Investigation into Ethanol Concentration Changes of Ethanol-Water Solution Using Polarization Property Associated with Surface Plasmon Resonance in Aluminum Grating in Conical Mounting

We experimentally investigated the changes in the ethanol concentration of an ethanol-water solution using a polarization property of the (specularly) reflected light associated with surface plasmon resonance (SPR) occurring in an aluminum diffraction grating in a conical mounting. The polarization property, which is observed as a rapid change in the normalized Stokes parameter s(3) of the reflected light, can be successfully applied to refractive index sensing: the zero-crossing point of s(3) precisely and easily determines the resonance angle theta(sp) at which SPR occurs; s(3) in the vicinity of theta(sp) varies largely in response to a small change in the refractive index of a sample. Using the features of the rapid change in s(3), we examined the ethanol concentration changes over a wide range from 0 to 100 percent by weight (wt%) through the measurement of theta(sp). Furthermore, we detected a small amount of ethanol component in water through the measurement of s(3) under the angle of incidence fixed at the resonance angle of pure water.