Impurities in synthetic fluorite for deep ultraviolet optical applications

Abstract

The impurity content of synthetic calciumdifluoride (fluorite, CaF2) single crystals produced for optical applications in the deep ultraviolet (DUV) and infrared (IR) spectral regions are investigated by luminescence and absorption spectroscopy. Lead, oxygen, and rare earth (RE) ions, namely, trivalent (Ce3+, Pr3+, Sm3+, Eu3+, Gd3+, Tb3+, Dy3+, Ho3+, Er3+, and Yb3+) and divalent (Eu2+) are identified as common impurities. RE ions are dominating impurities in the IR grade sample and the impurity concentration for this sample is found to be about 100 times higher than the one for DUV samples. Due to the strongly reduced RE luminescence background, we additionally identify lead and oxygen in DUV grade crystals. Remarkable differences in the luminescence spectra are found when comparing DUV grade crystals from different sources.