Organic silicone oil was photo–chemically oxidized to change into SiO2 by using a ultraviolet (UV) excimer lamp, which developed an over–coating for a nonlinear optical crystal.
In general, nonlinear optical crystals are used as a wavelength conversion element for laser. The crystals are, however, deliquescent and absorb moisture in the air, which causes clouding. They need heating in the oven to prevent it. It is desired to develop a protective film on the nonlinear optical crystals so as to be waterproof and transmit UV rays. We have, then, developed the waterproof and protective film on a KH2PO4 (KDP) surface, which was photo–chemically oxidized by using a silicone oil and Xe2 excimer lamp. The silicone oil was spin–coated on the KDP crystal surface, and then the Xe2 excimer lamp was vertically irradiated to the crystal surface. The O atoms in the air were photo–excited with the UV photon to generate high active O atoms. At the same time, the C—H and the Si—C bonds of the silicone oil were photo–dissociated because the photon energy of the UV photon is higher than the bonding energies of the C—H and Si—C. The siloxane of the silicone oil was consequently linked with the O2 that had been absorbed on the crystal surface to form SiOn. As a result, the SiO2 film was formed on the KDP crystal surface by the photo–oxidation of silicone oil.
The infrared spectroscopy analysis (ATR-FTIR) was carried out to investigate the modified film. The results showed that as the time of lamp irradiation extended, the absorption peak of the —CH3 groups at 2900 cm−1 decreased, but the absorption peak of the —SiO groups at 1050cm−1 increased. At the lamp irradiation time of 90 minutes, the UV transmittance of the treated silicone oil improved to 88.9 % at the 260 nm, which is the wavelength of the fourth harmonic generation, while that of the untreated silicone oil was 56 %.
The treated and untreated KDP crystals were tested for their waterproofing. The untreated crystal was completely dissolved thirty minutes after soaking in the water, and on the contrary, the KDP crystal with the film coating has neither been dissolved nor gotten cloudy for three months when soaking in water.