Wavelength insensitive phase-matched second-harmonic generation in partially deuterated KDP

We have experimentally determined the spectrally noncritical phase-matching behavior of type I frequency doubling in potassium dihydrogen phosphate (KDP) and its dependence on deuteration level in partially deuterated KD*P. Our results are in agreement with predictions based on available refractive index data. The first-order wavelength sensitivity parameter {partial derivative}{Delta}{ital k}/{partial derivative}{lambda} for type I doubling of 1.053-{mu}m light vanishes for a KD*P crystal with a deuteration level of 12{plus minus}2%. Calculations of this effect in other KDP analogs are presented, and the possibility of finding materials that have spectrally noncritical behavior for other phase-matched processes is discussed.