Originally Posted by jbeng

Hey _nox_ , I think the crystals you're talking about are known as NLO (Non-Linear Optic) crystals. They use them in laser systems to generate harmonics of laser wavelengths. For instance, the green laser pointers you can buy actually have a solid-state infrared laser diode operating at 808nm pumping a crystal of Nd:YV04 as the laser generator. Then they send the 1064nm IR beam produced through the NLO (usually made of a material called KTP). That doubles the frequency (halves the wavelength) to generate the green beam at 532nm. Here's a link to wiki regarding the Non-Linear Optics. Interesting stuff.
JB