Spin-separated metalens enables chiral metasurface for trace sensing

• The in-plane symmetry of the chiral metasurface is broken, causing an intensity difference in the circularly polarized cross-transmission component that excites the chiral response. The incorporation of the metalens modulates the optical field distribution of the chiral response. Finally, a high-resolution circular dichroism (CD) spectrum is obtained by extracting the intensity spectrum of the circularly polarized light field output from the cascade device. • The spin-separated metalens incorporates a geometric phase design, enabling opposite phase modulation for orthogonally circularly polarized light. This achieves spatial separation of distinct circular polarizations, effectively eliminating interpolarization crosstalk. • The metalens construct high-intensity localized fields by focusing the optical field, increasing the force in the interaction between light and matter. Metasurface-based biochemical sensing technology holds significant potential for applications in biomedical analysis. Conventional metallic chiral metasurfaces typically utilize a plane wave for polarization detection, which often suffer from limited polarization purity and suboptimal sensitivity. Furthermore, the non-uniform local field distribution inherent to monolayer metallic chiral metasurfaces restricts optical field confinement, thereby compromising the trace detection capability of these devices. To address these limitations, we propose a spin-separated metalens-enabled chiral metasurface sensor for enhancing sensing performance. The designed chiral metasurface features in-plane symmetry breaking, generating a chiral response characterized by transmission intensity differences in circularly polarized cross components. The integrated metalens independently modulates left-handed (LCP) and right-handed circularly polarized (RCP) light, creating distinct off-axis focal points. This configuration enables spatial separation of orthogonal circularly polarized states, effectively suppressing systematic random errors and improving the signal-to-noise ratio (SNR). The optical field focusing capability significantly enhances light-matter interactions, leading to a substantial enhancement in the circular dichroism (CD) Q-factor and substantially improved trace detection performance. Experimental validation demonstrates quantitative identification of homocysteine (Hcy) samples across various concentrations, achieving a detection limit of 12.5 μM. This approach establishes a novel paradigm for terahertz ultrasensitive sensing applications.