What are the difference between spontaneous & stimulated emissions?

Spontaneous emission: electron drops from an excited state to a lower state (no outside mechanism) - emitting a photon. Stimulated emission (lasers): photon of the same frequency interacts with electron in excited state which drops to lower state - the emitted photon is coherent with the incoming photon . Detailed description: Spontaneous emissionis the process by which a quantum system such as an atom, molecule, nanocrystal or nucleus in an excited state undergoes a transition to a state with a lower energy (e.g., the ground state) and emits quanta of energy. Light or luminescence from an atom is a fundamental process that plays an essential role in many phenomena in nature and forms the basis of many applications, such as fluorescent tubes, older television screens (cathode ray tubes), plasma display panels, lasers, and light emitting diodes. Lasers start by spontaneous emission, and then normal continuous operation works by stimulated emission. Stimulated emissionis the process by which an atomic electron (or an excited molecular state) interacting with an electromagnetic wave of a certain frequency may drop to a lower energy level, transferring its energy to that field. A new photon created in this manner has the same phase, frequency, polarization, and direction of travel as the photons of the incident wave. This is in contrast to spontaneous emission which occurs without regard to the ambient electromagnetic field. However, the process is identical in form to atomic absorption in which the 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 on the basis of stimulated emission. Stimulated emission was a theoretical discovery by Einstein within the framework of the old quantum theory, wherein the emission is described in terms of photons that are the quanta of the EM field. Stimulated emission can also be described classically, however, without reference to either photons, or the quantum-mechanics of matter read less