Water‑associated surface degradation of CsLiB6O10 crystal during harmonic generation in the ultraviolet region

3+ doped YLiF4 laser (4.7 eV was linearly proportional to the concentration of hydroxyl (OH groups in the CLBO crystal. In this study the formation of OH groups was recognized by infrared absorption spectroscopy and a method for determination of OH groups was proposed. The concentration of OH groups in the CLBO was linearly proportional to water vapor pressure and the CLBO containing OH groups absorbed UV light. We propose that one of the main mechanisms of surface degradation is the generation and growth of the mutants and that these mutants are generated by a UV photon‑assisted reaction via OH group formation associated with absorbing water molecules into CLBO crystal. It has recently been reported that the eighth harmonic (193 nm of the output of an Er‑doped fiber amplifier (1547 nm was generated by using CLBO crystal with 7% of the conversion efficiency. 5 Since more efficient conversion was desirable to generate the intense 4ω light the bulk laser‑damage threshold of CLBO crystal was investigated experimentally at a higher pulse energy. Surface finishing for the CLBO crystal by radiofrequency (rf plasma etching was improved to prevent the surface damage. 67 Even when the pulse energy was two or three orders of magnitude less than the damage threshold gradual degradation of the 4ω power occurred in our laboratory. The mechanism of the degradation seemed not to be established and was believed to be due to optical and mechanical stresses introduced by crystal hydration 4 or the accumulat ion of the surfa ce laser damage by the impact of high‑energy (UV laser light. Our laboratory has empirically shown than the hydration or reactivity of CLBO with water is extreme and its optical absorption rapidly changes. Moreover not only the UV beam spot on the exit surface of the crystal but also other parts of the surface were mutated and whitened in color as though the CLBO surface had been exposed to the air for a long time. We t herefore tentati vely hypothesized that the degradation would be associated with water vapor in the ambient atmosphere. To prove our hypothesis we decided to investigate the relationship between the surface damage and water vapor. However there is no existing report showing quantitative data for the solubility of water in CLBO crystal. Infrared (IR absorption spectroscopy is a useful technique to determine water contents for crystals and glasses. 8-11 In this study we confirmed that hydroxyl (OH groups are formed by the reaction of CLBO with water and established a conventional method for determining the relative concentration of OH groups in CLBO crystal taking account of the anisotropic structure of the crystal by means of a Fourier transform IR (FTIR spectrometer. We w ill s how t he d ependence o f O H g roups' concentration in CLBO crystal on water vapor pressure and/or temperature. Degradation rates during the 4ω generation by th e CLBO crystals were measured to examine the dependence of surface roughness. The a