Surface plasmon resonance sensor based on the measurement of differential phase

A differential phase detection technique has been used for measuring the optical phase change associated with the surface plasmon resonance (SPR) effect that exists in a glass-metal-dielectric stack. A typical prism-coupled SPR setup was constructed and a Mach-Zehnder interferometer was used to perform interferometric analysis between the two orthogonal polarizations in the exit beam. By stepping the optical phase of the reference arm, one can measure the phase change caused by the SPR effect. Since the reference and signal beams traverse identical optical paths except for the section where phase detection is performed, we expect that this scheme can be more robust in terms of noise immunity. The interrogation area can be enlarged to enable imaging of the SPR sensing surface. Initial phase measurement obtained from a salt-water mixture is presented to demonstrate the operation of this technique. Our SPR sensing scheme provides a refractive index measurement sensitivity of 3.1x10(-6) RIU which is comparable to the value reported for reflectivity-based SPR systems. The measured phase drift, which is a good indicator for the stability of the system, is within 6.6x10(-3) rad/h. Within our experiments, the effect of the incident angle deviated from resonance on the sensitivity of SPR system has also been studied, thus demonstrating that the choice of incident angle is an important factor for achieving highest sensitivity. (C) 2002 American Institute of Physics.