Population inversion

Population inversion (Latin inversio, reversing ') is a term from physics of systems (such as atoms), only certain states can take with discrete energies, as described by quantum mechanics. Population inversion is when more particles in a higher energy state E2 are as in the lower energy state E1. This is in thermal equilibrium by the Boltzmann distribution is not possible if a uniform temperature is provided. There is valid according to the Boltzmann equation:

Here, the density of the particles in the lower state, the density of the particles in the upper state, the statistical weights of the states and their energy. is the Boltzmann constant. Since the energy gap between the two levels is always greater than 0, the exponential function, therefore, can never be greater than 1. In the natural equilibrium, the population inversion is thus impossible:

A population inversion is present if and only if:. Each system seeks to maximize its entropy, ie to minimize its free energy. The population inversion is a deviation from local thermodynamic equilibrium and thus is not stable. It can therefore only with constant energy supply, known as pumps, are artificially created and maintained in non-equilibrium systems. The pump must be selective, that is, it may be supplied to only specific particle energy. So that it can be achieved that more reception levels are selected as would be the case in the natural balance. A common type of pumping is optical pumping, with flash lamps or other laser radiation can be used. The radiation of the pump source must be more energetic than the later emitted by the so pumped laser light.

The energy of the photon is proportional to its frequency, and the Planck's constant:

Population inversion in the optical pumping is achieved when the photon energy of the pump source and the energy difference between the bottom and correspond to a higher excited electronic state of the particle.

Some other form of selective excitation of the impact with another excited particles (B), the de-excitation can replace by the difference in energy in order to bring the first particles (A ) to the higher excited state. To bring the particles of type B to Stoßabregung back into the excited state, they are energy, for example by electron impact, fed (see He-Ne laser). The energy can be introduced into the medium in the form of an electrical discharge (such as corona discharge, hollow cathode, microwaves ).

If the excitation source (eg, optical pumping, gas discharge ) is switched off, the thermal occupation of the inverted electronic state by emission and collisions with other atoms or molecules is reduced. The local thermal equilibrium is reached when excited electronic states, degree of ionization, and the kinetic energy of motion of the atoms / molecules are redistributed according to the Boltzmann's statistics. Depending on the lifetime of the states and the particle density in the system, the process for some time (~ ms) can avail.

Laser

A laser is a device out to produce a beam of light whose photons by the same frequency, phase (collectively, coherence) and polarization distinguished. The useful radiation is coupled out of the radiation field of the resonator, for example, by partially transparent mirrors.

A necessary but not alone sufficient, condition for the operation of a laser is the gain of a beam by stimulated emission. To do this in the simplest case (3- level laser ) population inversion between the ground state and the laser level prevail. The picture shows a 4 - level laser, which works basically the same, but an additional level above (namely ) which in turn empties into the ground state quickly. In the 4- level laser is, therefore, to produce a population inversion easier, since it is virtually empty.

The population inversion can be achieved stationary only if both the state rapidly relaxes ( empties, happens in the ms range ), and, if present, has a short life span, or the excitation of fast enough. The laser-active level has, however, a large lifetime ( ms) have, as it will otherwise rapidly depopulated itself by spontaneous emission and a thermal equilibrium is established according to the Boltzmann distribution.

The detailed breakdown of the balances of individual radiation processes is as follows:

( spontaneous emission (low) stimulated emission = absorption)

A21: Einstein coefficient for spontaneous emission

B12: Einstein coefficient for absorption

B21: Einstein coefficient for stimulated emission

Uν: energy density of the radiation field

The Einstein coefficients represent transition probabilities between levels dar. The coefficient for stimulated emission is related to the absorption for related: B21 = ( g1/g2 ) B12.

The detailed balance applies in the non-equilibrium state only microscopically; the radiation density increases exponentially over the path length inside the cavity. In a laser, the laser radiation wavelength is optically amplified, while other wavelengths are suppressed for several reasons. These include on the one hand, the gain characteristic of the active laser medium ( gain only certain wavelength bands ), as well as the laser condition ( due to the formation of sharp wavelengths Resonatorabmessungen ).