A type of uniaxial crystal, such as calcite or BBO, in which light polarized parallel to the optic axis (the extraordinary wave) travels faster than light polarized perpendicular to it (the ordinary wave). This means the extraordinary refractive index is lower than the ordinary one. Laser engineers select negative crystals for specific phase-matching schemes in nonlinear optics, like second-harmonic generation.
BBO is a negative crystal, so we need to calculate the phase-matching angle for Type I generation accordingly.
Do not mix it up with
- Positive crystal: A positive crystal has an extraordinary index greater than its ordinary index, meaning the extraordinary wave travels slower.
Related terms
Positive crystal
A birefringent material where the extraordinary refractive index is greater than the ordinary index.
Phase matching
Keeping interacting waves in a nonlinear crystal in step so the generated light builds up instead of canceling.
SHG
Second Harmonic Generation: converting light to exactly double its frequency, half its wavelength, in a nonlinear crystal.