Terbium Gallium Garnet Crystal (TGG)
Terbium gallium garnet (TGG) is a synthetic cubic magneto-optical crystal with formula Tb3Ga5O12. In lasers and photonics it is the standard Faraday-rotator material for the visible and near-infrared (about 400–1100 nm, with a gap near 475–500 nm).
A rod of TGG in a strong axial magnetic field rotates the polarization of transmitted light by an angle that depends only on field direction, not on travel direction—the Faraday effect that makes isolators and circulators work.
How it is used:
Faraday rotation is:
θ=V B L
where V is the Verdet constant, B the magnetic flux density, and L the crystal length. Isolators place the TGG rod between two polarizers set 45° apart so forward light is transmitted and backward light is rejected.
TGG is grown by the Czochralski method and usually cut along [111]. Transparent ceramics of the same composition exist for larger apertures at high average power.
Basic properties:
Density ≈7.13 g cm−3; Mohs hardness 8; melting point ≈1725∘C.
Refractive index ≈1.95 at 1064 nm.
Thermal conductivity ≈7.4 W m−1 K−1 (much higher than Tb-doped glass).
Absorption typically <0.1%/cm in the useful window.
Laser damage threshold often quoted >1 GW cm−2 (pulse-dependent).
Extinction ratio of good rods >30 dB.
Verdet constant (wavelength-dependent; stronger at shorter λ):
about −134 rad T−1 m−1 at 632 nm.
about −35 to −40 rad T−1 m−1 at 1064 nm.
V increases at cryogenic temperature, so cooled isolators can use a shorter crystal or a weaker magnet. Related crystals (Gd:TGG, Tb-aluminum-gallium garnet) can have a modestly higher V than undoped TGG.
Compared with terbium glass, TGG has roughly twice the Verdet constant and about ten times the thermal conductivity, so it handles high average power with less thermal lensing and depolarization.
Limits at kilowatt class are still heat: absorption plus dn/dT causes lensing and stress birefringence; water cooling, ceramic apertures, and external depolarization compensation are used above a few hundred watts.
TGG is not used in the mid-IR (other Faraday materials take over) and is avoided in the blue-green absorption notch.
Applications:
Faraday isolators on Nd:YAG, Yb-fiber, Ti:sapphire, and other 0.5–1.1 µm lasers—especially multi-stage amplifiers, ring lasers, and injection-seeded systems that cannot tolerate back-reflections.
Optical circulators in fiber and free-space networks.
Faraday rotators for polarization control and multi-pass amplifier schemes.
Magneto-optical modulators and switches.
Current and magnetic-field sensors that read Faraday rotation.
High-average-power isolators using TGG ceramics or compensated TGG stages.
In short, TGG is the workhorse crystal that turns a magnetic field into a one-way polarization rotation for visible–NIR lasers.