In laser technology, the active medium refers to the light-generating laser medium in the laser. Along with the pump source and the resonator, it is one of the three essential elements of a laser. As a rule, lasers are categorized according to their laser medium: Solid state, for example doped glasses or also crystals are used and one speaks here of solid state lasers. When liquids such as dye solutions are used, they are referred to as dye lasers. When gases such as helium-neon mixtures are used, they are called gas lasers. Not all lasers use a laser-active medium in the sense of a material. In diode lasers or quantum cascade lasers, occupation inversion is generated with the aid of structures, often consisting of semiconductor material.