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Laser Crystals

Definition: transparent crystals with laser-active dopants, used as laser gain media

German: Laserkristalle

Categories: lasers, optical materials

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Laser crystals are typically single crystals (monocrystalline material) which are used as gain media for solid-state lasers. In most cases, they are doped with either 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 (→ optical pumping).

Compared with doped glasses, crystals usually have higher transition cross sections, a smaller absorption and emission bandwidth, a higher thermal conductivity, and possibly birefringence. (The article on laser crystals versus glasses discusses the differences in more detail.) In some cases, monocrystalline laser materials may be replaced with ceramic gain media, which have a fine polycrystalline structure.

Common Laser-active Dopants

The most frequently used laser-active rare earth ions and host media together with some typical emission wavelengths are shown in the following table:

Table 1: Common rare earth ions in laser-active crystals.

Ion Common host crystals Important emission wavelengths
neodymium (Nd3+) Y3Al5O12 (YAG), YAlO3 (YALO), YVO4 (yttrium vanadate), YLiF4 (YLF), tungstates (KGd(WO4)2, KY(WO4)2) 1064, 1047, 1053, 1342, 946 nm
ytterbium (Yb3+) YAG, tungstates (e.g. KGW, KYW, KLuW), YVO4, borates (BOYS, GdCOB), apatites (SYS), sesquioxides (Y2O3, Sc2O3) 1030, 1020–1070 nm
erbium (Er3+) YAG, YLF 2.9, 1.6 μm
thulium (Tm3+) YAG 1.9–2.1 μm
holmium (Ho3+) YAG 2.1, 2.94 μm
cerium (Ce3+) YLF, LiCAF, LiLuF, LiSAF, and similar fluorides 0.28–0.33 μm

The following tables lists common transition-metal doped crystals:

Table 2: Common transition metal ions in laser-active crystals.

Ion Common host crystals Important emission wavelengths
titanium (Ti3+) sapphire 650–1100 nm
chromium (II) (Cr2+) zinc chalcogenides such as ZnS, ZnSe, and ZnSxSe1−x 2–3.4 μm
chromium (III) (Cr3+) Al2O3 (ruby), LiSrAlF6 (LiSAF), LiCaAlF6 (LiCAF), LiSrGaF6 (LiSGAF) 0.8–0.9 μm
chromium (IV) (Cr4+) YAG, MgSiO4 (forsterite) 1.35–1.65 μm (YAG), 1.1–1.37 μm (forsterite)

These tables contain only the most common host crystals; many others exist, which are less frequently used.

Important Properties of Host Crystals

The host crystal is much more than just a means to fix the laser-active ions at certain positions in space. A number of properties of the host material are important:

It is apparent that different applications lead to very different requirements on laser gain media. For this reason, a broad range of different crystals are used, and making the right choice is essential for constructing lasers with optimum performance.

Common Crystalline Laser Host Media

There is a wide range of crystalline media, which can be grouped according to important atomic constituents and crystalline structures. Some important groups of crystals are:

Laser Crystals with Integrated Saturable Absorber

A few laser crystal materials have been demonstrated where some saturable absorber material is incorporated for passive Q switching of a laser. For example, Cr4+ ions can be incorporated into such Nd-doped crystals for emission in the 1-μm spectral region. This has been tried with Cr:Nd:YAG and Cr:Nd:YVO4, for example.

With that concept, one does not need an additional saturable absorber crystal, so that one may make more compact Q-switched laser setups with lower internal parasitic losses. However, unwanted side effects may also occur, such as obtaining unwanted valence states of the involved ions or energy transfers. In addition, some flexibility is lost in experiments if one cannot try out absorbers with different thickness or doping concentration, for example, without exchanging the laser crystal itself.

Geometries of Laser Crystals

Different geometric forms can be used in lasers:

Bulk Properties

For a given dopant and host medium, the doping concentration is the most important parameter. Other issues of interest are the uniformity of doping (influencing the tendency for quenching), the level of impurities (e.g. unwanted other rare earth ions), and the optical homogeneities. Several of these factors influence the absorption and scattering losses of the material, and/or the strength of thermal lensing.

Of course, it is very desirable that a given crystal quality is produced consistently, although different laser designs can have a different sensitivity to material parameters.

Optimization of Geometry and Parameters

Which geometry, dopant and doping concentration of the gain medium are most advantageous depend on several factors. The available pump source (type of laser diode or lamp) and the envisaged pumping arrangement are important factors, but the material itself also has some influence. For example, titanium–sapphire lasers have to be pumped with high intensities, for which the form of a transversely cooled rod, operated with relatively small pump and laser beam diameter, is more appropriate than e.g. a thin disk. As another example, Q-switched lasers reach a higher population density in the upper laser level and are therefore more sensitive to quenching effects and energy transfer processes; therefore, a lower doping density is often appropriate for these devices. For high-power lasers, lower doping densities are often used in order to limit the density of heat generation, although thin-disk lasers work best with highly doped crystals. Many laser products do not reach the full performance potential because such details have not been properly worked out.

Optical Surfaces

Those surfaces which are passed by the laser beam are normally either oriented at Brewster's angle or have an anti-reflection coating. Even AR-coated crystals are often slightly tilted against the beam so as to prevent back-reflections staying in the laser resonator. This is important for, e.g., mode-locked lasers and tunable single-frequency lasers.

A high surface quality is of course important. Specifications of surface flatness are often better than λ / 10. This helps to avoid both scattering losses and wavefront distortions which can degrade the laser's beam quality. In addition, scratch and dig specifications (cosmetic surface quality) limit the density of small-scale surface defects; they may read e.g. “80–50” for medium quality mass production, or “10–5” for particularly demanding laser applications. Proper surface treatment also influences the damage threshold, which is important e.g. for high-energy pulse amplifiers. Finally, a high degree of end face parallelism can be important for avoiding changes of beam direction in a crystal.


The RP Photonics Buyer's Guide contains 32 suppliers for laser crystals. Among them:


[1]A. A. Kaminskii, “Laser crystals and ceramics: recent advances”, Laser Photon. Rev. 1 (2), 93 (2007)
[2]A. A. Kaminskii, Laser Crystals, Springer, New York (1981)
[3]R. C. Powell, Physics of Solid-State Laser Materials, AIP Press, Springer (1998)
[4]W. Koechner, Solid-State Laser Engineering, 6th edn., Springer, Berlin (2006)
[5]F. Träger (ed.), Handbook of Lasers and Optics, Springer, Berlin (2007)

(Suggest additional literature!)

See also: gain media, laser crystals versus glasses, doped insulator lasers, rare-earth-doped gain media, transition-metal-doped gain media, rod lasers, slab lasers, thin-disk lasers, single-crystal fibers, Spotlight article 2007-02-09
and other articles in the categories lasers, optical materials

Dr. R. Paschotta

This encyclopedia is authored by Dr. Rüdiger Paschotta, the founder and executive of RP Photonics Consulting GmbH. Contact this distinguished expert in laser technology, nonlinear optics and fiber optics, and find out how his technical consulting services (e.g. product designs, problem solving, independent evaluations, or staff training) and software could become very valuable for your business!

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