Time-resolved photoluminescence for diagnosis of resistance to ArF excimer laser damage to CaF_2 single crystals

Abstract

Time-resolved photoluminescence (PL) on excitation with an ArF excimer laser and IR–vacuum UV optical absorption induced by irradiation of an ArF excimer laser of commercially available CaF2 single crystals fabricated by four different manufacturers were measured. Intrinsic PL that was due to self-trapped excitons generated by band-to-band excitation by two-photon absorption processes was observed in all samples. In addition, most samples showed PL originating from traces of lanthanoid (Ln) impurities, Ce3+,Eu2+ (blue emission), or Tb3+ (yellow-green emission). The PL of Ce3+ and Eu2+ had no correlation with color-center formation; however, intense absorption bands arising from an F- center associated with a trivalent yttrium ion (YFC) were necessarily induced in the samples that showed yellow-green PL of Tb3+ under irradiation by ArF excimer laser light. When the yellow-green fluorescent samples were irradiated with ArF excimer laser light, the PL intensity of Tb3+ and the induced absorption that was due to YFC increased with the irradiated pulse number, and their growth was found to conform to a first-order kinetics. It was concluded that photoelectrons were released from Tb2+ ions by ArF excimer laser irradiation, and the resulting electrons were trapped at the site of a fluorine vacancy neighboring on Y3+. Therefore the intensity of yellow-green PL of Tb3+ is proposed as an index for diagnosis of resistance to ArF excimer laser damage in CaF2 single crystals.