A High-Sensitivity Refractive Index Sensor with Period-Doubling Plasmonic Metasurfaces to Engineer the Radiation Losses

Plasmonic nanostructures with strong on-resonance local electric fields at the metal interfaces provide an excellent means for sensing applications. However, most plasmonic resonances suffer from relatively broad bandwidths, mainly due to the intrinsic material dissipation, restricting the optical performances. Moreover, little attention is paid on the radiation loss. In this work, we utilize quasi-guided modes supported by a metallic disk array and demonstrate the manipulation of radiation losses to further improve the resonance property. When a relative angle is introduced between every two adjacent metallic disks in a subwavelength-period array to form a zigzag metasurface, the period doubles leading to a transition of the true guided modes to be flipped above the light cone into the quasi-guided modes with finite yet relatively high-quality factors. We demonstrate the application of these quasi-guided modes with reduced bandwidths as refractive index sensors. Our experiment demonstrates that a refractive index sensitivity as high as 655 nm/RIU can be achieved with this structure, which has a good agreement with the numerical results. This sensitivity outperforms many other plasmonic sensors, e.g., those based on localized surface plasmon resonances or detuned plasmonic resonators. The proposed scheme of using quasi-guided modes as the refractive index sensor provides a prospective platform for biological and chemical sensing.