active laser medium

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.
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Within a laser, the material that emits coherent radiation or exhibits gain as the result of electronic or molecular transitions to a lower energy state or states, from a higher energy state or states to which it had been previously stimulated. Note: Examples of active laser media include certain crystals, gases, glasses, liquids, and semiconductors. Synonym laser medium.