Chirality enhancement and terahertz chiral biosensing in a planar metasurface empowered by rectangular lattices with achiral semicircle dimers

Terahertz (THz) chiral biosensing is critically limited by the inherently weak circular dichroism (CD) signals of molecules and the requirement for large sample quantities. Here, we propose and experimentally demonstrate an all-dielectric metasurface composed of achiral semicircle dimers arranged in a rectangular lattice to enhance THz chiral response. By breaking the mirror symmetry of the unit cell through lattice manipulation, the measured CD values of +43^∘ and -42^∘ are experimentally observed. More importantly, an accessible and single-sign near-field optical chirality enhancement is formed within the dimer gap, even under co-axis linear polarization incidence. This significant practical advantage circumvents the need for high-quality circularly polarized THz sources. Leveraging this enhanced chiral near-field, we achieve highly sensitive enantiomer discrimination of L- and D-threonines. A remarkable enantiomeric difference of 15.5° is experimentally realized with a minute sample amount of only 2 mg (0.1 g/mL). Our findings present a simple yet powerful strategy for achieving strong chirality from achiral structures, opening a new, to our knowledge, avenue for metasurface-based trace chiral biosensing with practical applicability.