Tunable 3D Dirac-semimetals supported mid-IR hybrid plasmonic waveguides

Dual Coupled Long-Range Hybrid Surface Plasmon Polariton Waveguide for Sub-Wavelength Confinement

In this paper, a long-range hybrid waveguide for subwavelength confinement based on double SPP coupling is proposed. The hybrid waveguide consists of a metal-based cylindrical hybrid waveguide and a silver nanowire. There are two coupling regions in the waveguide structure that enhance mode coupling. Strong mode coupling enables the waveguide to exhibit both a small effective mode area (0.01) and an extremely long transmission length (700 μm). The figure of merit (FOM) of the waveguide can be as high as 4000. In addition, the cross-sectional area of the waveguide is only 500 nm × 500 nm, allowing optical operation in the subwavelength range, which helps enhance the miniaturization of optoelectronic devices. The excellent characteristics of the hybrid waveguide make it have potential applications in photoelectric integrated systems.

Multi-Function Reflective Vector Light Fields Generated by All-Dielectric Encoding Metasurface

Traditional optics usually studies the uniform polarization state of light. Compared with uniform vector beams, non-uniform vector beams have more polarization information. Most of the research on generating cylindrical vector beams using metasurfaces focuses on generating transmitted beams using the geometric phase. However, the geometric phase requires the incident light to be circularly polarized, which limits the design freedom. Here, an all-dielectric reflective metasurface is designed to generate different output light according to the different polarization states of the incident light. By combining the two encoding arrangements of the dynamic phase and the geometric phase, the output light is a radial vector beam when the linearly polarized light is incident along the x-direction. Under the incidence of linearly polarized light along the y-direction, the generated output light is an azimuthal vector beam. Under the incidence of left-handed circularly polarized light, the generated output light is a vortex beam with a topological charge of -1. Under the incidence of right-handed circularly polarized light, the generated output light is a vortex beam with a topological charge of +1. The proposed reflective metasurface has potential applications in generating vector beams with high integration.

Hybrid Nanowire–Rectangular Plasmonic Waveguide for Subwavelength Confinement at 1550 Nm

This paper presents a hybrid waveguide based on metal surface plasmon polaritons (SPPs) at 1550 nm comprising two silver (Ag) nanowires and a rectangular silicon (Si) waveguide. Due to the strong coupling effect observed in both the metal SPP mode and Si waveguide mode, excellent waveguide characteristics, such as a small effective modal area and long transmission length, could be achieved. The research results revealed that the proposed hybrid waveguide could achieve an ultra-long transmission distance of 270 µm and normalized effective mode area of 0.01. Furthermore, the cross-sectional size of the waveguide was 500 nm × 500 nm, which helped in achieving a subwavelength size. In addition, the hybrid waveguide was resistant to manufacturing errors. These excellent performances indicate that the proposed waveguide has great application potential in optoelectronic integrated circuits.

Sodium-Based Cylindrical Plasmonic Waveguides in the Near-Infrared

Subwavelength optical field confinement and low-loss propagation are of great significance for compact photonic integration. However, the field confinement capability of plasmonic devices is always accompanied by the inherent Ohmic loss. Although recent studies have shown that sodium (Na) exhibits lower loss than noble metals in the near-infrared band, the field confinement ability has not been adequately assessed. Meanwhile, the high chemical reactivity of Na should be regulated for practical application. Two dielectric-coated Na nanowires, consisting of cylindrical Na nanowires with one or two dielectric layers as claddings, are proposed and investigated in this paper. Based on finite element calculations, we thoroughly study the modal fields and low-loss propagation properties of dielectric-coated Na nanowires. The results demonstrate that Na exhibits lower loss and stronger field confinement than the typical plasmonic material silver. These findings indicate the performance of plasmonic devices can be considerably improved by employing the metal Na compared with devices using noble metals, which may promote the applications in subwavelength photonic devices.

Improvement of beam shaping on a metasurface by eliminating the interaction between coding units

The interaction between subwavelength elements must be considered when constructing a metasurface. Generally, the interaction between cell structures is ignored when metasurface optoelectronic devices are designed, which results in a significant decrease in the design performance and efficiency of the overall metasurface structure. To reduce or further eliminate the interaction between cell structures, we propose a cell structure with borders to construct coded metasurface sequences. At the same time, we design a common frameless cell structure to construct a traditional coding metasurface. By numerical simulation of the near-field distribution and far-field scattering characteristics of these two types of coded metasurface sequences, we find that the element structure with a medium frame can attenuate the interaction between adjacent encoded particles. In the process of transmission on the encoding metasurface with a frame, different encoded particles can independently express their transmission phase and are not affected by adjacent structures, thus realizing a low coupling coding metasurface.

Highly sensitive biosensor based on an all-dielectric asymmetric ring metasurface

We propose an all-dielectric asymmetric ring-cylindrical metasurface. Based on the analysis of transmission characteristics and the calculation of electromagnetic field distribution of the metasurface with this element structure, it is found that the high Q resonance of the ultra-narrowband can be realized when the symmetry of the ring-cylindrical structure is broken. Meanwhile, it is found that the degree of asymmetry of the ring, the refractive index of the material, the radius of the ring, and the substrate have great influence on the Q value and resonant frequency of the metasurface. Our proposed metasurface structure is applied to the detection of biological molecules based on the change in refractive index of biomolecular solutions. The designed metasurface with high sensitivity to detect biomolecules with different refractive indices, the Q value can reach 365.03, and the sensitivity is increased by 90.36 GHz/RIU compared to that without substrate, while the figure of merit value is as high as 100.56, providing label-free detection of biomolecules.

Light-matter interactions enhanced by quasi-bound states in the continuum in a graphene-dielectric metasurface

In this paper, we propose a graphene-dielectric metasurface to enhance the light-matter interactions in graphene. The dielectric metasurface consists of periodically arranged silicon split rings placed on the silica substrate, which supports a symmetry-protected bound state in the continuum (BIC). When perturbation is introduced into the system to break symmetry, the BIC will transform into the quasi-BIC with high quality (Q)-factor. As the graphene layer is integrated with the dielectric metasurface, the absorption of graphene can be enhanced by the quasi-BIC resonance and a bandwidth-tunable absorber can be achieved by optimizing the Fermi energy of graphene and the asymmetry parameter of the metasurface to satisfy the critical coupling condition. By varying the Fermi energy of graphene, the quasi-BIC resonances can be effectively modulated and the max transmission intensity difference is up to 81% and a smaller asymmetry parameter will lead to better modulation performance. Our results may provide theoretical support for the design of absorber and modulator based on the quasi-BIC.

Mechanical modulation of multifunctional responses in three-dimensional terahertz metamaterials

Reconfigurable metamaterials have attracted a surge of attention for their formidable capability to dynamically manipulate the electromagnetic wave. Among the multifarious modulation methods, mechanical deformation is widely adopted to tune the electromagnetic response of the stereotype metamaterial owing to its straightforward and continuous controllability on the metamaterial structure. However, previous morphologic reconfigurations of metamaterials are typically confined in planar deformation that renders limited tunable functionalities. Here we have proposed a novel concept of out-of-plane deformation to broaden the functionalities of mechanically reconfigurable metamaterials via introducing a cross-shaped metamaterial. Our results show that the out-of-plane mechanical modulation dramatically enhances the magnetic response of the pristine metamaterial. Furthermore, by uncrossing the bars of cross-shaped meta-atoms, a L-shaped metamaterial is proposed to verify the effectiveness of such a mechanical method on the handedness switching via changing mechanical loading-paths. More importantly, the differential transmission for circularly polarized incidences can be continuously modulated from -0.45 to 0.45, and the polarization states of the transmission wave can be dynamically manipulated under the linearly polarized illumination. Our proposed mechanical modulation principle might open a novel avenue toward the three-dimensional reconfigurable metamaterials and shows their ample applications in the areas of chiroptical control, tunable polarization rotator and converter.

Defect and Doping Engineered Penta-graphene for Catalysis of Hydrogen Evolution Reaction

Water electrolysis is a sustainable and clean method to produce hydrogen fuel via hydrogen evolution reaction (HER). Using stable, effective and low-cost electrocatalysts for HER to substitute expensive noble metals is highly desired. In this paper, by using first-principles calculation, we designed a defect and N-, S-, P-doped penta-graphene (PG) as a two-dimensional (2D) electrocatalyst for HER, and its stability, electronic properties and catalytic performance were investigated. The Gibbs free energy (ΔG_H), which is the best descriptor for the HER, is calculated and optimized, the calculation results show that the ΔG_H can be 0 eV with C2 vacancies and P doping at C1 active sites, which should be the optimal performance for a HER catalyst. Moreover, we reveal that the larger charge transfer from PG to H, the closer ΔG_H is to zero according to the calculation of the electron charge density differences and Bader charges analysis. Ulteriorly, we demonstrated that the HER performance prefers the Volmer-Heyrovsky mechanism in this study.

Highly tunable thermal emitter with vanadium dioxide metamaterials for radiative cooling

An efficient thermal emitter for selective radiative cooling is realized with vanadium dioxide metamaterials. The novel structure consists of patterned VO₂ metamaterials on the multilayer substrate and a composite layer on it. To obtain the enhanced emissivity, the influence of the top composite layer and external thermal stimuli are comprehensively optimized. The emissivity can reach up to 0.952 in the metallic phase of VO₂ with a composite layer in the atmospheric window, which is due to strong localization of the electric field in the cavity. The influence on the emissivity with different incident angles and geometric parameters is investigated elaborately. Finally, the cooling power is calculated and achieves a high value of 710W/m² at 383 K, which is significantly higher than that of previous works. Thus, our proposed tunable emitter with high performance will be beneficial to the dynamic radiative cooling system and may open a potential application in building cooling and intelligent windows.

Highly confined dielectric guiding mode in nanoridges embedded in a conventional slot waveguide

Plasmonic waveguides can offer a promising solution beyond the optical diffraction limit. However, the cost of shrinking mode sizes reflects in metallic ohmic losses that lead to a short propagation distance of light, hindering the practical applications of plasmonic waveguides. Herein, we tackled the practicality of a novel CMOS-compatible all-dielectric waveguide structure that exploits electromagnetic boundary conditions of both the continuous normal component of the electric displacement field and the tangential component of the electric field at a high-index-contrast interface, which allows the attainment of mode areas comparable with those of plasmonic waveguides and theoretical lossless. The proposed waveguide comprises two oppositely contacted nanoridges with semicircular tops embedded in a conventional slot waveguide. By stepping on the strong electric field in the low-index slot region of the slot waveguides, the nanoridges squeeze the mode areas further with a guiding mechanism identical to that of a surrounding slot waveguide. Through the design of the geometry parameters, the calculated mode area of the reported structure achieved an unprecedented order of 4.21 × 10^-5A₀, where A₀ is the diffraction-limited area. The mode area dependence on fabrication imperfections and spectral response showed the robustness and broadband operation. Moreover, on the basis of extremely tight mode confinements, the present waveguide even outperformed the hybrid plasmonic waveguides in lower crosstalk. The proposed idea makes the realization of practically feasible nanoscale photonic integrated circuits without any obstructions by the limited propagation distance of light for plasmonic waveguides, thereby expanding its applications in various nanophotonic and optoelectronics devices requiring strong light-matter interaction within nanoscale regions.

Symmetric Graphene Dielectric Nanowaveguides as Ultra-Compact Photonic Structures

A symmetric graphene plasmon waveguide (SGPWG) is proposed here to achieve excellent subwavelength waveguiding performance of mid-infrared waves. The modal properties of the fundamental graphene plasmon mode are investigated by use of the finite element method. Due to the naturally rounded tips, the plasmon mode in SGPWG could achieve a normalized mode field area of ~10⁻⁵ (or less) and a figure of merit over 400 by tuning the key geometric structure parameters and the chemical potential of graphene. In addition, results show that the modal performance of SGPWG seems to improve over its circular counterparts. Besides the modal properties, crosstalk analysis indicates that the proposed waveguide exhibits extremely low crosstalk, even at a separation distance of 64 nm. Due to these excellent characteristics, the proposed waveguide has promising applications in ultra-compact integrated photonic components and other intriguing nanoscale devices.

Graphene-Based Polarization-Independent Mid-Infrared Electro-Absorption Modulator Integrated in a Chalcogenide Glass Waveguide

A polarization-insensitive graphene-based mid-infrared optical modulator is presented that comprised SiO2/ Ge23Sb7S70, in which two graphene layers are embedded with a semiellipse layout to support transverse magnetic (TM) and transverse electric (TE) polarizing modes with identical absorption. The key performance index for the polarization independent modulator is polarization-sensitivity loss (PSL). The waveguide of our device just supports basic TE and TM modes, and the PSL between two modes is of < 0.24 dB. The model can offer extinction ratio (ER) more than 16 dB and insertion loss less than 1 dB. The operation spectrum ranges from 2 to 2.4 μm with optical bandwidth of 400 nm. The 3 dB modulation bandwidth is as high as 136 GHz based on theoretical calculation.