2D Attogram Surface Plasmon Resonance Imaging
1st August 2014

Evanescent Wave Cavity Ring-Down Spectroscopy

Recent work at Exeter has started to investigate the optical properties of gold nanoparticles, in particular localised plasmons, to extend the sensitivity of SPR when interrogated by e-CRDS. Cavity ring-down spectroscopy (CRDS) is an ultra-sensitive absorption spectroscopy that has been used to detect molecules in the gas phase. Evanescent wave cavity ring-down spectroscopy (e-CRDS) is a new technique applied to interface photonics at Exeter, incorporating a new free-running cavity configuration in which we have demonstrated direct optical extinction detection with sensitivity in excess of 10-7, compared with bench-top sensitivities of order 10-3.

Two high reflectivity mirrors (99.995%) are placed optically opposite one another to form a stable optical resonator. Radiation from a cw broadband laser is admitted to the cavity allowing the radiation intensity within the cavity to rise with a ring-up time, τ, characteristic of the quality or Q-factor of the cavity. The radiation reaches a maximum level in the cavity within a few μs and the laser is switched off. The radiation in the cavity rings down with the same time constant, τ. A total internal reflection (TIR) prism is introduced into the cavity creating an evanescent wave that penetrates into the medium above the prism. The intensity of the evanescent field decays exponential with distance from the surface depending, on angle of incidence and the ratio of the refractive index of the glass and the medium above the prism (typically 200 nm). Plasmon particles in a nano-fabricated structure within the penetration depth will cause an extinction of the radiation and create an additional loss within the optical cavity that causes a decrease in the ring down time. The internal calibration of the time scale enables an absolute measure of the extinction to be determined also it is not sensitive to fluctuations in the laser source. Minimum extinction changes of order 5 x 10-6 are possible routinely.