A uniaxial birefringent crystal in which light polarized parallel to the optic axis (the extraordinary ray) travels slower than light polarized perpendicular to it (the ordinary ray). Quartz is a classic example. This property dictates the phase-matching angles required when cutting the crystal for nonlinear frequency conversion.
Since it's a positive crystal, we need to calculate the phase-matching angle differently than we did for the BBO.
Do not mix it up with
- Negative crystal: A positive crystal has a higher extraordinary index, while a negative crystal has a higher ordinary index.
Related terms
Negative crystal
A birefringent crystal where the extraordinary refractive index is less than the ordinary refractive index.
Phase matching
Keeping interacting waves in a nonlinear crystal in step so the generated light builds up instead of canceling.
Walk-off
The drifting apart of interacting beams in a crystal, either sideways from birefringence or in time from velocity mismatch.