Yttrium aluminium garnet

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Yttrium aluminium garnet (YAG, Y₃Al₅O₁₂) is a synthetic crystalline material of the garnet group. It is also one of three phases of the yttria-aluminum composite, the other two being yttrium aluminum monoclinic (YAM) and yttrium aluminum perovskite (YAP). YAG is commonly used as a host material in various solid-state lasers. Rare earth elements such as neodymium and erbium can be doped into YAG as active laser ions, yielding Nd:YAG and Er:YAG lasers, respectively. Cerium-doped YAG (YAG:Ce) is used as a phosphor in cathode ray tubes and white light-emitting diodes, and as a scintillator.

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Gemstone YAG

YAG for a period was used in jewelry as a diamond and other gemstone simulant. Colored variants and their doping elements include;^[1] green (chromium), blue (cobalt), red (manganese), yellow (titanium), purple (neodymium), pink, and orange. As faceted gems they are valued (as synthetics) for their clarity, durability, high refractive index and dispersion. The critical angle of YAG is 33 degrees. YAG cuts similar to natural garnet, with polishing being performed with alumina or diamond (50,000 or 100,000) on common polishing laps. YAG has low heat sensitivity.

As a synthetic gemstone YAG has numerous varietal and trade names, as well as a number of misnomers. Synonymous names include:^[1] alexite, amamite, circolite, dia-bud, diamite, diamogem, diamonair, diamone, diamonique, diamonite, diamonte, di'yag, geminair, gemonair, kimberly, Linde simulated diamond, nier-gem, regalair, replique, somerset, triamond, YAIG, and yttrium garnet. It's misnomers include;^[1] synthetic diamond, and synthetic demantoid, for green stones.

Production for the gem trade lapsed after the introduction of synthetic cubic zirconia, with little or no current production.^[1]

Technical-use varieties

Nd:YAG

Main article: Nd:YAG laser

Neodymium-doped YAG (Nd:YAG) was developed in 1960s. It is the most widely used active laser medium in solid-state lasers. It can be used in lasers utilizing frequency doubling and frequency tripling, and high-energy Q-switching. Its thermal conductivity is better and its fluorescence lifetime is about twice as long than Nd:YVO₄. It can be operated on power levels of up to kilowatts. It can be directly Q-switched with Cr⁴⁺:YAG.

Nd:YAG lases at 1064 nanometers and its best absorption band for pumping is 1 nm wide and located at 807.5 nm. [1]

The dopant concentration in commercially available Nd:YAG usually varies between 0.5-1.4 molar %. Higher dopant concentration is used for pulsed lasers, lower concentration is suitable for continuous wave lasers. Nd:YAG is pinkish-purple, lighter doped rods being less intensely colored than heavier-doped ones. As its absorption spectrum is narrow, the hue will depend on the type of light and will differ between sunlight and artificial light.

Nd:Cr:YAG

YAG doped with neodymium and chromium (Nd:Cr:YAG or Nd/Cr:YAG) has absorption characteristics which are superior to Nd:YAG. This is because energy is absorbed by the broad absorption bands of the Cr³⁺ dopant and then transferred to Nd³⁺ by dipole-dipole interactions.^[2] This material has been suggested for use in solar-pumped lasers, which could form part of a solar power satellite system.^[3]

Er:YAG

Main article: Er:YAG laser

Erbium-doped YAG (Er:YAG) is an active laser medium lasing at 2940 nm. Its absorption bands suitable for pumping are wide and located between 600 and 800 nm, allowing for efficient flashlamp pumping. The dopant concentration used is high: about 50% of the yttrium atoms are replaced. The Er:YAG laser wavelength couples well into water and body fluids, making this laser especially useful for medicine and dentistry uses; it is used for treatment of tooth enamel and in cosmetic surgery. Er:YAG is used for noninvasive monitoring of blood sugar. The mechanical properties of Er:YAG are essentially the same as Nd:YAG. Er:YAG operates at relatively eye-safe wavelengths (while it hits the lens, it is absorbed in the eye so it does not damage retina), works well at room temperature, and has high slope efficiency. Er:YAG is pale-green colored.

Yb:YAG

Ytterbium-doped YAG (Yb:YAG) is an active laser medium lasing at 1030 nm, with a broad, 18 nm wide absorption band at 940 nm. It is one of the most useful mediums for high-power diode-pumped solid state lasers. The dopant levels used range between 0.2-30% of replaced yttrium atoms. Yb:YAG has very low fractional heating, very high slope efficiency, and no excited-state absorption or up-conversion, high mechanical strength and high thermal conductivity. Yb:YAG can be pumped by reliable InGaAs laser diodes at 940 or 970 nm.

Yb:YAG is a good substitute for 1064 nm Nd:YAG in high-power applications, and its frequency-doubled 515 nm version can replace the 514 nm argon lasers.

Nd:Ce:YAG

Neodymium-cerium double-doped YAG (Nd:Ce:YAG, or Nd,Ce:YAG) is an active laser medium material very similar to Nd:YAG. The added cerium atoms strongly absorb in the ultraviolet region, and transfer their energy to the neodymium atoms, increasing the pumping efficiency; the result is lower thermal distortion and higher power output than Nd:YAG at the same pumping level. The lasing wavelength, 1064 nm, is the same as for Nd:YAG. The material has a good resistance to damage caused by UV from the pump source, and low lasing threshold. Usually 1.1-1.4 % of Y atoms are replaced with Nd, and 0.05-0.1% with Ce.

Ho:Cr:Tm:YAG

Holmium-chromium-thulium triple-doped YAG (Ho:Cr:Tm:YAG, or Ho,Cr,Tm:YAG) is an active laser medium material with high efficiency. It lases at 2097 nm and can be pumped by a flashlamp or a laser diode. It is widely used in military, medicine, and meteorology. It operates at relatively eye-safe wavelengths, works well at room temperature, and has high slope efficiency. When pumped by a diode, the 781 nm band is used. Other major pump bands are located between 400 and 800 nm. The dopant levels used are 0.35 atom.% Ho, 5.8 atom.% Tm, and 1.5 at.% Cr. The rods have green color, imparted by chromium(III).

Tm:YAG

Thulium-doped YAG (Tm:YAG) is an active laser medium operating at the relatively eye-safe wavelengh of 2010 nm. It is suitable for being diode-pumped. It can be tuned between 1930-2040 nm. A dual-mode Tm:YAG laser emits two frequencies separated by 1 GHz.

Cr⁴⁺:YAG

Chromium(IV)-doped YAG (Cr:YAG) provides a large absorption cross section in the 0.9-1.2 micrometer spectral region, which makes it an attractive choice as a passive Q-switch for Nd-doped lasers. The resulting devices are solid-state, compact and low-cost. It has high damage threshold, good thermal conductivity, good chemical stability, is resistant to ultraviolet radiation, and is easily machined. It is replacing the more traditional Q-switching materials like lithium fluoride and organic dyes. The dopant levels range between 0.5-3 molar %.

It can be used for passive Q-switching the diode or lamp pumped Nd:YAG, Nd:YLF, Nd:YVO₄, Yb:YAG, and other neodymium and yttrium doped lasers at wavelengths between 1000-1200 nm.

Cr:YAG can be also used for tunable lasers with output adjustable between 1350-1550 nm. It can generate ultrashort (down to femtoseconds range) pulses when pumped at 1064 nm by a Nd:YAG laser.

Cr:YAG was demonstrated in a non-linear optics application as a self-pumped phase-conjugated mirror in a Nd:YAG loop resonator. It provides compensation of both phase and polarization aberrations induced into the loop resonator.

Dy:YAG

Dysprosium-doped YAG (Dy:YAG) is a temperature-sensitive phosphor used in temperature measurements. The phosphor is excited by a laser pulse and its temperature-dependent fluorescence is observed. Dy:YAG is sensitive in ranges of 300-1700 K.^[4] The phosphor can be applied directly to the measured surface, or to an end of an optical fiber.

Sm:YAG

Samarium-doped YAG (Sm:YAG) is a temperature-sensitive phosphor similar to Dy:YAG.

Tb:YAG

Terbium-doped YAG (Tb:YAG) is a phosphor used in cathode ray tubes. It emits at yellow-green color, at 544 nm.

Ce:YAG

Cerium(III)-doped YAG (Ce:YAG or YAG:Ce) is a phosphor, or a scintillator when in pure single-crystal form, with wide range of uses. It emits yellow light when subjected to blue or ultraviolet light, or to x-ray light. It is used in white light-emitting diodes, as a coating on a high-brightness blue InGaN diode, converting part of the blue light into yellow, which then appears as white. Such light however gives suboptimal color fidelity of green and red colors, causing color distortion. The output color is also strongly dependent on temperature and current.

Ce:YAG is also used in some mercury-vapor lamps as one of the phosphors, often together with Eu:Y(P,V)O₄ (yttrium phosphate-vanadate). It is also used as a phosphor in cathode ray tubes, where it has green (530 nm) to yellow-green (550 nm). When excited by electrons, it has virtually no afterglow (70 ns decay time). It is suitable for use in photomultipliers.

Ce:YAG is used in PET scanners, high-energy gamma radiation and charged particle detectors, and high-resolution imaging screens for gamma, x-rays, beta radiation and ultraviolet radiation.

Ce:YAG can be further doped with gadolinium.

See also

• Yttrium iron garnet
• Gadolinium gallium garnet
• Nd:YAG laser
• Er:YAG laser

References

1. ^ ^{a b c d e f g h i j k l m n o p q r s} Gemological Institute of America, GIA Gem Reference Guide 1995, ISBN 0-87311-019-6
2. ^ §IIE, Crystalline solid lasers, Z. J. Kiss and R. J. Pressley, Proceedings of the IEEE, 54, #10 (October 1996), pp. 1236–1248.
3. ^ Disk-type Nd/Cr:YAG ceramic lasers pumped by arc-metal-halide-lamp, Taku Saiki, Kazuo Imasaki, Shinji Motokoshi, Chiyoe Yamanaka, Hisanori Fujita, Masahiro Nakatsuka and Yasukazu Izawa, Optics Communications 268, #1 (December 1, 2006), pp. 155–159. DOI 10.1016/j.optcom.2006.07.002.
4. ^ Goss, L.P.; Smith, A.A.; Post, M.E. (1989). "Surface thermometry by laser-induced fluorescence". Review of Scientific Instruments: 3702–3706. doi:10.1063/1.1140478. Retrieved on 2007-03-06.