Potassium dideuterium phosphate

Deuterated potassium dihydrogen phosphate (KD$2$PO$2$ or K$4$H$2$PO$4$) or DKDP single crystals are widely used in non-linear optics as the second, third and fourth harmonic generators for YAG and YLF lasers. They are also found in electro-optical applications as Q-switches for YAG, YLF, alexandrite and Ti-sapphire lasers, as well as for Pockels cells.

DKDP is monopotassium phosphate (KDP, or KH$2$PO$2$), but using deuterium. Replacement of hydrogen by deuterium in DKDP lowers the frequency of O–H vibrations and their overtones (high-order harmonics). Absorption of light by those overtones is detrimental for the infrared lasers, which DKDP and KDP crystals are used for. Consequently, despite higher cost, DKDP is more popular than KDP.

DKDP crystals are grown by a water-solution method at usual level of deuteration >98%.