Transparent crystals with laser-active dopants, used as Laser gain media.
Laser crystals are typically single crystals (monocrystalline material) which are used as gain media for solid state lasers. In most cases, they are doped either with trivalent rare-earth ions or transition metal ions. These ions enable the crystal to amplify Light at the laser Wavelength via stimulated emission, when energy is supplied to the crystal via Absorption of pump light.
Compared with doped glasses, crystals usually have higher transition cross sections, a smaller absorption and Emission bandwidth, a higher thermal conductivity, and possibly birefringence. In some cases, monocrystalline laser materials may be replaced with ceramic gain media, which have a fine polycrystalline structure.