The complexity of lithography methods and other methods utilizing chiral templates to produce three-dimensional structures restricts further research on chiral plasmonics. Herein, a plasmonic nanostructure with a strong chiroptical response and enhanced near-fields generated on a large and highly ordered achiral tapered nanopore anodic aluminum oxide template is proposed and fabricated via a low-cost and efficient glancing angle deposition method. The plasmonic chiral conic nanoshell metallic nanostructure (CCNM), which is composed of three nanoshells of different heights, is obtained by varying the incidence and orientation angles of deposition to achieve symmetry breaking. Such a conic nanoshell nanostructure can couple incident light into the nanostructure, thus reducing the reflection and localizing the electromagnetic energy inside the nanoshell. The experimental circular dichroism spectra of the CCNM in the visible range shows that the chiroptical response is amplified with an increased height difference of the three nanoshells and period of the nanopore. The dissymmetry factor of the CCNM is up to 0.45, which results from the helix-like electron oscillation characteristics on the surface of the three nanoshells. The simulation result shows that the enhancement of the chiral near-fields of the CCNM reaches 155 times with respect to the circularly polarized light due to the small angle between electric and magnetic fields. The chiral signal is enhanced by about 2 orders of magnitude using the CCNM to detect chiral molecules. This study offers a concise and large-area regular method for fabricating plasmonic chiral nanostructures with a tunable chiroptical response and provides an effective and convenient idea to control the chiral near-fields for sensitive biomolecule detection.

The circular dichroism (CD) effect characterized by different optical responses between left and right circularly polarized lights is widely applied for polarization-resolved detection and imaging. The tunable CD effect is of substantial importance in improving the detection sensitivity and imaging resolution. In this paper, we show that planar Z-shaped composite metamaterial embedded with VO2 that exhibits insulator-metal transition (IMT) can enable thermally tunable chirality. Simulated by the finite element method, the tunable CD effect can be achieved by changing the environment temperature to initiate the IMT of VO2. We also demonstrate that the underlying mechanism of the CD effect generation is the electric multipole oscillation response in the vicinity of the VO2 gap in vertical direction. These findings not only provide a new strategy to change chirality and tune the effect of CD dynamically but also broaden its potential applications in polarization-resolved detection and imaging.

Chirality, which has long been known as an intrinsic property of living organisms, has caught the interest of researchers due to the rapid emergence of chiral metamaterials. The chiroptical response of noble metal nanostructures in visible and near-infrared regions has been widely investigated. Herein, we propose a bilayer Ag metastructure, in which a chiral L-shaped nanostructure at the bottom is coupled with an achiral nanorod acquiring different positions in the top layer with respect to the long and/or short arm of the chiral L-shaped nanostructure at the bottom layer. The metastructure generates a giant circular dichroism (CD) signal resulting from the strong coupling of the multipolar and dipolar resonant modes on the two layers, in the visible and near-infrared regions. With changing the position of the achiral nanorod, an unusual reversal of the CD spectra is observed, along with a fourfold increase in CD intensity in the short wavelength range due to the multipolar resonant modes. The position of the achiral nanorod is tailored by the azimuthal angle of the substrate during the fabrication of the metastructure using the oblique angle deposition method. This study provides insights into the variation of the coupling strength between a chiral L-shaped nanostructure and an achiral nanorod. The results can be useful in designing chiral–achiral composite nanoantennas for sensing devices.

Circular dichroism (CD) spectroscopy is widely used in the conformational analysis of biomolecules. The CD of chiral molecules is weak. Therefore, plasmon-generated near field is used to enhance the CD of chiral molecules. In this study, we propose a chiral molecule-nanodimer coupled system to investigate the CD enhancement that originates from molecular CD () and induced CD (). Calculation results show that is generated from different optical rotations and attenuation rates of the near field under right circularly polarized (RCP) and left circularly polarized (LCP) light. In the case of oriented molecular assembly, becomes twice that in the case of isotropic molecular assembly or even larger. Moreover, acquires the maximum value when the orientation of the molecular assembly is parallel to the strongest component of the electric field. Different components of the electric field generate opposite CD, thereby reversing the spectra. These findings can be helpful in realizing ultrasensitive probing for chiral molecules.

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