Potassium dihydrogen phosphate (KH2PO4, KDP) and potassium dideuterium phosphate (KD2PO4, DKDP) crystals are among the most long-used nonlinear materials. They both have high optical damage thresholds, strong birefringence, and high nonlinearity coefficients in addition to outstanding UV transmission. These crystals may be produced in huge diameters and their nonlinearity is rather modest.
Alfa Chemistry provides a large selection of KDP and DKDP to its customers for nonlinear applications. In addition to being Q-switched devices for Ti:Sapphire, Alexandrite, and Nd-doped lasers, they are regarded as nonlinear multipliers, triplet multipliers, and quantizers for Nd-doped lasers.
Fig 1. Structural comparison of the tetragonal DKDP and the HTmonoclinic KDP crystals.
KDP/KD*P crystals have the drawback of being highly hygroscopic; as a result, it is advised to cover the polished crystal surface with a protective coating or house it in a sealed container with a light-transmitting window. The water solubility method is used to generate DKDP crystals, which are typically deuterated to a level of >98%.
DKDP is closely related to monopotassium phosphate or KDP or KH2PO4. In the DKDP, deuterium takes the place of hydrogen, which lowers the frequency of the O-H vibrations and their higher order harmonics. The use of DKDP and KDP crystals in infrared lasers is not favored by the light absorption caused by these overtones. Therefore, DKDP is chosen over KDP despite the higher price.
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Reference
- Zhang L, et al. (2015). "New Annealing Method to Improve KD2PO4 Crystal Quality: Learning from High Temperature Phase Transition." Cryst Eng Comm. 17(25): 4705-4711.