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. 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 possesses centric symmetry and thus does not have the same harmonic generation capabilities as the β phase.

Advantages of BBO

Broad phase-matchable range from 409.6nm - 3500nm.

Wide transmission region from 190nm - 3500nm.

Large effective second-harmonic-generation (SHG) coefficient: approximately 6 times greater than that of KDP. High damage threshold of 10 GW/cm2 for 100 ps pulse-width at 1064nm. High optical homogeneity with delta n=10-6/cm. Wide temperature-bandwidth of about 55°C.

Physical Properties

Crystal structure: Trigonal, space group R3c

Lattice parameters: a=b=1.253nm, c=1.272nm, Z=6

Melting point: 1095±5°C

Phase transition temperature: 925±5°C

Mohs hardness: 4.5

Density: 3.85 g/cm3

Absorption coefficient @ 1064nm: <0.1%/cm3

Hygroscopic susceptibility: low

Resistivity: >1011 ohm-cm

Optical Properties

Refractive index, 1064nm: $n\_o=1.65,\; \backslash quad\; n\_e=1.54$

Refractive index, 532nm: $n\_o=1.67,\; \backslash quad\; n\_e=1.55$

Refractive index, 266nm: $n\_o=1.75,\; \backslash quad\; n\_e=1.61$

Optical homogeneity Dn: $10^\{-6\}\; cm^\{-1\}$

Transparency range: 189-3500 nm

Phase matchable range (SHG): 189-1750 nm

Effective nonlinear coefficient: 4-6 x KDP

Acceptance angle: 0.2-1 mrad-cm

Walk-off angle, from SHG-5HG of Nd:YAG laser: 3.2°-5.5°

Thermo-optic coefficients:

$\backslash frac\{\backslash partial\; n\_o\}\{\backslash partial\; T\}=\; -9.3\; \backslash times\; 10^\{-6\}\; /\; \{\}^\backslash circ\; C$

$\backslash frac\{\backslash partial\; n\_e\}\{\backslash partial\; T\}\; =\; -16.6\; \backslash times\; 10^\{-6\}\; /\; \{\}^\backslash circ\; C$

Sellmeier equations

$n^2\_o(\backslash lambda)\; =\; 2.7359+0.01878/(\backslash lambda^2\; -\; 0.01822)-0.01354\; \backslash times\; \backslash lambda^2$ in micrometers

$n^2\_e(\backslash lambda)\; =\; 2.3753+0.01224/(\backslash lambda^2-0.01667)-0.01516\; \backslash times\; \backslash lambda^2$ in micrometers

Source

http://www.photox.co.uk/bbo.htm Retrieved: June 28th, 2006

Nonlinear optical materials