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.