Abstract
For the application of F<sub>2</sub> lasers to micro-/nano-fabrication including photolithography, one of the important research subjects is to eliminate the chromatic aberration, and combination of lenses made of two optical materials with different refractive indices is the effective way. However, only CaF<sub>2</sub> is widely applied to lenses for a vacuum ultraviolet (VUV) beam, and a "second material" to be paired with CaF<sub>2</sub> is strongly requested. Here, we examined two fluoride crystals which are transparent in the VUV region, BaF<sub>2</sub> and LiCaAlF<sub>6</sub> grown by the Czochralski technique, as candidates for VUV optics. We investigated the change of the VUV transmittance and the onset of optical damage of these fluoride crystals against F<sub>2</sub> laser irradiation. These crystals showed good optical tolerance against cumulative F<sub>2</sub> laser irradiation, and the damage threshold of LiCaAlF<sub>6</sub> was similar to that of VUV grade CaF<sub>2</sub> while that of BaF2 was about half of the other two fluoride crystals.
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