Zhongyue Zhang

Plasmonic chirality of L-shaped nanostructure composed of two slices with different thickness

A concise method is proposed to fabricate L-shaped Ag nanostructures (LSANs) for generating chirality. Prepared by glancing angle deposition, the LSAN composed of two slices with different thickness is stacked on self-assembled monolayer polystyrene nanosphere arrays by controlling substrate azimuth and deposition time. The strong optical chirality of LSANs is achieved in visible and near-IR regions by measurement. For the circular dichroism spectrum of LSANs, the intensity is enlarged, and its peaks red-shift with increasing thickness difference. When LSANs are stacked on polystyrene spheres of different diameters, enlargement and red-shift are also observed in their circular dichroism spectra with increasing thickness difference. The numerical calculations of finite element method show that the two slices composing LSAN provide cross-electric dipoles and their thickness difference provides phase difference for generating optical chirality. This study not only provides a concise and scalable method for fabricating chiral plasmonic nanostructures but also contributes to understand the knowledge of the mechanism of circular dichroism.

Plasmon resonances and strong electric field enhancements in side-by-side tangent nanospheroid homodimers.

The plasmon resonance and electric field enhancement in a side-by-side tangent nanospheroid homodimer (TNSHD) have been investigated theoretically by using DDA and FDTD methods, respectively. The simulation results indicate that this side-by-side TNSHD has its novel optical properties. We find that the plasmon resonance with a distinct Fano lineshape can be achieved and the electric field intensity can be enhanced strongly. The tunability of the Fano resonance could provide important applications in biosensing. The obtained electric field enhancement might open a promising pathway for surface-enhanced Raman scattering (SERS) and light trapping in solar cells.

Giant circular dichroism induced by tunable resonance in twisted Z-shaped nanostructure

Circular dichroism (CD) is useful in molecular chemistry, pharmaceuticals, and bio-sensing. In this paper, twisted Z-shaped nanostructure (TZN) is proposed to achieve giant CD. The TZN is composed of three vertical and twisted nanorods. Given that the resonance of vertical nanorod is only observable for left circularly polarized light excitation but is subdued for right circularly polarized light excitation, which leads to the giant CD effect approaching 88%. The subdued resonance of vertical nanorod can be excited by rotating the bottom nanorod. The CD properties can be tuned by the length of nanorods and the gap between them. These results would guide the design of plasmonic chiral nanostructures for achieving giant CD effect.

Co-occurrence of circular dichroism and asymmetric transmission in twist nanoslit-nanorod Arrays

Circular dichroism (CD) and asymmetric transmission (AT) are important in the field of negative refractive index media and perfect polarization converters. A large difference between T++ and T-- in the transmission matrix T leads to a large CD effect, whereas a large difference between T-+ and T+- leads to a large AT effect. To achieve large CD and AT effects simultaneously, we theoretically analyzed the transmission matrix T and proposed the chiral plasmonic nanostructure of twist nanoslit-nanorod arrays (TNNAs) in this study. Results calculated by the finite element method show that, at around resonant wavelengths, the spectra of T++ and T-- correspondingly present peaks and valleys leading to a large CD effect. Meanwhile one of the spectra for T-+ and T+- presents valleys and another presents peaks leading to a large AT effect. More importantly, the magnitude of CD is equivalent to that of AT. In addition, the CD and AT effects strongly depend on the geometric parameters of TNNAs. Overall, these results are useful for designing chiral plasmonic nanostructures with large CD and AT effects.

Tunable Circular Dichroism of Achiral Graphene Plasmonic Structures

Graphene bilayered split rings (BSRs) are proposed to generate tunable circular dichroism (CD). BSRs with traditional materials do not elicit the CD effect because of C4 symmetry. By contrast, BSRs with graphene can achieve the CD effect by varying the Fermi energy of each part of the split rings. The CD signal can be reversed from positive to negative or vice versa by exchanging the Fermi energy. CD effects can also be tuned by varying the Fermi energies of the different parts. This phenomenon indicates that the chirality of BSRs gradually change as the Fermi energy varies. This concept provides a method to change chirality and dynamically vary the CD effect without rebuilding the structures.

Extraordinary Optical Transmission of Broadband Through Tapered Multilayer Slits

The nonresonant, enhanced optical transmission of subwavelength metallic slits on a thin film is significant in broadband light harvesting devices. To improve transmission efficiency, this paper established tapered multilayer slits in which thin dielectric layers are sandwiched between two metallic layers. The transmission properties of these slits are then investigated using the finite element method. Results show that transmission is improved in the tapered multilayer slits relative to that in the tapered monolayer slits. The effects of structural parameters on these transmission properties are also examined.

Broadband Extraordinary Optical Transmission Through a Multilayer Structure With a Periodic Nanoslit Array

Extraordinary optical transmission (EOT) of metallic film perforated by a periodic array of subwavelength holes is significant in photoelectric devices. In this paper, a multilayer structure with periodic nanoslit arrays is proposed to achieve broadband EOT in infrared. The optical transmission properties of such structure are simulated through the finite-element method. Greatly enhanced transmissions over a broad spectral range are observed in near-infrared wavelengths, and many enhanced electric fields occur in the narrower slit. In addition, the effects of structural parameters on transmission properties are also investigated. All these findings could guide the design of devices with broadband-enhanced transmission and high electric-field concentration.

Giant circular dichroism induced by silver nanocuboid heterodimers.

Metallic nanocuboid heterodimers are proposed to generate a giant circular dichroism (CD) effect. Two cuboids in the heterodimers have different heights. The dipole and quadrupole charge oscillation modes in the cuboids occur under left- and right-handed circular polarizations. The height difference generates phase difference between charge oscillations in the two cuboids. The two charge oscillations and the phase difference between them are consistent with the Born-Kuhn model for the CD effect. The CD effect of the nanocuboid heterodimers can be tuned by changing the structural parameters of the nanocuboid heterodimers, especially the height difference between two cuboids. The results of this research are not only useful for designing plasmonic structures to generate the CD effect but also for understanding the physical mechanisms of the CD effect.

Circular dichroism of a tilted U-shaped nanostructure

The circular dichroism (CD) effect plays an important role in biological detection, analytical chemistry, and plasmonic sensing. Tilted 3D structures can generate CD signals under normal illumination. However, fabricating tilted 3D structures is complex and expensive. In this study, we fabricate a tilted U-shaped nanostructure (TUSN) on a polystyrene (PS) nanosphere through the glancing angle deposition method. One branch of the U-shaped nanostructure is tilted by raising a sheet of SiO2 on the PS nanosphere. And the CD signal of the TUSN is enhanced because the phase difference increases with increasing thickness of the SiO2 sheet. This work proposes a method for fabricating tilted nanostructures and elucidates the mechanism of the CD effect for future research.

Direct and indirect coupling mechanisms in a chiral plasmonic system

Artificial chiral plasmonic nanostructures (ACPNs) are widely studied and used in biological monitoring, analytical chemistry, and negative-refractive-index media. The mechanism of direct coupling between two twist metal nanorods has been obtained in usual ACPNs. In this work, we proposed a nanosystem of twist nanorods separated by a metal film (TNMF). By analyzing the charge distributions, a new indirect coupling mechanism is found. According to the equivalent LC resonant circuits, gold nanorods on the two sides of the gold film can be regarded as a receiver and an emitter. These components enhanced transmittance and provided direct and indirect coupling mechanisms for the circular dichroism (CD). The direct coupling mode cannot be explained by impedance matching and can be tuned monotonously by monotonously varying geometric dimensions. However, the CD signal of indirect coupling can be explained by impedance matching and can be tuned to its maximum by varying geometric dimensions when the impedances of both sides of the gold film match. These results can help design novel chiral optical structures and promote combined applications between photons and electrons when a gold film is powered on.