Resonant chiroptical effects in all-dielectric metasurface with monoclinic lattices for terahertz sensing applications

Over recent years, chiral metasurfaces have emerged as pivotal platforms for ultrasensitive biosensing, particularly in detecting and identifying chiral biochemical samples. However, the excitation of chiroptical responses in conventional chiral metasurfaces predominantly rely on intricate asymmetric meta-atoms or elaborate geometric configurations, which complicate fabrication process and limit scalability. Here, we report an all-dielectric chiral metasurface consisting of an array of circular air-hole in monoclinic lattice. By breaking in-plane mirror symmetries through introducing side-length differences δ and changing lattice angle θ, the proposed metasurface achieves strong intrinsic chirality, yielding an enhanced circular dichroism (CD) signal with a simulated peak value of 0.89 at 0.254THz and a quality factor of ~50.4. Theoretical analyses of induced electromagnetic fields reveal that the observed chiroptical responses originate from spin-selective multipole excitations and localized field enhancements, where the left-handed circularly polarized (LCP) waves exhibit significantly stronger optical chirality (C_E=251) than that of the right-handed circularly polarized (RCP) waves. Experimental validations based on the fabricated sample and terahertz characterization confirm the excitation of resonant CD signals with peak value of ~0.52 around 0.25THz. Furthermore, we demonstrate practical biosensing capabilities of the proposed chiral metasurface by detecting bovine serum albumin (BSA) samples with different concentrations, which manifests good sensing performances including high sensitivity (0.023(mg/mL)^(-1)) and wide detection range (0.3-50 mg/mL). This work paves a new avenue for designing chiral metasurfaces with simple structures but high-performance, advancing practical applications in label-free biosensing and chiral nanophotonics.