Ultrasensitive Refractive Index Sensing Based on the Quasi‐Bound States in the Continuum of All‐Dielectric Metasurfaces

Abstract Symmetry‐protected quasi‐bound states in the continuum (BIC) controlled by metasurfaces with broken in‐plane symmetry are widely exploited to achieve highly surface‐sensitive and spectrally sharp resonances for nanophotonic biosensors. Through the engineering of silicon‐based asymmetric nanobar pairs, a quasi‐BIC mode is excited showing a dominant toroidal dipole (TD) and electric quadrupole (EQ) resonant feature in the near‐infrared and performs ultrahigh sensitivity in the refractometric monitoring of local environment changes. Contrary to the typical electric and magnetic Mie‐type resonances of dielectric resonators with the enhanced field mostly inside the resonator volume, the TD‐EQ quasi‐BIC mode is found to exhibit strong and tightly confined optical fields at the surface of tilted nanobar pairs, and its refractive‐index (RI) sensitivity can be dramatically increased for nanopillars with larger aspect‐ratio. The measured (simulated) sensitivity and figure of merit for nanobar pairs with a height of 450 nm reach 608 nm/RIU and 46 (612 nm/RIU and 85), respectively. Such ultrahigh sensitive all‐dielectric platform can be fabricated through complementary metal‐oxide‐semiconductor compatible process and is promising for on‐chip integration and sensor miniaturization to a wide range of diagnostic applications.