Inversing Plasmonic Chirality through Locally Turning Building Blocks in a Trident-Shaped Nanostructure Array

Structure-sensitive chiroptical properties favor applications such as polarization detection and nonlinearity; however, large-scale tuning of circular dichroism (CD), for instance, flipped signal, is typically realized in two enantiomers with distinct handedness and rarely in a single planar chiral nanostructure. This paper numerically studies a trident-shaped nanostructure array that allows magnetic-field enhancement by forming semiclosed loop currents in quadrupolar localized plasmon (QL) to generate substantial chiroptical responses with a maximum CD magnitude of 33.1%. Locally changing the rotation of the building block at a small angle of 15° can efficiently modify the structural chirality of the unit cells and thus dramatically inverse the chiroptical responses from negative to positive. We arranged two trident-shaped nanostructures with slightly different configurations but opposite optical chirality into a unit to achieve a high-contrast CD signal with a narrowed line width. Calculations show that the new metamaterial with large electromagnetic-field enhancement in sharp corners enables the detection of low-concentration bovine serum albumin protein with the largest sensitivity of SCD = 795 RIU–1.