Multi‐Band Polarization‐Independent Quasi‐Bound States in the Continuum Based on Tetramer‐Based Metasurfaces and Their Potential Application in Terahertz Microfluidic Biosensing

Abstract Quasi‐bound state in the continuum (quasi‐BIC) has drawn increasing attention in terahertz biosensing and filtering owing to an extremely narrow linewidth and ultra‐high quality factor. However, it is usually polarization‐sensitive due to broken structure or incidence symmetry and mainly holds for polarization‐dependent single or dual bands, which sets a limitation on practical applications. Here, this article proposes and experimentally demonstrates terahertz multi‐band polarization‐independent quasi‐BICs in tetramer‐based metasurfaces by using the laser etching method. Employing a tetramer cluster with C4v symmetry as the structural unit, which consists of four L‐type copper particles on a flexible and optically‐transparent substrate, the metasurfaces can support dual‐band or tri‐band polarization‐independent quasi‐BICs after performing collective perturbation, specific displacement, or both to the constituent particles. Furthermore, the metasurfaces are integrated with microfluidic channels to explore their potential application in terahertz biosensing. Experimental results indicate that when the concentration of bovine serum albumin solutions is below 4 mg mL −1 , both resonance modes of the dual‐band terahertz microfluidic biosensor exhibit a sensitivity of ≈12 GHz mg −1 mL each, and the detection limits reach 0.17 mg mL −1 . This work provides a general and simple strategy to construct metasurfaces supporting multi‐band polarization‐independent quasi‐BICs, which have promising applications in biosensing, filtering, and lasing.