Yongkai Wang

Extraordinary Optical Transmission Property of X-Shaped Plasmonic Nanohole Arrays

Optical transmission properties of periodic X-shaped plasmonic nanohole arrays in a silver film are investigated by performing the finite element method. Obvious peaks appear in the transmission spectra due to surface plasmon polaritons (SPPs) on the top surface of the silver film, to the Fabry–Ferot resonance effect of SPPs in the nanohole, and to the localized surface plasmon resonance of the nanohole. Besides the topologic shape parameters of the X-shaped nanohole, transmission properties strongly depend on incident polarization. The results of this study not only present a tunable plasmonic filter, but also aid in the understanding of the mechanisms of the extraordinary optical transmission phenomenon.

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.

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.

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.

Asymmetric transmission of obliquely intersecting nanoslit arrays in a gold film.

Asymmetric transmission (AT) has significant applications in optical polarization control. In this paper, we propose a kind of periodic nanoslit rather than the protruding planar structures, such as G-shaped structure and coupled split-ring resonators, to realize the AT effect. The planar periodic obliquely intersecting nanoslits (OINs) in the gold film, composed of gratings with an infinite length and tilted nanoslits with a finite length, are proposed to realize the AT effect by performing the finite element method. Obvious dips in the AT spectra result from the circular localized surface plasmon resonance around the two terminals of the tilted nanoslits and from the surface plasmon polariton resonances on the film and in the gratings or tilted nanoslits. In addition, the AT effect strongly depends on the geometric parameters of the OINs. The film can be straightly powered on as an in-plane electrical conductor, which broadens its applications in optoelectronic devices. Overall, these results are beneficial in designing devices to achieve AT for polarization transformation.

Transmission characteristics of surface plasmon polaritons through a metallic rectangle above a metallic film

The effects of a silver rectangle on the transmission characteristics of surface plasmon polaritons (SPPs) that propagate at the air–silver interface are investigated using the finite-element method. Results show that the structural parameters of the rectangle and distance between rectangle and film significantly influence SPP-transmission characteristics. These effects are due to the restriction of SPPs at the air–silver interface and resonance around the rectangle.

Broadband Extraordinary Optical Transmission Through Gold Diamond-Shaped Nanohole Arrays

Extraordinary optical transmission (EOT) is widely accepted as a resonant phenomenon. However, the nonresonant EOT phenomenon of subwavelength metallic nanohole arrays on a metallic thin film is significant for harvesting of broadband light, confining optical power in small area, and enhancing local electric field. To achieve nonresonant EOT, a novel paradigm structure comprising periodic diamond-shaped nanohole arrays engraved on a thin gold film is proposed. The transmission properties of the diamond-shaped nanohole arrays are calculated using the finite-element method. Results show that this paradigm structure facilitates a broadband and enhanced transmission in the infrared region. In addition, the effects of incident polarization and structural parameters on the transmission property are also studied.

Tunable asymmetric transmission through tilted rectangular nanohole arrays in a square lattice

Asymmetric transmission (AT) holds significant applications in controlling polarization and propagation directions of electromagnetic waves. In this paper, tilted rectangular nanohole (TRNH) arrays in a square lattice are proposed to realize an AT effect. Numerical results show two AT modes in the transmission spectrum, and they are ascribed to the localized surface plasmon resonances around the two ends of TRNH and surface plasmon polaritons on the golden film. AT properties of the TRNH strongly depend on structural parameters, such as width, length, thickness, and tilted angle of TRNH. Results provide a novel mechanism for generating AT effect and offer potential plasmonic device applications, such as asymmetric wave splitters and optical isolators.