Circular dichroism of a tilted U-shaped nanostructure

Coexistence of circular dichroism and asymmetric transmission in Babinet-complementary metamaterials

Chiral metamaterials with circular dichroism (CD) or asymmetric transmission (AT) draw enormous attention for their attractive applications in polarization transformers, circular polarizers, and biosensing. In this study, a feasible trilayer chiral metamaterials (TCM) is designed and investigated in theory and simulation. The proposed TCM is composed of a nanoslit layer and a Babinet-complementary nanorod layer separated by a nanoslit spacer. Owing to symmetry breaking by the tilted nanoslit in metal film, the TCM shows simultaneous CD and AT effects in the near-infrared region. The simulated electric charge distributions prove that the chirality arises from the excitation of asymmetric electric dipole resonant modes due to the coupling of adjacent unit cells. Moreover, CD and AT can be tuned by the tilted angle of the nanoslit and the thickness of the spacer, the fitting functions of which are consistent with the theoretical formulas based on transmittance matrix analysis. The proposed nanostructure offers a potential strategy for manipulating metamaterials with simultaneous CD and AT effects, allowing a multitude of exciting applications such as ultra-sensitive polarization transformer and biosensor.

Engineering a strong and stable ultraviolet chiroptical effect in a large-area chiral plasmonic shell

Ultraviolet chiral metamaterials (UCM) are highly desired for their strong interaction with the intrinsic resonance of molecules and ability in manipulating the polarization state of high energy photons, but rarely reported to date due to their small feature size and complex geometry. Herein, we design and fabricate a kind of novel ultraviolet chiral plasmonic shell (UCPS) by combing the stepwise Al deposition and colloid-sphere assembled techniques. The cancellation effect originated from the disorder lattices of micro-domains in the colloid monolayer has been successfully overcome by optimizing the deposition parameters, and a strong CD signal of larger than 1 deg in the UV region is demonstrated both in simulation and experiment. This strong ultraviolet chiroptical resonances mainly come from the surface chiral lattice resonance mode, the whispering gallery mode and also the interaction between neighbor shells, and can be effectively tuned by changing structural parameters, for example, the sphere diameter, or even slightly increasing the deposition temperature in experiment. To improve the stability, the fabricated UCPSs are protected by N₂ in the deposition chamber and then passivated by UV-ozone immediately after each deposition step. The formed UCPS show an excellent stability when exposing in the atmospheric environment. The computer-aided geometrical model, electromagnetic modes, and the tunable chiroptical resonance modes have been systematically investigated.

Anisotropy of DNA molecule detection and enhancement by GaN-based electronic sensor

An electrical characterization approach with a newly, to the best of our knowledge, defined electrical anisotropy (η) was proposed to characterize and enhance the anisotropy signals of DNA molecules. This approach utilizes L-shaped aluminum gratings on a gallium nitride PiN electronic sensor system to adjust and improve the η signals. Using this approach, the η signals of DNA molecules can be adjusted more easily and efficiently by changing the electrical parameters of the sensor. For instance, the η modes of DNA were enhanced more than 22 times with the change of the incident power and the reverse bias voltage of the PiN structure.

Plasmonic Chiral Metasurface Absorber Based on Bilayer Fourfold Twisted Semicircle Nanostructure at Optical Frequency

In this paper, we present a plasmonic chiral metasurface absorber (CMSA), which can achieve high selective absorption for right-handed and left-handed circular polarization (RCP, "+", and LCP, "-") lights at optical frequency. The CMSA is composed of a dielectric substrate sandwiched with bi-layer fourfold twisted semicircle metal nanostructure. The proposed CMSA has a strong selective absorption band, where absorption peaks for LCP and RCP lights occur at different resonance frequencies, reflecting the existence of a significant circular dichroism (CD) effect. It is shown that the absorbance of the CMSA can reach to 93.2% for LCP light and 91.6% for RCP light, and the maximum CD magnitude is up to 0.85 and 0.91 around 288.5 THz and 404 THz, respectively. The mechanism of the strong chiroptical response of the CMSA is illustrated by electric fields distributions of the unit-cell nanostructure. Furthermore, the influence of the geometry of the proposed CMSA on the circular polarization selective absorption characterization is studied systematically.

Giant circular dichroism of chiral L-shaped nanostructure coupled with achiral nanorod: anomalous behavior of multipolar and dipolar resonant modes

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.