Enhanced circular dichroism induced by connectivity effect of rectangular metal nanorods

Abstract

Compared to natural chiral structures, planar chiral plasmonic nanostructures, which are two-dimensional artificial structures composed of noble metals that break mirror symmetry, are widely applied in fields such as analytical chemistry, pharmaceutical production, and bioanalytical monitoring. Understanding circular dichroism (CD) and its enhancement mechanisms is crucial for these applications. Although a variety of chiral structures have been extensively studied, a deep understanding of the tunability of the CD effect remains insufficient. In particular, helical structures face challenges such as difficult fabrication and poor tunability. In this study, we designed a chiral structure composed of rectangular metal nanorods and metallic spheres, aiming to achieve a significant tunable CD effect by utilizing the connectivity effect of the metal nanorods, reaching an impressive CD value of 0.7. Results calculated by the finite element method show that, near the resonant wavelengths of 710 nm and 730 nm, the spectral responses of \({T}_{++}\) and \({T}_{--}\), respectively, exhibit peak and valley patterns, thereby generating a substantial CD effect. Fundamentally, this is due to the shifting of the resonance modes at these specific wavelengths under RCP and LCP light. The extent of this shift can be precisely manipulated by altering the width of the rectangular metal nanorods, thus enabling controlled CD effects. Moreover, the CD effect is found to be highly dependent upon the geometric parameters of the designed structures. In summary, these findings contribute significantly to the development of planar chiral plasmonic nanostructures with tunable and large CD effects, providing valuable insights for their optimization and practical applications.