Resolution of a grating-based plasmon index sensor with efficiency-alone interrogation

High-resolution measurement of refractive index has a wide range of application and employment of the plasmon resonance absorption combined with phase detection is a promising way to achieve 8-digit resolution. However, because the phase detection in light-wave frequency is not simple, we would like to report on a method of refractive-index measurement by using efficiency-alone interrogation. The basic idea comes from the fact that the TM-wave diffraction efficiency of a metal grating placed in conical mounting can be extremely small at the resonance angle (the polar angle of incidence at which the resonance absorption occurs). We have found in our previous works that (1) there is an appropriate azimuth (a parameter of the conical mounting) determined by a rough value of the index; (2) the TM efficiency at the resonance angle can be 10^-8 or less. Hence, if we use a rotating stage with an instrumental resolution of 1/4000 degrees in finding the resonance angle, the expected resolution in refractive index would be a little less than 7-digit.