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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]

BBO is a common material used to split photons to produce the phenomenon of quantum entanglement.

Contents

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.
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Sellmeier equations (for λ in micrometres)

  • 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

  1. ^ a b c D. N. Nikogosyan. "Beta Barium Borate (BBO)". Applied Physics A 52: 359--368.  
  2. ^ V.G. Dmitriev, G. G. Gurzadyan, and D. N. Nikogosyan (1999). Handbook of Nonlinear Optical Crystals, 3rd edition. Springer. ISBN 0342-4111.  
  3. ^ a b Kato, K. (1986). "Second-harmonic generation to 2048 Å in β-BaB2O4". IEEE Journal of Quantum Electronics 22 (7): 1013--1014. doi:10.1109/JQE.1986.1073097.  

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