Near-field Coupling Research Articles

Lattice plasmon mode excitation via near-field coupling

The optical response of metal nanoparticles can be modified through near-field or far-field interaction, yet the lattice plasmon modes (LPMs) considered can only be excited from the latter. Here instead, we present a theoretical evaluation for LPM excitation via the near-field coupling process. The sample is an arrayed structure with specific units composed of upper metal disks, a lower metal hole and a sandwiched dielectric post. The excitation process and underlying mechanism of the LPM and the influence of the structure parameters on the optical properties have been investigated in detail by using a finite-difference time-domain (FDTD) numerical method. Our investigation presented here should advance the understanding of near-field interaction of plasmon modes for LPM excitation, and LPMs could find some potential applications, such as in near-field optical microscopes, biosensors, optical filters and plasmonic lasers.

Low-profile, wideband dual-polarized 1 × 2 MIMO antenna with FSS decoupling technique

AbstractA low-profile, wideband dual-polarized 1 × 2 multiple-input-multiple-output (MIMO) antenna with frequency selective surface (FSS) decoupling technique is presented. Low profile is realized with two different artificial magnetic conductor (AMC) cells out of which one operates at 3.5 GHz and other with dual band at 3.1 and 4.5 GHz. The proposed antenna height is maintained at 0.125λ0 which is significantly reduced when compared with the conventional perfect electric conductor (PEC) ground plane. Wideband dual polarization is enabled by two pairs of bow-tie antenna elements surrounded by a square ring placed in the orthogonal orientation. To mitigate the near-field coupling in 1 × 2 MIMO an FSS wall is constructed with wide band stop characteristics from 2.85 to 4.75 GHz. Measured results show by inserting FSS wall vertically, coupling reduction is improved by 27 dB over the existing coupling and the antenna exhibits a bandwidth of 57.14% (2.95–4.95 GHz) for VSWR<2 with port isolation of more than 25 dB for entire band of operation.

Numerical Simulations of Laser Pulse Propagation in Quantum Active Media: Using a semiclassical model

Active media are materials that consist of quantum systems, such as atoms, impurities, or quantum dots, and are characterized by strong resonant absorption and re-emission of radiation. In this article, we present a general framework for the numerical simulation and analysis of time-dependent radiation-induced phenomena in active media. The formulation used is based on the solution of semiclassical Maxwell–Bloch equations describing the evolution of each quantum element under the effect of external and re-emitted radiation. Within these Maxwell–Bloch equations, the coupling of quantum systems to classical Maxwellian fields poses computational challenges due to the strong nonlinearities involved. In contrast to traditional mesh-based solvers, we adopt an electric field integral-operator approach that can reliably account for near-field effects—including self-radiation—and is scalable to large systems. We then focus on media based on large numbers of quantum dots and demonstrate various physical effects arising from the near-field coupling, including polarization modulations and superradiance.

Investigation of multiple metal nanoparticles near-field coupling on the surface by discrete dipole approximation method

Investigation of multiple metal nanoparticles near-field coupling on the surface by discrete dipole approximation method

Cooling Metals via Gap Plasmon Resonance.

We report highly emissive and radiatively cooled metallic surfaces that sustain multiple and high-amplitude gap plasmon cavity modes within the principal thermal radiation spectrum at room temperature (i.e., 8-13 μm). A square-lattice array of Cu/ZnS/Cu gap plasmon cavities with five different widths was designed to avoid the near-field coupling between adjacent cavities and the anticrossing of a cavity mode and the first diffraction mode. The gap plasmon cavities fabricated on a Si substrate exhibited an effective emissivity of >0.62, up to an incidence of 60°. Outdoor solar heating experiments showed that the Cu/ZnS/Cu multicavity array lowered the Si substrate temperature by 4 °C at a maximum solar irradiance of 800 W/m2, which is equivalent to a near-one-sun intensity, relative to a planar Cu/ZnS/Cu multilayer. Such mid-infrared spectrum management of metals enables heat dissipation via radiation, which will be further utilized for designing electrodes that cool optoelectronic devices with the same metal/dielectric/metal configuration.

Active control of terahertz waves based on p-Si hybrid PIT metasurface device under avalanche breakdown.

Active control of terahertz waves is a critical application for terahertz devices. Silicon is widely used in large-scale integrated circuit and optoelectronic devices, and also shows great potential in the terahertz field. In this paper, a p-Si hybrid metasurface device is proposed and its terahertz characteristics under avalanche breakdown effect is investigated. In the study, a plasmon-induced transparency (PIT) effect caused by the near-field coupling of the bright mode and the dark mode is observed in the transmission spectrum. Due to avalanche breakdown effect, the resonance of the PIT metamaterial disappears as the current increased. Carriers existed in the interface between the metasurface and substrate result to a dipole resonance suppression. When the current continues increasing, the maximal modulation depth can reach up to 99.9%, caused by the avalanche effect of p-Si. Experimental results demonstrate that the avalanche breakdown p-Si can achieve a performance modulation depth, bringing much more possibilities for terahertz devices.

Sensitivity of topological edge states in a non-Hermitian dimer chain

Photonic topological edge states in one-dimensional dimer chains have long been thought to be robust to structural perturbations by mapping the topological Su–Schrieffer–Heeger model of a solid-state system. However, the edge states at the two ends of a finite topological dimer chain will interact as a result of near-field coupling. This leads to deviation from topological protection by the chiral symmetry from the exact zero energy, weakening the robustness of the topological edge state. With the aid of non-Hermitian physics, the splitting frequencies of edge states can be degenerated again, with topological protection recovered by altering the gain or loss strength of the structure. This point of coalescence is known as the exceptional point (EP). The intriguing physical properties of EPs in topological structures give rise to many fascinating and counterintuitive phenomena. In this work, based on a finite non-Hermitian dimer chain composed of ultra-subwavelength resonators, we propose theoretically and verify experimentally that the sensitivity of topological edge states is greatly affected when the system passes through the EP. Using the EP of a non-Hermitian dimer chain, we realize a new sensor that is sensitive to perturbation of on-site frequency at the end of the structure and yet topologically protected from internal perturbation of site-to-site couplings. Our demonstration of a non-Hermitian topological structure with an EP paves the way for the development of novel sensors that are not sensitive to internal manufacturing errors but are highly sensitive to changes in the external environment.

Heterointerface-Enhanced Ultrafast Optical Switching via Manipulating Metamaterial-Induced Transparency in a Hybrid Terahertz Graphene Metamaterial.

We have demonstrated the active manipulation of metamaterial-induced transparency (MIT) in a terahertz hybrid metamaterial with graphene overlayer under photoexcitation. It is found that the introduction of graphene can greatly modify the resonant dips and transparency window through the formed depolarization field around unequal-length double bars to weaken dipole resonances and their destructive interference. Transient control of MIT behaviors is determined by the photogenerated carrier dynamics, which influences the distributions of currents and electric fields in the resonant region to hinder the near-field coupling of two bright modes. Optical modulation depth is sensitive to bar spacing due to an anomalous increased double-bar coupling involving intracell and intercell interaction. Heterointerface formed by the added graphene with substrate could further enhance terahertz response via effective separation of the photoexcited carriers. Theoretical calculation based on the coupled Lorentz oscillator model reveals that the photoinduced terahertz response mainly originates from the coupling and damping in hybrid structures. Our findings could facilitate the development of graphene-based dynamical terahertz modulators and optoelectronic devices.

Transmitting Surface Plasmon Polaritons across Nanometer-Sized Gaps by Optical near-Field Coupling

Metallic nanostructures can transport electromagnetic fields in the form of surface plasmon polariton (SPP) excitations, focus them into nanometric spots, and transfer them to nearby nanostructures by near-field coupling. This provides a basic functionality for designing new plasmonic devices that can greatly enhance light–matter coupling and facilitate ultrafast and efficient all-optical switching on the nanoscale. Here, we study a prototypical device geometry, a bow-tie antenna equipped with curved line gratings, for the efficient coupling of light into and across the antenna nanogap. We experimentally demonstrate the spectrally broadband launching and propagation of SPP waves over more than 10 μm on one arm of the antenna, their focusing into and transmission across the gap being studied by the plasmon outcoupling on the other arm. A substantial increase in the coupling efficiency for antennas with gap widths below 20 nm proves that the optical near-field coupling between the two antenna arms dominates the gap transmission. We find overall transmission efficiencies for nanofocusing, gap transmission, and plasmon outcoupling of up to 4%. A finite-difference time-domain simulation supports our experimental findings. This makes such bow-tie couplers an interesting platform for sensitively probing near-field coupling to single quantum emitters and for the ultrafast switching of light by light on the nanoscale.

Tuning Electrogenerated Chemiluminescence Intensity Enhancement Using Hexagonal Lattice Arrays of Gold Nanodisks.

Electrogenerated chemiluminescence (ECL) microscopy shows promise as a technique for mapping chemical reactions on single nanoparticles. The technique's spatial resolution is limited by the quantum yield of the emission and the diffusive nature of the ECL process. To improve signal intensity, ECL dyes have been coupled with plasmonic nanoparticles, which act as nanoantennas. Here, we characterize the optical properties of hexagonal arrays of gold nanodisks and how they impact the enhancement of ECL from the coreaction of tris(2,2'-bipyridyl)dichlororuthenium(II) hexahydrate and tripropylamine. We find that varying the lattice spacing results in a 23-fold enhancement of ECL intensity because of increased dye-array near-field coupling as modeled using finite element method simulations.

Super-Absorbers by Randomly Distributed Titanium Spheres

Electromagnetic wave absorbers have been widely investigated in these years. Nevertheless, the structural models were mainly based on the metal-insulator-metal metamaterials. In this work, we show a study on the achievement of ultra-broadband perfect absorption via a randomly distributed titanium spheres in the hydrogel layer. In the visible and near-infrared range ( $0.38-2.0\,\,\mu \text{m}$ ), the spectral average absorption reaches 93.6%. Taking the absorption efficiency above 90% into account, the bandwidth of the spectral window is 1386 nm. Moreover, the maximal absorption is even up to 98.9% in this open structure since there is no metal reflector back. In particular, these impressive absorption properties can be well maintained under complex electromagnetic surroundings, indicating the insensitive response to the polarization and incident angles. The particle plasmons and the near-field coupling effects for the adjacent spheres in-plane and out-of-plane are the main contributions for the ultra-broadband absorption. The absorption properties together with the simple structural characteristics including the flexible hydrogel layer will hold wide applications in the solar energy related technologies such as the solar thermal steam generation, desalination, macula-removing, and biological sterilization.

“Meta-atomless” architecture based on an irregular continuous fabric of coupling-tuned identical nanopillars enables highly efficient and achromatic metasurfaces

Metasurfaces are subwavelength-thick constructs, consisting of discrete meta-atoms, providing discretized levels of phase accumulation that collectively approximate a designed optical functionality. The meta-atoms utilizing the Pancharatnam–Berry phase with polarization-converting structures produced encouraging implementations of optical components including metalenses. However, to date, a pending and fundamental problem of this approach has been the low device efficiency that such resulting metasurface components suffer, an unwanted side effect of large lattice constants that are used for preventing intercoupling of their meta-atoms. Although the use of near-field coupling for tuning electromagnetic resonances found its use in constructing efficient narrow-band designs, such structures fell short of providing high efficiency over a broad spectrum. Here, we propose and show that tightly packed fabrics of identical dielectric nanopillar waveguides with continuously tuned intercoupling distances make excellent and complete achromatic metasurface elements. This architecture enables the scatterers to interact with the incoming wave extremely efficiently. As a proof-of-concept demonstration, we showed an achromatic cylindrical metalens, constructed from strongly coupled dielectric nanopillars of a single geometry as continuously set phase elements in a meta-atomless fashion, working in the entirety of the 400–700 nm band. This metalens achieves over 85% focusing efficiency across this whole spectral range. To combat polarization sensitivity, we used hexagonally stacked nanopillars to build up a polarization-independent scatterer library. Finally, a circular metalens with polarization-independent operation and achromatic focusing was obtained. This is a paradigm shift in making an achromatic metasurface architecture by weaving identical nanopillars coupled into an irregular lattice laterally constructed via carefully tuned near-field coupling.

Multi-Band Electromagnetically-Induced-Transparency Metamaterial Based on the Near-Field Coupling of Asymmetric Split-Ring and Cut-Wire Resonators in the GHz Regime

A metamaterial (MM), mimicking electromagnetically-induced transparency (EIT) in the GHz regime, was demonstrated numerically and experimentally by exploiting the near-field coupling of asymmetric split-ring and cut-wire resonators. By moving the resonators towards each other, the original resonance dip was transformed to a multi-band EIT. The phenomenon was explained clearly through the excitation of bright and dark modes. The dispersion characteristic of the proposed MM was also investigated, which showed a strongly-dispersive behavior, leading to a high group index and a time delay of the MM. Our work is expected to contribute a simple way to develop the potential devices based on the multi-band EIT effect.

Normal-Mode Helical Antennas With Near-Field Coupling: Improving Design Flexibility [Antenna Applications Corner

The near-field coupling between closely placed metallic structures has been utilized to cope with the dilemma of a normal-mode helical antenna (NMHA) design. For small NMHAs, the impedance-matching difficulty is mitigated by designing appropriated coupling feeds, either electrically or magnetically. For long NMHAs, the radiation pattern distortion is rectified by dividing the complete helix into properly designed small sections. With mutual section coupling, the currents on all of the sections can be nearly in-phase, dictating an omnidirectional radiation pattern. The design principle has been described as two-element models and a corresponding equivalent circuit analysis. The simulated and measured results show that, compared to conventional NMHAs, it is possible to design an NMHA of simultaneous good matching and radiation properties but with a much more flexible helix length and size.

Demonstration of topological wireless power transfer

Recent advances in non-radiative wireless power transfer (WPT) technique essentially relying on magnetic resonance and near-field coupling have successfully enabled a wide range of applications. However, WPT systems based on double resonators are severely limited to short- or mid-range distance, due to the deteriorating efficiency and power with long transfer distance. WPT systems based on multi-relay resonators can overcome this problem, which, however, suffer from sensitivity to perturbations and fabrication imperfections. Here, we experimentally demonstrate a concept of topological wireless power transfer (TWPT), where energy is transferred efficiently via the near-field coupling between two topological edge states localized at the ends of a one-dimensional radiowave topological insulator. Such a TWPT system can be modelled as a parity-time-symmetric Su-Schrieffer-Heeger (SSH) chain with complex boundary potentials. Besides, the coil configurations are judiciously designed, which significantly suppress the unwanted cross-couplings between nonadjacent coils that could break the chiral symmetry of the SSH chain. By tuning the inter- and intra-cell coupling strengths, we theoretically and experimentally demonstrate high energy transfer efficiency near the exceptional point of the topological edge states, even in the presence of disorder. The combination of topological metamaterials, non-Hermitian physics, and WPT techniques could promise a variety of robust, efficient WPT applications over long distances in electronics, transportation, and industry.

Tunable Plasmon-Induced-Transparency Effect in a Simple Planar Composite Structure

A planar metamaterial structure has been designed to obtain surface plasmon-induced-transparency (PIT) effect, which is composed of a nanodisk sandwiched between double rods. From theoretical perspective, the three-level plasmonic system has been utilized to analyze the near-field coupling mechanism. Besides, the simulation results show that PIT resonance can be tailored by structure parameters. Particularly, when the length of the rods is more than twice of the diameter of the disk, two PIT windows are generated simultaneously, based on the coupling between the bright mode of the nanodisk and two multipole dark modes of rods. Furthermore, a metamaterial structure, composed by a disk and two pairs of nanorods, is suggested to achieve the polarization-independent plasmon-induced-transparency effect. As a result, this work shows great application prospect in the area of compact optical devices, such as multiband tunable filters, plasmonic switches, and slow light devices.

Measurements and Characterization of a Newly Developed Novel Miniature WIPT System

This article reports, for the first time, the design and experimental evaluation of a compact wireless information and power transfer (WIPT) system operating at radio frequency (RF). The design of WIPT system is facilitated by the development of a novel tri-band defected ground structure (DGS) based bandstop filter (BSF) operating at three different frequencies of 300, 433, and 700 MHz. Then, two similar BSFs are arranged in a back-to-back configuration to achieve near-field resonant inductive coupling for transfer of information and power. The measurement carried out on a prototyped system shows an excellent agreement between the measured and simulation results and provides power transfer efficiencies of 70.1%, 66%, and 65% at 300, 433, and 700 MHz, respectively, at a separation distance of 23 mm. These figures are a significant advancement over the existing state of the art. Furthermore, up-link and down-link of data transfer at 433 and 700 MHz are demonstrated using universal software radio peripheral (USRP), and power transfer is shown by illuminating a light emitting diode (LED) at 300 MHz.

Switched Beam SIW Horn Arrays at 60 GHz for 360° Reconfigurable Chip-to-Chip Communications With Interference Considerations

Traditional wired interconnects in multicore multichip (MCMC) systems can be replaced with chip-integrated switched beam antenna arrays to provide reconfigurable chip-to-chip communications. By simply switching the directive beams of the arrays, the chips can be made to dynamically communicate with their surrounding neighbors. Radiation from the arrays in the unintended direction, however, causes interference when multiple pairs of chips are simultaneously communicating. In this paper, the noise and interference limitations of such concurrent chip-to-chip communication systems are analyzed at 60 GHz for the channels defined in the IEEE 802.11ay standard. Each array consists of eight substrate integrated waveguide (SIW) horn elements printed on a thin dielectric substrate. The elements can be individually excited to produce eight directive endfire beams, providing full 360° coverage, in the horizontal plane. The substrate acts as a hybrid space-surface wave interconnect (HSSW-I) that allows near-field, space and surface wave coupling to occur between the arrays, and thus, increases interchip transmission. The antenna and link parameters that dictate the signal-to-noise-plus-interference ratio (SNIR) of such systems are established by considering all the major components. The maximum achievable data rates are then determined based on the SNIR, calculated from the measured and simulated transmission coefficients between the arrays.

RF Systems Design for Simultaneous Wireless Information and Power Transfer (SWIPT) in Automation and Transportation

This work presents some recent solutions that exploit the wireless power transfer (WPT) technology for energizing moving vehicles and machinery tools. Such technology is currently experiencing unprecedented interests in non-traditional RF/microwave sectors fields, such the industrial automation and the railway transportation safety. Near-field electromagnetic coupling solutions are presented showing that, in order to obtain efficient performances for broad ranges of operating conditions, the nonlinear electromagnetic co-design of the entire WPT system, from the energy source to the receiver load, needs to be carried out. This technology can be combined with wireless data transfer, thus realizing integrated systems able to simultaneously control the energy transfer and the transmission of data. The adopted operating frequencies are in the MHz range, which is only recently considered for this kind of applications. In particular this work focuses on three different systems: the first one demonstrates the constant powering of “on the move” industrial charts at 6.78 MHz, regardless of the relative position of the transmitter and the receiver sub-systems; the second one presents a novel design of a balise transportation system adopting a high efficiency GaN-based transmitter designed to keep its performance over a wide range of loading conditions; the last one consists of the simultaneous wireless power and data transfer, to a rotating machinery tool, automatically controlled by the powering system based on the coexistence of frequency-diverse inductive and capacitive couplings.

3D fluorescence confocal microscopy of InGaN/GaN multiple quantum well nanorods from a light absorption perspective.

A nanostructure of In0.18Ga0.82N/GaN multiple quantum well (MQW) nanorods (NRs) was fabricated using top-down etching with self-organized nickel (Ni) nanoparticles as masks on the wafer. The optical properties of In0.18Ga0.82N/GaN MQW NRs were discussed by experiment and theory from a light absorption perspective. Three-dimensional (3D) optical images of NRs were successfully obtained by confocal laser scanning microscopy (CLSM) for physical observation of the optical phenomenon of InGaN/GaN MQW NRs. Moreover, optical simulations were performed by COMSOL Multiphysics via the three-dimensional finite-element method to explore the influences of NR geometrical parameters on optical absorption. The simulated results demonstrate that the absorption of NRs is higher than that of the film due to the waveguide properties of NRs resulting from their higher refractive index than embedding medium and higher aspect ratio than bulk. In addition, an increase in the diameter results in a red-shift of the absorption peak position of In0.18Ga0.82N/GaN MQW NRs. The smaller pitch enhances the near-field coupling of the nanorods and broadens the absorption peak. These results clearly illustrate the optical properties of In0.18Ga0.82N/GaN MQW NRs from the perspective of 3D confocal laser scanning microscopy. This work is promising for the applications of III–V optoelectronic devices.