Surface-plasmon resonance sensing with cavity-enhanced methods
Optical surface-plasmon resonance (SPR) sensors are nowadays a well-established tool for chemical and biological analysis. The readout of state-of-the-art instruments is commonly based on spectroscopic and interferometric techniques as well as direct power measurements. However, for the best reported SPR sensors for refractive index (RI), improvements beyond the resolution limit of 10<sup>−7</sup> RIU do not seem to be possible with these conventional measurement schemes. One of the main limiting factors is represented by the conversion of light intensity fluctuations into noise on the sensor output signal.
We have proposed a novel approach where an SPR chip serves as a variable-reflectivity mirror in an optical resonator. In this system, any RI variation occurring on the SPR chip changes the amount of light coupled to the SPR and thus the finesse of the resonator. Therefore, a RI change can be detected by measuring the cavity photon lifetime, which is insensitive to the amplitude fluctuations of the light source. A first experimental proof of concept pointed to a RI resolution of 10<sup>-5</sup> RIU although, with a 1-Hz detection-bandwidth extrapolation, a resolution of 10<sup>-7</sup> is within reach. Furthermore, we investigated the performance of a surface-plasmon-resonance refractive-index sensor based on a polarization-sensitive detection scheme: here the SPR turns refractive index (RI) changes into variations of the cavity birefringence, which thereby is converted by the optical cavity in a shift of its orthogonally-polarized inter-mode distance. Heterodyne beat signals of this distance are possible in such a way that the SPR sensor is trasnformed into an optical RF microbalance. A preliminary theoretical analysis shows that a resolution of 10<sup>-8</sup>-10<sup>-9</sup> RIU is achievable over observation times of 1-30 s. Work is in progress to test detection performance of biomolecules.