Finishing surface treatment of potassium titanyl phosphate single crystals by argon cluster ions

It is demonstrated that a argon cluster ion beam can be used for the finishing treatment of ultrasmooth surfaces (with initial root-mean-square roughness below 0.3 nm) of potassium titanyl phosphate single crystals. Analysis of the power spectral density function of surface roughness showed that cluster-ion irradiation leads to removal of residual chemical contaminations from the subsurface layer and produces additional smoothing of the crystal surface in a broad range of spatial frequencies ($\nu$ = 0.2–100 $\mu$m$^{-1}$).