Novel etching fluids for potassium dihydrogen phosphate

Absorbing defects such as fractures and contaminants are a leading cause of surface damage in nanosecond pulsed lasers. Etching such defects has proven to be a powerful technique for increasing the laser damage threshold of fused silica, but to date no etching process has been reported for potassium dihydrogen phosphate (KH2PO4 or KDP) or its deuterated analog (DKDP). We show that physical dissolution in water is a viable strategy for etching DKDP surfaces but surface-redeposited byproducts can serve as laser damage precursors. We use a water-in-oil microemulsion to etch engineered surface fractures in DKDP. Etching widens surface fractures laterally and decreases their optical activity, as measured by photoluminescence. The removal of 1 mu m of the surface of a DKDP crystal increases the laser damage threshold (lambda = 355 nm, 7 ns) of the engineered surface fractures by 2-4 J/cm(2) (15-30%).