All-Dielectric Chiral Metasurfaces Based on Crossed-Bowtie Nanoantennas
Authors: J Ricardo Mejía-Salazar
Faustino Reyes Gómez
Pablo Albella
Journal: ACS Omega (2019)
Discipline: CHEMISTRY
Keywords: Circular dichroism spectroscopy; molecular chirality; chiral agents; plasmons; chiral plasmonic structures
Affiliations: Departamento de Física, Universidad del Valle, Cali AA 25360, Colombia, National Institute of Telecommunications (Inatel), Santa Rita do Sapucaí, MG 37540-000, Brazil, Department of Applied Physics, University of Cantabria, Avda. Los Castros, s/n, Santander 39005, Spain

Abstract:  Circular dichroism spectroscopy is a technique used to discriminate molecular chirality, which is essential in fields like biology, chemistry, or pharmacology where different chiral agents often show different biological activities. Nevertheless, due to the inherently weak molecular-chiroptical activity, this technique is limited to high concentrations or large analyte volumes. Finding novel ways to enhance the circular dichroism would boost the performance of these techniques. So far, the enhancement of light-matter interaction mediated by plasmons is the most common way to develop chiral plasmonic structures with extraordinarily strong chiroptical responses. However, absorptive losses of metals at optical frequencies has hindered its practical use in many scenarios. In this work, we propose an all-dielectric low-loss chiral metasurface with unit cells built by high-refractive-index crossed-bowtie nanoantennas. These unit cells, built of silicon, strongly increase the chiroptical effect through the simultaneous interaction of their electric and magnetic modes, which in contrast to other recent proposals shows at the same time a high concentration of the electric field in its gap that leads to the presence of hotspots. The proposed structure exhibits a circular dichroism spectra up to 3-fold higher than that of previous proposals that use complex plasmonic or hybrid nanostructures, making it a clear alternative to develop low-loss metasurfaces with potential applications in chiral target sensing/biosensing. For completeness, single triangular shaped and symmetric (achiral) bowtie nanostructures were also studied as possible candidates for a detection up to the single-molecule level due the lack of a circular dichroism background of the nanostructures themselves. 

Retracted: No

PubPeer comments:
 ![file] 1.tIn this paper, the crossed bowtie is proposed for enhancing circular dichroism signal of chiral molecules. The induced circular dichroism also generates from interaction of chiral molecules and nanostructure, and the induced circular dichroism is larger than enhanced circular dichroism by optical chiral field. Why authors did not consider induced circular dichroism? Actually, the circular dichroism of molecule is negligible. Therefore, this deviates from the problem to be solved. 2.tThe dissymmetry facture is calculated by equation 2, but the equation 2 is written wrong, the dominator must be sum of absorption of right-handed and left-handed circularly polarized light. In addition, the authors should be considered dissymmetry factor of whole system. Specifically, must be consider all of induced circular dichroism, molecular dichroism and circular dichroism of crossed boetie. Unless, how to realize the separately measure the dissymmetry facture of molecular circular dichroism? in the second paragraph of introduction, the authors write u201cHowever, absorptive losses associated with the inter- and intraband transitions in metals at optical frequencies constitute a major hurdle for practical use in many scenarios.u201d, clearly the authors mentioning the absorption of metals are large, then the dissymmetry factor is small, but why neglect circular dichroism of crossed bowtie when calculating $g$? 3.tFor chiral structure, equation 2 and equation 3 are incorrect due to molecular dichroism not only arisen from optical chiral field, dissymmetry of electric field also contributes the molecular circular dichroism. ,