Yttrium aluminium garnet
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Yttrium aluminium garnet (YAG, Y3Al5O12) is a synthetic crystalline material of the garnet group, used as the active laser medium in various solid-state lasers. YAG is commonly doped with other elements to obtain a specific laser wavelength. For example, neodymium and erbium are commonly used, yielding Nd:YAG and Er:YAG lasers, respectively. YAG is also used in jewelry as a diamond simulant. Colored variants are faceted and valued (as synthetics) for their clarity, durability, high refractive index and dispersion. 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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[edit] Properties
- Color: Wide range of colors
- Mohs’ hardness: 8.5
- Refractive index: 1.833
- Critical angle: 33°
- Specific gravity: 4.55
- Cleavage: Imperfect
- Fracture: Conchoidal
- Dispersion: Moderately high (.028)
- Heat sensitivity: Low
- Pleochroism: None
- Birefringence: None
- Crystal structure: Isometric
- Treatments: YAG is a synthetic stone.
- Cutting: Cuts similar to natural garnet.
- Polishing: Alumina or diamond (50,000 or 100,000) common polishing laps.
[edit] Doped variations
[edit] Nd:YAG
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.
[edit] Er:YAG
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 600-800 nm, allowing for efficient flashlamp pumping. The dopant concentration used is high, about 50% of yttrium atoms is replaced. The Er:YAG laser crystals couple well into water and body fluids, making them 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.
[edit] 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.
[edit] 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 ultraviolet region, and transfer their energy to the neodymium atoms, increasing the pumping efficiency; its result is lower thermal distortion and higher power output than Nd:YAG at the same pumping. The lasign wavelength, 1064 nm, is the same as for Nd:YAG. The material has a good anti-UV irradiation property and low lasing threshold. Usually 1.1-1.4 % of Y atoms are replaced with Nd, and 0.05-0.1% with Ce.
[edit] 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 400-800 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).
[edit] 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.
[edit] 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.
[edit] 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 2950-1800 K. The phosphor can be applied directly to the measured surface, or to an end of an optical fiber.
[edit] Sm:YAG
Samarium-doped YAG (Sm:YAG) is a temperature-sensitive phosphor similar to Dy:YAG.
[edit] Tb:YAG
Terbium-doped YAG (Tb:YAG) is a phosphor used in cathode ray tubes. It emits at yellow-green color, at 544 nm.
[edit] 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.
