Ultra-sensitive refractive index sensing enabled by accidental bound states in the continuum on ultrathin dielectric grating metasurfaces.

We propose a suspended high-contrast grating metasurface for refractive index sensing, composed of silicon bars and air slots, with an ultrathin thickness of less than one-tenth of the operating wavelength. The grating geometry is designed to excite a quasi-accidental bound state in the continuum (quasi-A-BIC) by manipulating the coupling and interference of four symmetric waveguide-array modes (TM-TM). This quasi-A-BIC achieves a high factor of 10, enabling significant field enhancement on the metasurface. As a result, the normalized index-change sensitivity is improved to 0.99, approaching the theoretical upper limit. By tuning the grating's duty cycle, the electric field penetration into the surrounding medium can be flexibly adjusted, ranging from 0.5 to 30 times the operating wavelength. This tunability enhances the device's versatility, making it suitable for various applications such as biomedical sensing and environmental gas detection. Furthermore, the effects of material dielectric loss, refractive index contrast, and finite grating size are analyzed, revealing a moderate impact on the factor but negligible influence on index-change sensitivity. Notably, as the incident angle varies from 0° to approximately 90°, the A-BIC persists, exhibiting a wide-range redshift in its resonant wavelength across more than one octave (> 67% fractional bandwidth), while maintaining stable sensitivity. This unique behavior highlights the inherent potential for broadband fingerprint spectroscopy applications.