Nanoscale refractive index sensor with ultrahigh figure of merit based on toroidal dielectric metasurfaces

Dielectric metasurfaces exhibiting low intrinsic loss and strong localized field are an emergent platform in light manipulation and sensing. Toroidal dipole, a new eigenmode distinct to the common electric and magnetic multipoles, expands the toolbox of nonlinear optics, filters, and multi-channel biosensors. Here, we propose a dielectric metasurface with two semicircle disks, which supports the high Q-factor toroidal dipole for the refractive index sensing. The performance of toroidal dipole is sensitive to the parameters of two semicircle disks. Via optimizing the geometrical parameters, we realize a Q-factor up to 69000. Moreover, the corresponding sensitivity and figure of merit can reach up to 70nm/RIU and 2970/RIU, respectively. The figure of merit is substantially higher than the previous works in near-infrared region. The enhancement of electric field located in the gap of two semicircles enables the strong interaction of light and the analyte. The introduction of the analyte will introduce the redshift of the resonant frequency of toroidal dipoles. Our findings expand the scope of toroidal dipoles and may serve as a new platform for high sensitivity sensing.