Chirality enhancement and threonine enantiomers recognition in a planar terahertz metasurface driven by quasi-bound states in the continuum

Chiral quasi-bound state in the continuum (QBIC) resonances have garnered considerable interest in recent years. However, the design and application of chiral QBIC resonances are often challenged by out-of-plane symmetry breaking. Here, a strategy synergistically integrating two in-plane implementations is proposed to enhance the chirality of QBIC in purely planar metasurfaces, with an elliptic cylinder trimer (ECT) as an example. An in-plane symmetry perturbation is first introduced into the oligomer structure to generate QBIC and only a weak chirality is observed. Then, chirality enhancement is achieved by an in-plane rotation arrangement, which is experimentally verified in the terahertz band. The chirality enhancement mechanism is conjointly elucidated via coupled-mode theory and multipole decomposition analysis (MDA). A chirality-sensitive parameter is introduced into MDA to unravel the chirality generation mechanism. It is indicated that chirality enhancement in ECT originates from a pronounced disparity in electric dipole moments excited by right- and left-circularly polarized lights. Furthermore, a circular dichroism difference of approximately 10° is experimentally observed between minute amounts of L- and D-threonines, which can be attributed to tenfold optical chiral field enhancement from the ECT structure. Our findings provide, to our knowledge, a new chiral QBIC design paradigm in planar metasurfaces and provoke its applications in chiral biosensing.