Active laser medium

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Within a laser, the active laser medium is the material that exhibits optical gain. This gain is generally generated by stimulated emission on electronic or molecular transitions to a lower energy state, starting from a higher energy state to which it had been previously stimulated by means of a pump source.

Examples of active laser media include:

certain crystals, typically doped with some rare-earth ions (e.g. of neodymium, ytterbium, or erbium) or transition metal ions (e.g. of titanium or chromium)
glasses, e.g. silicate or phosphate glasses, also doped with some laser-active ions
gases, e.g. mixtures of helium and neon, or argon or carbon dioxide
semiconductors, e.g. gallium arsenide (GaAs), indium gallium arsenide (InGaAs), or gallium nitride (GaN)
liquid solutions of certain dyes

Pumping of gain media (i.e., the supply of energy) can be achieved with electrical currents (e.g. in semiconductors, or in gases via high-voltage discharges) or with light, which may be generated with discharge lamps or with other lasers. More exotic gain media can be pumped by chemical reactions, nuclear fission, or with high-energy electron beams.

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