Waterproof coating on nonlinear optical crystal surfaces by photo-oxidation of silicone oil
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Waterproof coating on nonlinear optical crystal surfaces by photo-oxidation of silicone oil
Nobuhiro Sato1, Yuji Sato2, Yoshiaki Okamoto3 and Masataka Murahara2 1 Department of Electrical and Electronics Engineering, Tokai University 1117 Kitakaname, Hiratsuka, Kanagawa, 259-1292, Japan 2 Entropia Laser Initiative, Tokyo Institute of Technology 2-12-1, O-okayama, Meguro-ku, Tokyo 152-8552, Japan 3 Okamoto Optics Co. 8-34 Haramachi, Isogo-ku, Yokohama, Kanagawa, 235-0008 Japan ABSTRACT In general, nonlinear-optical crystals are used as a wavelength conversion element for laser. The crystals are, however, deliquescent and absorb moisture in air, which causes clouding. They need to be heated in the oven for humidity control. The development of a protective film, which is moistureproof and transmits UV rays, is expected. We have, therefore, developed a waterproof and hard protective coating on a KH2PO4 (KDP) surface at room temperature by photo-oxidation of silicone oil using a Xe2 excimer lamp. A test of waterproofing was conducted for the treated and untreated KDP crystals. The results showed that the untreated crystal completely dissolved in water thirty minutes after immersing, whereas the coated KDP crystal has neither dissolved nor clouded for three months since immersing in water.
INTRODUCTION In general, a fused silica is cooled to vitrify after melting crystal powder at high temperatures above 2000 degrees centigrade. The fused silica is regarded as promising since it has a low refractive index, a high dielectric constant, and VUV permeability. Therefore, it is used as a protective film for a mirror, lens, and crystal. Mahajan et al. deposited a SiO2 film on the silicon wafer by Plasma-enhanced Chemical Vapor Deposition (PECVD) method using tetraethoxysilane (TEOS) in O2 atmosphere when maintaining the substrate temperature at 300 degrees centigrade [1]. Dennler et al. also deposited a SiO2 film on the polyimide substrate by PECVD method in the presence of the gases, which consists of 1:9 mixture of hexamethyldisiloxane (HMDSO) and O2 [2]. On the other hand, Orlowski et al. produced a SiO2 film on the silicon wafer by irradiating a XeCl excimer laser (308 nm) in the oxygen atmosphere (1 atm) when maintaining the substrate temperature at 400 degrees centigrade [3]. Koichi et al. formed the 200 nm and 1000 nm thick SiO2 films on the silicon wafer by laser ablation in O2 atmosphere or in the presence of O2 and H2 mixture with the chamber temperature of 1000 degrees centigrade [4]. These methods require the substrates
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at high temperatures above 300 degrees centigrade, which damages the substrates. In order to prevent the damages, Hozumi et al. deposited a SiO2 film on the PMMA or single crystalline Al2O3 substrate by Chemical Vapor Deposition (CVD) method of a Xe2 excimer lamp using TEOS at low temperature [5-8]. Boyer et al. formed a SiO2 film on the silicon substrate, of which temperature changes from room temperature to 600 degrees centigrade, by Laser-induced Chemical Vapor Dep
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