We report a theoretical investigation of a surface plasmon resonance (SPR)-based acoustic sensor for optical detection of ultrasound. The structure being studied is arranged in the Krestchmann configuration and the detection is performed by observing the change of refractive index of water next to the SPR metal. The acoustic pressure is simulated using COMSOL. The simulation results illustrate an insight into mechanism of pressure variation on the surface of SPR sensor due to a constructive interference of the ultrasound. This leads to a local refractive index change of water. The local refractive index change is calculated by converting the incident pressure to water density using IAPWS-95 formulation. Then, the water density is converted to the refractive index using Lorentz-Lorenz formulation. Here we report the change in the refractive index of the water to pressure, dn/dp, which is calculated to be 1.4 x 10-10 Pa-1, which is very close to the dn/dp reported by M. W. Sigrist 1986. We also investigated the effect of temperature and wavelength on the dn/dp and found that the variation in temperature and wavelength does not show any significant effect on the dn/dp relationship. We also discuss the effect of quality factor (Q) and possible improvements to enhance the sensitivity of SPR-based acoustic sensor.
Modeling refractive index change in writing long-period fiber gratings using mid-infrared laser radiation
