Permittivity-asymmetric qBIC metasurfaces for refractive index sensing.

Area of Science:

• Photonics and Metasurfaces
• Optical Sensing Technologies
• Nanophotonics

Background:

• Bound states in the continuum (BICs) offer superior light confinement but are sensitive to symmetry breaking.
• Quasi-BICs (q-BICs) enable high-quality factor resonances crucial for refractive index (RI) sensing.
• Existing geometric asymmetry methods in q-BICs limit sensing response to spectral shifts.

Purpose of the Study:

• To develop a novel permittivity-asymmetric q-BIC (ε-qBIC) metasurface for enhanced RI sensing.
• To demonstrate an RI sensing mechanism that modulates both resonance wavelength and intensity.
• To improve the linearity and robustness of q-BIC-based RI sensors.

Main Methods:

• Fabrication of an ε-qBIC metasurface.
• Characterization of resonance shifts and modulation variations with changing RI.
• Numerical simulations to analyze optical restoration of geometry-asymmetric q-BICs (g-qBIC) to BIC conditions.

Main Results:

• The ε-qBIC metasurface achieved a transmittance sensitivity of ~5,300%/RIU.
• The linear response window of the ε-qBIC was 104 times larger than that of g-qBIC.
• Numerical results showed environmental permittivity asymmetry can restore g-qBIC to ultra-high-Q states (>10^7), approaching BIC.

Conclusions:

• The ε-qBIC metasurface provides a significantly improved linear response for RI sensing, enabling more reliable single-wavelength readout.
• This design offers a new strategy for developing high-performance photonic sensors.
• The concept of permittivity-restored BICs opens new avenues for tunable photonic devices.