High Quality Content by WIKIPEDIA articles! The active laser medium is the source of optical gain within a laser. The gain results from the stimulated emission of electronic or molecular transitions to a lower energy state from a higher energy state previously populated by a pump source. Examples of active laser media include: Certain crystals, typically doped with rare-earth ions or transition metal ions; most often yttrium aluminium garnet, yttrium orthovanadate, or sapphire; Glasses, e.g. silicate or phosphate glasses, doped with 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, indium gallium arsenide, or gallium nitride. In order to lase, the active gain medium must be in a nonthermal energy distribution known as a population inversion. The preparation of this state requires an external energy source and is known as laser pumping. Pumping may be achieved with electrical currents or with light, generated by discharge lamps or by other lasers. More exotic gain media can be pumped by chemical reactions, nuclear fission, or with high-energy electron beams.

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