Angular Momentum-Reversible Near-Unity Bisignate Circular Dichroism

Bisignate circular dichroism, originated from the Cotton effect, with opposite signs of the chiroptical response is an indispensable property for protein sensing, drug discovery and design, and liquid crystallinity. However, naturally occurring and artificially synthetic chiral structures with random orientations in a bulk sample often exhibit ensemble-averaged bisignate circular dichroism in absorption with a magnitude less than 0.01. Even though vertically stacked achiral plasmonic nanostructures could enhance the absorption-based bisignate circular dichroism, the resonance nature of this approach restricts its magnitude less than 0.5. Here, a novel concept of non-absorption-based bisignate circular dichroism is demonstrated through the nonresonant angular momentum mode-sorting sensitivity of a single nanoring slit enclosed by a concentric plasmonic nanogroove coupler. Quasi-flat bisignate circular dichroism with a near-unity magnitude is achieved in the visible and near-infrared wavelength range. Additionally, the new bisignate circular dichroism reveals complete reversibility when the sign of the orbital angular momentum mode of incident light and of the geometrical topological charge of the nanogroove coupler is changed. This work opens exciting avenues for ultrasensitive angular momentum-dependent stereochemistry and may find promising applications in integrated photonics harnessing switchable chiroptical devices.