Stimulated emission

Stimulated emission or stimulated emission is called the emission ( emission ) of a photon, if it does not occur spontaneously, but is triggered by another photon.

She is one of the prerequisites for the functioning of a laser or maser.

Expiration

Is a quantum- mechanical system, - in the simplest case an atom - energy is supplied by, for example, a photon is absorbed or atom collide with other atoms, the atom is transferred to an excited state.

Now, a photon whose energy is exactly equal to the energy difference between the current state and an energy level of lower energy, the excited atom, the atom can switch to the state of low energy, and emit the energy difference in addition to the sunken photon as another photon. However, this is usually only when the transition in question is allowed by the selection rules. In the adjacent figure, the stimulated emission by the example of a laser at the transition from is to be shown.

The newly created photon has the same energy and wavelength as the sunken photon. It moves in the same direction, has the same polarization direction and also in the same phase, so that it is less like a copy of the original photon behaves, which is called coherence.

History

The stimulated emission was postulated in 1916 by Albert Einstein in his article to the quantum theory of radiation. In his hypotheses about the exchange of energy by radiation he assumed that under the action of the radiation takes place and the reverse process of absorption. He summed up this process with the absorption collectively, the " change of state by irradiation ." A name did not exist Einstein this process. 1928 was made by Rudolf Ladenburg in gas discharges the first experimental evidence.

Although the term induced emission is still used, has established itself by taking over from the English, the term stimulated emission.

Occurrence

Stimulated emission in addition to the spontaneous emission and the absorption A of the three kinds of interactions of electromagnetic radiation with matter.

In local thermodynamic equilibrium, so in most situations that occur in nature, the stimulated emission is more improbable than the spontaneous emission and the absorption. The relationship between the probabilities of these three processes is described by the Einstein coefficients. Since the ratio of stimulated to spontaneous emission is determined by the Boltzmann distribution, it also depends on the wavelength in the field of microwave radiation by stimulated emission is much more common than in the region of visible light, or X-rays.

While the spontaneous emission happens without prior irradiation of a photon exist depending on the current state of the system two ways in which the photon can interact with this system: the system is in the lower energy state of a triggerable by the photon transition, it is absorption of the photon stimulated. Is it already at the excited state, the photon causes a stimulated emission. In local thermodynamic equilibrium, the state of low energy according to the Boltzmann distribution is more common than the excited state. However, for the operation of a laser or maser, it is provided that the stimulated emission is more common than the absorption, which is called population inversion, and a non-equilibrium state.