Sensitivity enhancement based on application of multi-pass interferometry in phase-sensitive surface plasmon resonance biosensor

A novel design of multi-pass surface plasmon resonance (SPR) biosensor with differential phase interrogation based on multi-pass interferometry is presented. This new configuration provides an intrinsic phase amplification effect of over twofold by placing the SPR sensor head in a signal arm of the interferometer so that the interrogating optical beam will traverse the sensor surface infinite number of times. Experimental interferometers based on the Michelson and Fabry–Perot configurations have been employed to experimentally verify this amplification effect when they were compared with Mach–Zehnder configuration, results obtained from salt–water mixtures, antibody–antigen, and protein–DNA binding reaction confirmed the expected phase measurement enhancement, thus leading to the possibility of direct detection of small sized bio-molecules using SPR.