Optics 101

Metastable States - Definition and Applications in Optics

Metastable state is an excited state of an atom or other system with a longer lifetime than the other excited states. However, it has a shorter lifetime than the stable ground state. Atoms in the metastable state remain excited for a considerable time in the order of 10-6 to 10-3. During metastable state, all the parameters associated with state hold stationary values. A large number of excited atoms are accumulated in the metastable state.

The population of metastable state can exceed the population at a lower level thereby establishing population inversion in a lasing medium. Population inversion could not be created without a metastable state. The metastable electronic state of the solid gain media is same as that of an upper laser level.

The additional metastable states of the media occur under conditions where a rapid depopulation of a particular state does not take place either through radiative or non-radiative processes. Radiative processes can be reduced if the lower-level transitions are weakly allowed transitions that do not involve dipole processes.

In general, metastable levels do not exist in the laser gain media. However, it is possible to use a short-lived level as the upper laser level in the presence of large emission cross sections. Metastable levels with long lifetimes allow storage of significant amount of energy, and hence they are critical for Q-switched lasers. They also have a large effect on the laser kinetics such as spiking phenomena. Moreover, three-level laser transitions occur only in the presence of metastable states which help in achieving a substantial upper-state population for positive net gain.

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