Handedness-switchable chiral field in the 1D metal grooves for plasmonic circular dichroism spectroscopy

Abstract

The electromagnetic fields possessing optical chirality are very important to circular dichroism (CD) analysis for chiral molecules. Plasmonic nanostructures are often adopted to generate the chiral near-fields for enhancing the weak CD signal of the chiral molecules, which is called plasmonic-enhanced CD spectroscopy. However, due to the conservation of optical chirality the general nanostructures cannot provide sign-uniform chiral near-fields close to the structures, which dramatically reduce the performance of plasmonic-enhanced CD spectroscopy. The prerequisite of plasmonic-based CD analysis is to generate two enantiomeric chiral fields with opposite optical chirality close to the nanostructures in two independent measurements. Therefore, the design that can provide a handedness-switchable and equivalent optical chirality of near-field of plasmonic structure is desired. In this paper, we show that simple 1D periodic metal grooves can provide handedness-switchable, sign-uniform chiral fields inside the grooves with enhanced optical chirality through adjusting the linear polarization direction of incident light from +45° to −45° off the periodic direction. A reflection-type plasmonic CD spectroscopy scheme was proposed based on metal grooves, which works under illumination of linearly polarized light. Moreover, the numerical simulation including chiral analytes was implemented to verify the feasibility. The proposed CD spectroscopy scheme has the potential for rapid and low-cost chiroptical sensing for a small quantity of chiral molecules.