ArF excimer laser irradiation effects in AlF3-based fluoride glasses for vacuum ultraviolet optics

Color center formation in AlF3–YF3–RF2 (R=alkaline earth metal) glasses doped with PO2.5 (0–3 mol %) by irradiation with ArF excimer laser light was examined by optical absorption (vacuum ultraviolet–visible) and electron paramagnetic resonance (EPR) spectroscopies. Concentrations of impurity iron in all the glasses and oxygen in the P-free samples were ∼0.2 and ∼450 ppm, respectively. Optical absorption ranging from 2–8 eV was induced via one-photon absorption processes. The optical band dominating the transmission loss at 193 nm has a peak at 5 eV in the P-free glasses or a peak at 6.9 eV in the P-doped glasses. The origins of the 5 and 6.9 eV bands were tentatively ascribed to an oxygen-related hole center giving an EPR signal with a width of 7.2 mT at g=2.0097 and P E′ center giving a hyperfine doublet with a separation of ∼70 mT. Provided these correspondences, then the values of oscillator strength for the 5 and 6.9 eV bands were calculated as ∼0.15 and ∼0.1, respectively. No formation of color centers associated with intrinsic constituents such as Al, Y, R, and F ions, was observed. The present results suggest that the reduction of impurity oxygen content is a route to effectively suppress the solarization in the P-free AlF3-based glasses.