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Point defects are known to limit the performance of many nonlinear optical materials. For example, gray tracks form along the beam path in some KTP crystals when they are subjected to lasers operating at high peak powers. Transient absorption bands in the visible and ultraviolet can be induced in KDP and BBO crystals by intense pulsed laser beams. Chalcopyrite crystals such as ZnGeP2 and AgGaS2 can have unwanted absorption bands which overlap their desirable 2-μm OPO pump region. All of these device-limiting extrinsic absorption bands are associated with point defects in the bulk of the crystals. Among the responsible defects are transition-metal-ion impurities, other nonmagnetic impurities, cation vacancies, anion vacancies, and antisites. In some materials, the point defects present in the as-grown crystals may already exhibit absorption bands, while in other cases, the existing point defects may need to trap an electron or hole during device operation before the absorption band is activated. Eliminating the optically active point defects during crystal growth will lead to materials with higher damage thresholds. With this as a goal, numerous investigators have used electron paramagnetic resonance (EPR) and electron-nuclear double resonance (ENDOR) to identify and characterize point defects which give rise to the unwanted absorption bands. EPR can detect concentrations of defects at levels as low as tens of parts per billion. Furthermore, each defect has a unique g-value signature which allows a variety of defects to be monitored simultaneously. Superhyperfine interactions with surrounding nuclei permits a "mapping" of the wave function for each defect and this results in a detailed model of the defect. The following review provides examples of how EPR and ENDOR are used to characterize commercially available nonlinear optical materials.
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