Optical cavity-enhanced surface plasmon resonance refractive index sensing

A novel type of high-sensitivity Surface Plasmon Resonance (SPR)-based refractive index sensor is demonstrated, with a Kretschmann prism-coupling setup enclosed in an optical cavity and interrogated by a telecom wavelength laser source. Two thin layers of Au and SiO2 are deposited on the prism (SPR chip). Analogously to the sensors commonly referred as "Intensity modulated SPR sensors", in the presented setup refractive index variations of the sample induce a shift of the SPR central wavelength, which in turn leads to a variation of the SPR chip reflectivity. Since the SPR chip is one of the cavity mirrors, the corresponding loss can be sensitively determined by measuring the internal photon lifetime by a cavity ring-down technique [1]. The interrogating laser is periodically shut down and the subsequent exponential decay of the on-resonance transmitted intensity is recorded to retrieve the ring-down time. In order to improve the repetability of the measurement and allow for long-term averaging, the laser is frequency locked to a cavity resonance by means of the Pound-Drever-Hall scheme[2]. The setup is sketched in fig.1