Active laser medium
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Within a laser, the active laser medium or gain 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), most often yttrium aluminium garnet (YAG), yttrium orthovanadate (YVO4), or sapphire (Al2O3);
- glasses, e.g. silicate or phosphate glasses, also doped with some laser-active ions;
- gases, e.g. mixtures of helium and neon, nitrogen, argon, carbon monoxide, carbon dioxide, or metal vapors;
- semiconductors, e.g. gallium arsenide (GaAs), indium gallium arsenide (InGaAs), or gallium nitride (GaN);
- liquid solutions of certain dyes (see dye laser);
- a beam of electrons (see free electron laser).
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 (often semiconductor lasers, see DPSS laser). More exotic gain media can be pumped by chemical reactions (see chemical laser), nuclear fission (see nuclear pumped laser), or with high-energy electron beams.
