Yttrium aluminum garnet (YAG) is a synthetic crystal doped with neodymium or other rare earths for use in bulk solid-state lasers. Although neodymium lasers were first demonstrated in a calcium tungstate host, YAG has long been the most common host for solid-state lasers. YAG also has been used in jewelry, and may be doped with other rare earths for use in lasers or phosphors.
YAG can be a puzzling acronym to decode if you think of crystals as chemical compounds. The Y stands for yttrium and the A for aluminum, but G is for garnet, which is a class of minerals with a particular cubic crystalline structure, not an element. In fact, the chemical formula of YAG, Y3Al5O12, does not fit the usual definition of garnet. Dictionaries define a garnet as a silicate mineral consisting of three SiO4 groups plus three atoms of a divalent metal (A) and two atoms of a trivalent metal (B), with a chemical formula A3B2(SiO4)3. Yet YAG contains no silicate groups, and no divalent atoms. Both yttrium and aluminum are trivalent, but they combine with a dozen oxygen atoms to form a unit cell containing the same number of atoms as a unit cell of a standard garnet, producing a crystal with a garnet-like structure.
The optical and mechanical properties that make YAG attractive for laser use include high thermal conductivity, high energy storage, and long fluorescence lifetime. For use in Nd:YAG lasers, YAG is doped with a molar concentration of roughly 1% neodymium atoms, which replace yttrium atoms in the crystal. Other rare earths including ytterbium, erbium, holmium, and thulium also can be doped into YAG to make lasers, and additional dopants may be added to aid energy transfer. An important emerging application for cerium-doped YAG is as a yellow-emitting phosphor used with blue LEDs to produce white light.
Traditionally, YAG laser rods have been fabricated from crystalline boules, limiting their size. New processes can produce ceramic YAG in much larger sizes, for use as laser slabs, disks, or rods. Ceramic slab lasers have reached 100-kilowatt powers in experimental military lasers.
FIGURE. 10-centimeter-square slab of ceramic Nd:YAG glows in 808-nm pump light during tests of the Lawrence Livermore National Laboratory's Heat Capacity Laser.