The formation of metallic nanoparticles in single crystal CaF2 under 157 nm excimer laser irradiation

Single crystal calcium fluoride (CaF2) is an important material for vacuum-ultraviolet optical components. Unfortunately, all metal halides tend to form defects when exposed to energetic particles and laser radiation, and these defects can degrade optical performance. Here we examine the consequences of exposing CaF2 to 157 nm excimer laser radiation and show that several tens of thousands of pulses at fluences near 1 J/cm(2) can color the material. Absorption spectra of the exposed material confirm the formation of metallic calcium nanoparticles similar to those produced by other forms of energetic radiation. The rate of nanoparticle formation depends on the bulk temperature and displays a local maximum near 50 degrees C. Absorption measurements at 157 nm display a transient absorption component that grows during prolonged irradiation and disappears on time scales of several minutes after irradiation ceases. The implications of these effects in optical components are discussed.