A 3-D Building Block Shape Refractive Index Sensor Based on Quasi-Bound States in the Continuum

In the all-dielectric metasurfaces, leveraging high-Q resonance excited by quasi-bound states in the continuums (Q-BICs) holds significant promise for refractive index sensing. These metasurfaces introduce broken geometric symmetry to effect the transformation from bound states in the continuums (BICs) to Q-BICs. Typically, metasurface sensors focus on studying sensing performance through 1-D or 2-D changes in the XY-direction, often overlooking the impact of the Z-direction on Q-BICs excitation and sensing capabilities. In this work, we design a building block-like metasurface to induce Q-BICs excitation by breaking both structural and angular symmetries. We explore the influence of structural changes in the metasurface along the X, Y, and Z-axes on the spectra of excited Q-BICs, while also investigating the spectral characteristics of the metasurface in 3-D structures. Through simulations of two different Q-BICs modes for metasurfaces, we find that under asymmetric Q-BICs structures, the sensor exhibits a sensitivity of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${S} =184.29$ </tex-math></inline-formula> nm/RIU, with a minimum full width at half maximum (FWHM) of 0.0014 nm and a quality factor of FOM <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\approx 1.32\times 10^{{5}}$ </tex-math></inline-formula>. Similarly, when considering angle-asymmetric Q-BICs, the sensor demonstrates a sensitivity of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${S} =168.37$ </tex-math></inline-formula> nm/RIU, with a minimum FWHM of 0.00021 nm and a quality factor of FOM <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\approx 8.02\times 10^{{5}}$ </tex-math></inline-formula>. The linearity is <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${R} ^{{2}} =0.99982$ </tex-math></inline-formula>, showcasing excellent sensing properties across both excitation modes. These findings significantly contribute to the application of BICs in refractive index sensors and hold great potential for advancing biosensing research. Moreover, they offer valuable insights for designing and developing metasurfaces based on 3-D structures.