Oxides

Sapphire. The first laser material to be discovered (ruby laser) employed sapphire as a host. The Al₂O₃ (sapphire) host is hard, with high thermal conductivity, and transition metals can readily be incorporated substitutionally for the Al. The Al site is too small for rare earths, and it is not possible to incorporate appreciable concentrations of these impurities into sapphire. Besides ruby which is still used today, Ti-doped sapphire has gained significance as a tunable-laser material. The properties of ruby.
Garnets. Some of the most useful laser hosts are the synthetic garnets: yttrium aluminum garnet, Y₃Al₅O₁₂ (YAG); gadolinium gallium garnet, Gd₃Ga₅O₁₂ (GGG), and gadolinium scandium aluminum garnet Gd₃Sc₂Al₃O₁₂ (GSGG). These garnets have many properties that are desirable in a laser host material. They are stable, hard, optically isotropic, and have good thermal conductivities, which permits laser operation at high average power levels.
In particular, yttrium aluminum garnet doped with neodymium (Nd :YAG) has achieved a position of dominance among solid-state laser materials. YAG is a very hard, isotropic crystal, which can be grown and fabricated in a manner that yields rods of high optical quality. At the present time, it is the best commercially available crystalline laser host for Nd³⁺, offering low threshold and high gain. The properties of Nd :YAG are discussed in more detail. Besides Nd³⁺, the host crystal YAG has been doped with Tm³⁺, Er³⁺, Ho³⁺, and Yb³⁺. The Yb :YAG laser material and Er :YAG, Tm:YAG, and Ho :YAG crystals are briefly covered under the appropriate active ion.
In recent years, Nd : GSGG co-doped with Cr³⁺ has been employed in a number of laser systems. Cr³⁺ considerably increases the absorption of flashlamp radiation and transfers the energy very efficiently to Nd. A comparison with Nd :YAG reveals a lower thermal conductivity and a lower heat capacity for GSGG but, in general, the materials parameters are fairly close. The stimulated emission cross section, and therefore the gain of Cr : Nd :GSGG is about half of that in YAG.
This can be an advantage for Q-switch operation since more energy can be stored before amplified spontaneous emission (ASE) starts to deplete the metastable level.