Jia-Xian Wang

Surface-plasmon-polariton whispering-gallery mode analysis of the graphene monolayer coated InGaAs nanowire cavity

In this article, we proposed and numerically studied the surface plasmon polariton whispering gallery mode properties of the graphene coated InGaAs nanowire cavity. The quality factor and the mode area were investigated as a function of the chemical potential, the cavity radius and the wavelength. A high cavity quality factor of 235 is predicted for a 5 nm radius cavity, accompanied by a mode area as small as3.75×10(-5)(λ(0))(2), when the chemical potential is 1.2 eV. The proposed structure offers a potential solution to high density integration of the nanophotonic devices with an ultra-compact footprint.

Nanofocusing of mid-infrared electromagnetic waves on graphene monolayer

Nanofocusing of mid-infrared (MIR) electromagnetic waves on graphene monolayer with gradient chemical potential is investigated with numerical simulation. On an isolated freestanding monolayer graphene sheet with spatially varied chemical potential, the focusing spot sizes of frequencies between 44 THz and 56 THz can reach around 1.6 nm and the intensity enhancement factors are between 2178 and 654. For 56 THz infrared, a group velocity as slow as 5×10−5 times of the light speed in vacuum is obtained at the focusing point. When the graphene sheet is placed on top of an aluminum oxide substrate, the focusing spot size of 56 THz infrared reduces to 1.1 nm and the intensity enhancement factor is still as high as 220. This structure offers an approach for focusing light in the MIR regime beyond the diffraction limit without complicated device geometry engineering.

Investigation of plasmonic whispering-gallery mode characteristics for graphene monolayer coated dielectric nanodisks

In this Letter, we theoretically studied high-quality (Q) factor plasmonic whispering-gallery modes (WGMs) with ultrasmall mode volumes in graphene monolayer coated semiconductor nanodisks in the mid-infrared range. The influence of the chemical potential, the relaxation time of graphene, and the radius of the nanodisk on the cavity Q factor and the mode volume was numerically investigated. The numerical simulations showed that the plasmonic WGMs excited in this cavity had a deep subwavelength mode volume of 1.4×10(-5)(λ(0)/2n)(3), a cavity Q factor as high as 266 at a temperature lower than 250 K, and, consequently, a large Purcell factor of ∼1.2×10(7) when the chemical potential and relaxation time were assumed to be 0.9 eV and 1.4 ps, respectively. The results provide a possible application of plasmonic WGMs in the integration of nano-optoelectronic devices based on graphene.

An ultrabroad band omni-directional anti-reflective coating with quasi-gradient refractive index distribution based on Si–SiOxNy–SiO2 materials system

An ultrabroad band omni-directional antireflective coating was fabricated using both reactive magnetron sputtering and glancing angled electron beam evaporation methods. Gradient index amorphous Si, SiNx, and SiOxNy thin films were deposited by tuning the flow rate of the reactive gases, while the gradient index distribution of the nanoporous SiO2 stacks was obtained by rotating the angle of the substrate. A low average reflectivity of less than 2% at normal incidence in the wavelength range 280 to 3300 nm was achieved, and the average reflectivity over the angle range 15 to 89° was 3.7% for the wavelength between 300 and 1700 nm.

Top-emitting organic light-emitting devices with improved light outcoupling and angle-independence

An organic capping layer has been used to increase the light output of the top-emitting organic light-emitting device (TEOLED) employing highly reflective Ag as anode and semitransparent Ag as cathode. The transmittance of the cathode with a 50 nm thick optimized top-capping layer for a wavelength of 520 nm is nearly twice that of the controlled device. Accordingly, an efficient TEOLED capped with a 50 nm thick refractive index-matching layer had been fabricated. Especially, together with the independence of the view angle for the emission peak, the luminance and efficiency for the TEOLED at low voltage (4 V) are 677 cd m−2 and 15 cd A−1, respectively, as was desired for the displays.

Optimization of the Fano Resonance Lineshape Based on Graphene Plasmonic Hexamer in Mid-Infrared Frequencies

In this article, the lineshape of Fano-like resonance of graphene plasmonic oligomers is investigated as a function of the parameters of the nanostructures, such as disk size, chemical potential and electron momentum relaxation time in mid-infrared frequencies. Also, the mechanism of the optimization is discussed. Furthermore, the environmental index sensing effect of the proposed structure is revealed, and a figure of merit of 25.58 is achieved with the optimized graphene oligomer. The proposed nanostructure could find applications in the fields of chemical or biochemical sensing.

Topologically protected edge states in graphene plasmonic crystals

A two-dimensional graphene plasmonic crystal composed of periodically arranged graphene nanodisks is proposed. We show that the band topology effect due to inversion symmetry broken in the proposed plasmonic crystals is obtained by tuning the chemical potential of graphene nanodisks. Utilizing this kind of plasmonic crystal, we constructed N-shaped channels and realized topologically edged transmission within the band gap. Furthermore, topologically protected exterior boundary propagation, which is immune to backscattering, was also achieved by modifying the chemical potential of graphene nanodisks. The proposed graphene plasmonic crystals with ultracompact size are subject only to intrinsic material loss, which may find potential applications in the fields of topological plasmonics and high density nanophotonic integrated systems.

Investigation of plasmonic whispering gallery modes of graphene equilateral triangle nanocavities

In this paper, a graphene-based equilateral triangle nanocavity is proposed and numerically investigated. The relationship between the mode characteristics and the nanocavity parameters, such as the geometry of nanocavity and the chemical potential of graphene, is systematically explored. A high-order plasmonic WGM (whispering gallery mode) with a high quality factor of 147.93 is obtained in our nanocavity with a wavelength of around 1.415 µm in free space, with a corresponding Purcell factor as high as 7.067 × 108. The proposed plasmonic WGM nanocavity could be a key component of the high density plasmonic integrated circuits due to its ultra-compactness and performances.

Investigation of the Band Structure of Graphene-Based Plasmonic Photonic Crystals

In this paper, one-dimensional (1D) and two-dimensional (2D) graphene-based plasmonic photonic crystals (PhCs) are proposed. The band structures and density of states (DOS) have been numerically investigated. Photonic band gaps (PBGs) are found in both 1D and 2D PhCs. Meanwhile, graphene-based plasmonic PhC nanocavity with resonant frequency around 175 THz, is realized by introducing point defect, where the chemical potential is from 0.085 to 0.25 eV, in a 2D PhC. Also, the bending wvaguide and the beam splitter are realized by introducing the line defect into the 2D PhC.

A Chirped Subwavelength Grating With Both Reflection and Transmission Focusing

A planar lens composed of a chirped subwavelength grating (CSG) structure with high numerical aperture (NA) was designed and analyzed in this paper. The reflectivity, transmission, and phase were calibrated as a function of the grating dimension using rigorous coupled wave analysis, while the focusing properties were numerically simulated by finite-element method. The designed CSG focused the reflected and transmitted waves that have approximately the same power ratios simultaneously. Numerical aperture values of the planar lens as high as 0.91 and 0.92 were obtained for normal incidence of TM and TE polarization light, respectively.