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

摘要

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 CaF_2 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, Ce~(3+), Eu~(2+) (blue emission), or Tb~(3+) (yellow-green emission). The PL of Ce~(3+) and Eu~(2+) 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 Tb~(3+) under irradiation by ArF excimer laser light. when the yellow-green fluorescent samples were irradiated with ArF excimer laser light, the PL intensity of Tb~(3+) and the induced absorption that was due th 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 Tb~(2+) ions by ArF excimer laser irradiation, and the resulting electrons were trapped at the site of a fluorine vacancy neighboring on Y~(3+). Therefore the intensity of yellow-green PL of Tb~(3+) is proposed as an index for diagnosis of resistance to ArF excimer laser damage in CaF_2 single crystals.