Silicon nitride slotted-disk metasurfaces for refractive index sensing based on quasi-bound states in the continuum

Recent advances in nanophotonics have opened new pathways for developing optical biosensors with exceptional sensitivity and tunability. Dielectric metasurfaces based on quasi-bound states in the continuum (q-BICs) are of particular interest as they offer potential for high quality factor resonances and large near-field enhancement. It is demonstrated that a transparent all-dielectric Si₃N₄ metasurface, comprised of meta-atoms with an asymmetric slotted-disk geometry, supports a symmetry-broken q-BIC resonance. The q-BIC resonance can be tuned across a broad spectral range from the ultra-violet to the near-infrared. By adjusting the offset and width of the slot, the Q-factor, bulk refractive index sensitivity and figure of merit can be systematically controlled. Large electric field enhancements can be achieved and exploited for sensing applications. Fabricated metasurfaces confirm the q-BIC resonance and high Q-factor at sub-micron wavelengths. A Q-factor of 204, sensitivity of 118 nm/RIU and figure of merit of 33 are experimentally observed for a 50 μm × 50 μm array with a q-BIC resonance at 727 nm. Numerically it is shown that the sensitivity can increase to 127 nm/RIU and the figure of merit to 35000 for low-loss larger arrays based on a slot width of 45 nm which can be easily fabricated. These findings establish the Si₃N₄ slotted-disk metasurface as a robust CMOS-compatible platform for next-generation, high-performance optical sensors.