Lasing medium
The active laser medium (also called gain medium or lasing 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:
In order to fire a laser, 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 (e.g. semiconductors, or gases via high-voltage discharges) or with light, generated by discharge lamps or by other lasers (semiconductor lasers). More exotic gain media can be pumped by chemical reactions, nuclear fission, or with high-energy electron beams.
A universal model valid for all laser types does not exist. The simplest model includes two systems of sub-levels: upper and lower. Within each sub-level system, the fast transitions ensure that thermal equilibrium is reached quickly, leading to the Maxwell–Boltzmann statistics of excitations among sub-levels in each system (fig.1). The upper level is assumed to be metastable. Also, gain and refractive index are assumed independent of a particular way of excitation.
For good performance of the gain medium, the separation between sub-levels should be larger than working temperature; then, at pump frequency , the absorption dominates.
...
Wikipedia