Recent advances in integrated photonics technologies have enabled miniaturized on-chip optical devices to be used as sensitive chemical and biological molecular sensors. In particular, optical resonators offer a preferred device platform for sensing applications as they claim simultaneously small footprint, strong optical confinement, and resonantly enhanced light-molecule interactions, attractive features for molecular sensing. To date, optical resonators are predominantly fabricated in semiconductor or dielectric materials with low optical loss to ensure high Q (quality factor) performance. Since most resonator sensors rely on evanescent optical interactions to detect the presence of molecules, engineering optical field distribution to maximize field overlap with the sensing region without compromising the low-loss optical performance is an important design consideration for these devices.

In their paper in Applied Optics entitled “Silicon hybrid plasmonic microring resonator for sensing applications”, Zhang et al. present a new sensor design based on hybrid dielectric-plasmonic resonator. The design consists of a cylindrical metal wire ring-shaped cavity sitting on top of a concentric silicon micro-ring resonator, both of 5 μm radius. The whispering galley mode supported by the device, formed through hybridization of the dielectric strip waveguide mode and the plasmonic wire mode, has a considerable fraction of optical field residing outside the semiconductor and metal structure. Consequently, a high refractive index sensitivity (defined as the ratio of wavelength shift over the refractive index change in the sensing region) of over 500 nm/RIU (refractive index unit) result, according to the authors’ theoretical models. This refractive index sensitivity figure represents a significant improvement compared to other resonator technologies such as silicon-on-insulator or silicon nitride devices, which generally exhibit refractive index sensitivity figures of less than 200 nm/RIU.

In addition to the high refractive index sensitivity, the hybrid resonator also claims a small mode volume of less than 15 μm³ and a comparatively high quality factor of about 600. It is anticipated that the device design may also find interesting applications such as nonlinear optics, which can also benefit from the tight optical confinement and resonant enhancement effects.

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