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

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 ${\mathrm{CaF}}_2$ 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, ${\mathrm{Ce}}^{3+},$ ${\mathrm{Eu}}^{2+}$ (blue emission), or ${\mathrm{Tb}}^{3+}$ (yellow-green emission). The PL of ${\mathrm{Ce}}^{3+}$ and ${\mathrm{Eu}}^{2+}$ had no correlation with color-center formation; however, intense absorption bands arising from an ${\mathrm{F}}^{-}$ center associated with a trivalent yttrium ion (YFC) were necessarily induced in the samples that showed yellow-green PL of ${\mathrm{Tb}}^{3+}$ under irradiation by ArF excimer laser light. When the yellow-green fluorescent samples were irradiated with ArF excimer laser light, the PL intensity of ${\mathrm{Tb}}^{3+}$ 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 ${\mathrm{Tb}}^{2+}$ ions by ArF excimer laser irradiation, and the resulting electrons were trapped at the site of a fluorine vacancy neighboring on ${\mathrm{Y}}^{3+}.$ Therefore the intensity of yellow-green PL of ${\mathrm{Tb}}^{3+}$ is proposed as an index for diagnosis of resistance to ArF excimer laser damage in ${\mathrm{CaF}}_2$ single crystals.