectronics with practical applications like lens design, fabrication and testing of optical components, and image processing. Some of these fields overlap, with nebulous boundaries between the subjects terms that mean slightly different things in different parts of the world and in different areas of industry. A professional community of researchers in nonlinear optics has developed in the last several decades due to advances in laser technology. A laser is a device that emits light (electromagnetic radiation) through a process called "stimulated emission". The term "laser" is an acronym for "Light Amplification by Stimulated Emission of
Sound amplification by stimulated emission of radiation one quantum well to another neighbouring well it produces a phonon. External energy pumping (e.g. a light beam or voltage) can help to the excitation of an electron. Relaxation of an electron from one of the upper states may occur by emission of either a photon or a phonon. This is determined by the density of states of phonons and photons. Density of states is the number of states per volume unit in an interval of energy (E, E+dE) that are available to be occupied by electrons. Both phonons and photons are bosons and thus, they obey Bose–Einstein statistics. This