Interferometric sensors based on surface plasmon resonance

A common-path, heterodyne interferometric system for studying the phase variations under surface plasmon resonance (SPR) is presented. The reflected beam from SPR is further going into a total internal reflection device (TIR) for increasing the sensitivity by the phase shift between TE-wave and TM-wave, as described by the Fresnel's equation. With the combination of a SPR prism and a TIR prism, the system can avoid the change of direction in the output light, which is always happened when only a SPR prism has been used. An unaltered output light is convenient for the detection devices. The system utilizes a pair of orthogonally linearly polarized beams with heterodyne frequency of 60 kHz as the light source. They are perfectly collinear so that the noises resulting from the ambient conditions are greatly reduced. Compared with the technique of reflectivity variation measurement, which is widely used in traditional SPR, the phase variation measurement using common-path, heterodyne techniques is estimated to be higher in sensitivity and thus can be used as a high-sensitivity-demanded biosensor.