Lossy mode resonance-based method for evaluating the vanadium oxide thin films for fiber optic devices

An original technique for studying the optical characteristics of thin-film coatings made from vanadium oxides during their deposition using vanadyl isopropoxide has been demonstrated. The synthesis was carried out in argon atmosphere at low temperatures ranging from 180 °C to 290 °C. The technique was based on using chemically thinned optic fiber , with thin film coatings applied to the side surface. When a specific film thickness was achieved, a lossy mode resonance (LMR) occurred in the transmission spectrum of the optical pathway. Using real-time monitoring of the deposition process , we studied the shape, depth, and velocity of the resonance spectral motion. We evaluated optical parameters of deposited coatings and growth process characteristics based on LMR features. Plane-parallel plates of silica glass and sapphire were used in the reactor for additional determination of optical properties and coating composition. It was found that the deposited coatings consisted mainly of a mixture of V 2 O 5 and V 6 O 13 oxides, but under some deposition conditions , small amounts of V 2 O 3 , V 3 O 5 , and VO 2 phases were present in the films. It was also found that additional annealing in a hydrogen atmosphere could produce a predominant amount of each phase by changing the temperature and time. Films with a predominance of VO 2 had an electrical resistance jump of 4 orders of magnitude at 69 °C. The presence of VO 2 in optical fiber coatings led to an optical transmission change of up to 17 %, but optimal conditions for forming coatings with high jumps in electrical resistance and optical transmission did not coincide. A sensor of the refractive index of aqueous solutions was demonstrated for the first time on a coating with a dominant V 2 O 5 phase. The maximum sensitivity of the sensors implemented for liquids with a refractive index between 1.331 and 1.429 is equal to 2050 nm/RIU.