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Beta barium borate

Beta barium borate (β-barium borate, BBO or β-BaB₂O₄) is a crystal frequently used for frequency mixing and other nonlinear optics applications. It has a wide transmission range, from approximately 200 nanometres to beyond 3 micrometres, and is used particularly for its transparency in the deep ultraviolet.

BBO has a trigonal crystal system, belongs to crystallographic point group 3m, and belongs to the crystallographic space group R3c. BBO has strong negative uniaxial birefringence and can be phase-matched for type I (ooe) second harmonic generation from 409.6–3500 nm. The temperature sensitivity of the indices of refraction is low, leading to an unusually large (55°C) temperature phase-matching bandwidth. The crystal is mildly hygroscopic.

β-Barium borate differs from α-barium borate in the positions of the barium ions within the crystal. Both phases are birefringent, however the α phase, which exists above 925 C, possesses centric symmetry and thus does not have the same nonlinear properties as the β phase.^[1]

Additional recommended knowledge

1 Physical properties
2 Optical properties
- 2.1 Sellmeier equations
3 References

Physical properties

Lattice parameters: a=b=1.253 nm, c=1.272 nm, Z=6
Melting point: 1095±5°C
Phase transition temperature: 925±5°C
Mohs hardness: 4.5
Density: 3.85 g/cm³

Optical properties

Refractive index, 1064 nm: $n_o=1.65, \quad n_e=1.54$
Refractive index, 532 nm: $n_o=1.67, \quad n_e=1.55$
Refractive index, 266 nm: $n_o=1.75, \quad n_e=1.61$
Transparency range (> 0.5 cm^-1): 198–2600 nm^[1]
Thermo-optic coefficients:^[1]

$\frac{\partial n_o}{\partial T}= -9.3 \times 10^{-6} / \mathrm{^\circ C}$
$\frac{\partial n_e}{\partial T} = -16.6 \times 10^{-6} / \mathrm{^\circ C}$

Nonlinear optical coefficients: ^[2]

d₂₂ = 2.3 pV / m.

d₃₁ = -0.16 pV / m.

High damage threshold of 10 GW/cm² for 100 ps pulse-width at 1064 nm.

Sellmeier equations

$n^2_o(\lambda)$ = 2.7359+0.01878/(λ² - 0.01822)-0.01354 λ² ^[3]
$n^2_e(\lambda)$ = 2.3753+0.01224/(λ²-0.01667)-0.01516 λ² ^[3]

References

^ ^a ^b ^c D. N. Nikogosyan. "Beta Barium Borate (BBO)". Applied Physics A 52.
^ V.G. Dmitriev, G. G. Gurzadyan, and D. N. Nikogosyan (1999). Handbook of Nonlinear Optical Crystals, 3rd edition. Springer. ISBN 0342-4111.
^ ^a ^b Kato, K. (1986). "Second-harmonic generation to 2048 Å in β-BaB₂O₄". IEEE Journal of Quantum Electronics 22.