Advances in optical ring resonator based bio/chemical sensing

Optical ring resonators emerge as a new class of evanescent label-free sensors that have been extensively explored in recent years. Optical ring resonators have a variety of configurations, including microspheres, planar ring resonators, microtoroids, and capillary based opto-fluidic ring resonators (OFRRs), as shown in Fig. 1 [1-8]. In a ring resonator, the light propagates in the form of whispering gallery modes (WGMs) or circulating waveguide modes, which result from total internal reflection of light along the curved boundary between the high and low refractive index (RI) media. The WGM has the evanescent field present at the ring resonator surface and responds to the binding of biomolecules. In contrast to the straight waveguide, the effective light-analyte interaction length of a ring resonator sensor is no longer determined by the sensorpsilas physical size, but rather by the number of revolutions of the light supported by the resonator. Consequently, despite its small physical size, a ring resonator can deliver sensing performance similar or superior to a waveguide while using orders of magnitude less surface area and sample volume. Furthermore, due to the small size of ring resonators, high density sensor integration becomes possible.