Study on detection of contamination of pure water using silica microsphere

The adsorption of nanoparticles on the surface of a whispering-gallery mode (WGM) cavity causes the resonant wavelength to shift because it changes the effective refractive index of the cavity material. By taking advantage of this effect, WGM cavities can be used for such applications as the highly sensitive detection of nanoparticles and label-free sensing of bio-molecules [1]. The use of an ultrahigh-Q microcavity for single molecule detection has already been reported [2]. In this study, we demonstrate the detection of the contamination of pure water using a silica microsphere. Pure water contamination is usually detected by monitoring its conductance or measuring the residual silica concentration. However, here we conduct a preliminary study on the detection of contamination in water by monitoring the resonant wavelength shift of a WGM microcavity. We observed a large difference in the wavelength shift of the silica microsphere depending on whether we placed the cavity in city water or pure water.