Also, doping with Yb, Er, Nd, Tm, Cr of optical crystals, ceramics and glasses is used to make the solid-state lasers; the elements mentioned are the most common dopants. In many optically-transparent ghosts, such active centers may keep their excitation for a time on the order of milliseconds, and relax with stimulated emission, providing the laser action. The amount of dopant is usually measured in atomic percent; usually the relative atomic percent is assumed, taking into account that the dopant ion can substitute the only part of site in a crystalline lattice. The doping can be also used to change the refraction index in optical fibers, especially in the double-clad fibers. The optical dopants are characterized with lifetime of excitation and the effective absorption and emission cross-sections, which are main parameters of an active dopant. Usually the concentration of optical dopant is of order of few percent or even lower. At large density of excitation, the cooperative quenching (cross-relaxation) reduces the efficiency of the laser action.
The process of introducing dopants into a semiconductor is called doping.
The addition of a dopant to a semiconductor has the effect of shifting the Fermi level within the material. This results in a material with predominantly negative (n type) or positive (p type) charge carriers depending on the dopant species. Pure semiconductors altered by the presence of dopants are known as extrinsic semiconductors (cf. intrinsic semiconductor). Dopants are introduced into semiconductors in a variety of techniques: solid sources, gases, spin on liquid and ion implanting. See ion implantation, surface diffusion, and solid sources footnote.