Near-field Coupling Effects Research Articles

Enhancing near-field heat transfer between composite structures through strongly coupled surface modes

In this work, we study the near-field heat transfer between composite nanostructures. It is demonstrated that thermally excited surface plasmon polaritons, surface phonon polaritons, and hyperbolic phonon polaritons in such composite nanostructures significantly enhance the near-field heat transfer. To further analyze the underlying mechanisms, we calculate energy transmission coefficients and obtain the near-field dispersion relations. The dispersion relations of composite nanostructures are substantially different from those of isolated graphene, silicon carbide (SiC) films, and SiC nanowire arrays due to the strong coupling effects among surface polaritonic modes. We identify four pairs of strongly coupled polaritonic modes with considerable Rabi frequencies in graphene/SiC film composite structures that greatly broaden the spectral peak. We confirm that near-field strong coupling effects between surface plasmon polaritons and hyperbolic phonon polaritons in the in-plane Reststrahlen band are different from those in the out-of-plane Reststrahlen band due to the different types of hyperbolicity. In addition, we analyze the effective tunability of the near-field heat transfer of graphene/SiC nanowire arrays composite structures by adjusting the chemical potential of graphene, the height and volume filling factor of the SiC nanowire arrays. This work provides a method to manipulate the near-field heat transfer with the use of strongly coupled surface polaritonic modes.

Accurate Metasurface Synthesis Incorporating Near-Field Coupling Effects

One of the most promising metasurface architectures for the microwave and terahertz frequency ranges consists of three patterned metallic layers separated by dielectrics. Such metasurfaces are well suited to planar fabrication techniques and their synthesis is facilitated by modelling them as impedance sheets separated by transmission lines. We show that this model can be significantly inaccurate in some cases, due to near-field coupling between metallic layers. This problem is particularly severe for higher frequency designs, where fabrication tolerances prevent the patterns from being highly-subwavelength in size. Since the near-field coupling is difficult to describe analytically, correcting for it in a design typically requires numerical optimization. We propose an extension of the widely used equivalent-circuit model to incorporate near-field coupling and show that the extended model can predict the scattering parameters of a metasurface accurately. Based on our extended model, we introduce an improved metasurface synthesis algorithm that gives physical insight to the problem and efficiently compensates for the perturbations induced by near-field coupling. Using the proposed algorithm, a Huygens metasurface for beam refraction is synthesized showing a performance close to the theoretical efficiency limit despite the presence of strong near-field coupling.

Enhanced Coupling of Light From Subwavelength Sources Into a Hyperbolic Metamaterial Fiber

A hyperbolic metamaterial has one or two negative components of the dielectric permittivity tensor while the others are positive. In an optical fiber inhomogenously filled with hyperbolic inhomogeneous metamaterials, modes with large angular momentum are localized near the centre of the fiber. Such a hyperbolic metamaterial fiber with radial anisotropy has no cutoff frequency for the transverse electric or the transverse magnetic modes simultaneously. This makes it possible to couple the near-field evanescent modes, associated with large transverse wavevectors of small subwavelength sized sources to these high angular momentum fiber modes well below the conventional cutoff frequencies in the butt-coupling geometry. Such hyperbolic and inhomogeneous fibers can out-couple power from a subwavelength sized source very efficiently and the coupling can be more than seven orders of magnitude larger compared to a conventional step index waveguides over lengthscales of few wavelengths. Specifically, we show that radial hyperbolicity $(\varepsilon _r 0,\varepsilon _{\phi }>0)$ is more advantageous than axial hyperbolicity $ (\varepsilon _z 0,\varepsilon _{\phi }>0)$ for such near-field coupling effects. These inhomogeneous hyperbolic fibers can be synthesized from nanoporous alumina microtubes and have immense potential to be used as near-field couplers for nanophotonic devices and near-field imaging probes.

Effects of near-field electromagnetic coupling in dimers of nanoparticles with a silver core and a J-aggregate dye shell

We report a theoretical study of the plasmon-exciton coupling effect on the absorption spectra of pairs of closely spaced double-layer hybrid nanoparticles consisting of a metallic core and a J-aggregate dye shell. The effect of frequency conversion of plasmonic lines due to the near-field interaction between plasmons and Frenkel excitons of the organic shell is demonstrated. The effect leads to the appearance of additional spectral lines in the long-wavelength part of the spectrum of the system of hybrid particles. The shapes and the relative intensities of the additional lines exactly reproduce the specific features of the original spectrum of plasmonic absorption bands in uncoated metallic nanoparticles. The discovered phenomenon can be used to design new types of high-sensitivity nanosensors, based on plasmon-exciton effects and principles of near-field optics.

A Centimeter-Scale Inorganic Nanoparticle Superlattice Monolayer with Non-Close-Packing and its High Performance in Memory Devices.

Due to the near-field coupling effect, non-close-packed nanoparticle (NP) assemblies with tunable interparticle distance (d) attract great attention and show huge potential applications in various functional devices, e.g., organic nano-floating-gate memory (NFGM) devices. Unfortunately, the fabrication of device-scale non-close-packed 2D NPs material still remains a challenge, limiting its practical applications. Here, a facile yet robust "rapid liquid-liquid interface assembly" strategy is reported to generate a non-close-packed AuNP superlattice monolayer (SM) on a centimeter scale for high-performance pentacene-based NFGM. The d and hence the surface plasmon resonance spectra of SM can be tailored by adjusting the molecular weight of tethered polymers. Precise control over the d value allows the successful fabrication of photosensitive NFGM devices with highly tunable performances from short-term memory to nonvolatile data storage. The best performing nonvolatile memory device shows remarkable 8-level (3-bit) storage and a memory ratio over 105 even after 10 years compared with traditional devices with a AuNP amorphous monolayer. This work provides a new opportunity to obtain large area 2D NPs materials with non-close-packed structure, which is significantly meaningful to microelectronic, photovoltaics devices, and biochemical sensors.

Near-Field Coupling Effects in Mie-Resonant Photonic Structures and All-Dielectric Metasurfaces

We reveal that strong near-field coupling effects can be observed for dissimilar Mie-resonant dielectric meta-atoms and demonstrate that both properties and functionalities of high-index all-dielectric photonic structures can be controlled by engineering their geometry and changing the distance between meta-atoms thus enhancing the effective magnetic response. We describe dielectric dimers, quadrumers, and metasurfaces with a staggered structure of optically induced magnetic moments (the so-called ”optical antiferromagnetism”) and also demonstrate that a strong toroidal response can be introduced in metasurfaces by engineering asymmetric nanoparticle quadrumers as building blocks for novel designs in all-dielectric resonant meta-optics.

Thermally generated metals for plasmonic coloring and surface-enhanced Raman sensing

Spectral coloring glass and its application on the surface-enhanced Raman scattering are demonstrated experimentally via a simple and moderate heat-treating of the top ultrathin gold film to create discrete nanoparticles, which can produce localized surface plasmon resonances and strong plasmonic near-field coupling effects. Ultrathin metal films with a wide range of thicknesses are investigated by different heat-treatment processes. The annealed metal films have been demonstrated with a series of spectral coloring responses. Moreover, the microscopy images of the metal film structures confirm the formation of distinct geometry features in these operation procedures. Densely packed nanoparticles are observed for the ultrathin metal film with the single-digit level of thickness. With increasing the film thickness over 10 nm, metallic clusters and porous morphologies can be obtained. Importantly, the metallic resonators can provide enhanced Raman scattering with the detection limit down to 10 − 7 molL − 1 of Rhodamine 6G molecules due to the excitation of plasmon resonances and strong near-field coupling effects. These features hold great potential for large-scale and low-cost production of colored glass and Raman substrate.

Geometry and near-field coupling effects on the refractive-index sensitivities of individual Ag nanoparticle sensors

The refractive-index sensitivities of sensors of individual Ag nanoparticles (including nanodisks, nanocubes, and nanoprisms) and nanoparticle clusters are investigated numerically and analytically. It is demonstrated that the refractive-index sensitivities are a nonlinear function of the peak positions of localized surface plasmon resonances (LSPRs) with fixed surrounding mediums and metal materials at optical wavelengths. Moreover, this nonlinear function is independent of Ag nanoparticle geometries and near-field couplings between components within nanoparticle clusters. In addition, the discrepancies between our results and those of literatures are also discussed. The present work facilitates choosing appropriate peak positions of LSPRs for plasmonic sensing and detections by only analytically calculating with an equation under LSPR condition.

Plasmonic Near-Field Effect on Visible and Near-Infrared Emissions from Self-Assembled Gold Nanoparticle Films

In this work, we have performed a systematic investigation of the plasmon near-field effect on photoluminescence (PL) behavior of the annealed self-assembled gold nanostructured films. For this purpose, PL spectra of the films in different morphologies are compared. This allows us to identify the role of plasmon near-field enhancement and coupling effects in visible and near-infrared emissions as modulating the energy transfer mechanism between excited electrons and emitted photons. Our results indicate that the films’ near-infrared emission is generated by allowed intraband transition related to the breakdown of the symmetry and momentum selection rules due to the strongly confined near-field distribution. In addition, it is found that the efficiency of the near-infrared emission is directly proportional to the strength of the coupled near-field effect. The observed visible emission of the films is well explained by the interband transition of electrons into the conduction band and subsequent radiation by particle plasmons. The influence of the coupled plasmon resonance of the samples on the characteristics of the visible PL emission is discussed. The observed emission properties of the gold nanostructured films can make them very attractive material for many bio-imaging and surface-enhanced Raman scattering (SERS) applications.

Characterization of Near-Field Coupling Effects From Complicated Three-Dimensional Structures in Rectangular Cavities Using Fast Integral Equation Method

This paper characterizes the frequency domain near-field coupling effects from complicated three-dimensional transmission lines, monopole/dipole radiators, and printed circuit boards (PCBs) located in a metallic rectangular cavity with an efficient iterative fast Fourier transform-accelerated integral-equation method. The simulation method combines the rectangular-cavity Green's functions with the frequency-domain electric field integral equation, which is further accelerated by the extension of adaptive integral method for rectangular cavities. By using this method, the near-field coupling effects and internal field distributions are captured and compared with independent references for various structures, including transmission lines, monopole/dipole radiators, and PCBs in rectangular cavities. Numerical results show that strong couplings do appear at multiple resonant frequencies of the structures. In addition, this method can be further employed to mitigate and suppress near-field coupling effects in the optimization of electromagnetic compatibility/interference designs.

Improved Hybrid Leapfrog ADI-FDTD Method for Simulating Near-Field Coupling Effects Among Multiple Thin Wire Monopole Antennas on a Complex Platform

A hybrid one-step leapfrog alternating-direction-implicit finite-difference time-domain (ADI-FDTD) method integrated with a thin wire algorithm is developed for simulating the near-field mutual coupling effects among multiple wire antennas on an electrically large warship platform, where induced surface current and electric field distributions are also captured. Its stability analysis, based on von Neumann method, is performed in a semi-analytical way and verified numerically. In order to demonstrate its accuracy and efficiency, the near-field mutual coupling effect between two thin wire antennas is at first investigated. Then, it is applied to accurately characterize complex electromagnetic environment effects on a warship with multiple thin wire antennas, and the results are compared with those obtained by the other commercial simulators.

Metallic Metasurfaces for Light Absorbers

Multi-band light perfect absorption by all-metal resonant structure is of particular interest for applications in a wide variety of technologies. Here, we propose and demonstrate a new strategy for achieving multi-band light absorber based on the metallic sub-wavelength structure, which consists of a hexagonally packed nanoring-nanodisk composite array on an opaque metal substrate. By introducing asymmetry to the nanoring-nanodisk hybrid system, new absorption bands emerge. The optical properties are enabled by strong plasmon resonances of the metallic resonators and their near-field coupling effects. Designing such nanostructures is a promising route for all-metal multi-band light absorption platform, which can be used as absorption filters, photo-thermal therapy, multispectral thermal emitters, and plasmonic biosensors.

Annealing effects on electrical behavior of gold nanoparticle film: Conversion of ohmic to non-ohmic conductivity

This paper reports on the electrical behavior of self-assembled gold nanoparticle films before and after high-temperature annealing in ambient environment. These films are made by depositing gold nanoparticles from a colloidal solution on glass substrates using centrifuge deposition technique. The current-voltage (I–V) characteristics of these films exhibits ohmic and non-ohmic properties for un-annealed and annealed films respectively. As the annealing time duration increases, the onset of non-ohmic behavior occurs at higher voltages. To understand the underlying mechanisms for the observed electrical conduction behavior in these films and how electrical conduction is effected by film morphology and structural properties before and after annealing, systematic comparative studies based on scanning electron microscopy (SEM), UV–vis absorption spectroscopy and X-ray photoelectron spectroscopy (XPS) have been performed. The morphology of the films shows that the assembled gold nanoparticles are distributed on the substrate in a random way before annealing. After 2h annealing gold nanoparticles exhibit a higher filling fraction when examined by SEM, which means that they coalesce, upon annealing, with respect to un-annealed films. The UV–vis absorption spectra of the films show that there is a red-shift and broadening in the absorption band for the annealed films. The observed phenomenon is related to the plasmon near-field coupling effect and suggests that the nanoparticle ensembles interspacing has decreased. The structural and crystallinity of the films exhibit amorphous structure before annealing and pure crystalline phases with a preferential growth direction along the (111) plane after annealing. The XPS analysis further suggests the existence of the stable thin oxide layer in the phase of Au2O3 in the annealed films. The I–V characteristics have been described by Simmons’ model for tunnel transport through metal-insulator-metal (MIM) junctions. The Fowler-Nordheim (F-N) plots show the transition of the in-plane charge transport mechanism from direct tunneling to field emission in annealed films. Our results suggest that, the formation of a thin layer of Au2O3, the proximity of the nanoparticles as well as their higher filling fraction are important parameters related with the tunneling process enhancement. The observed non-ohmic conductivity property can make these self-assembled gold nanoparticle films very useful structures in different applications such as sensing, resistors and other nanoelectronic applications.

The Influence of Element Deformation on Terahertz Mode Interaction in Split-Ring Resonator-Based Meta-Atoms

The interaction between terahertz (THz) resonance modes and element deformation in rectangular split-ring resonator (RSRR)-based meta-atoms (MAs) is investigated experimentally. Two types of RSRR-based MAs are presented: lateral-varied SRR (LV-SRR) and arm-twisted SRR (AT-SRR). When the distances from the gaps to the opposite sides of above meta-atoms increase from 10 to 40 μm, the inductive-capacitive (LC) resonance modes and dipole oscillation modes exhibit redshift behavior. The quality factor (Q factor) of LC resonance decreases while that of dipole oscillation modes increases. The THz mode interaction is subject to the distance between the gap and opposite side. An extension of lateral side contributes much more to the enhancement of Q factor of dipole oscillation mode than the twisted arms. The relationship between the near-field coupling effect and THz modes is revealed by the analysis of surface currents as well as the electric energy density distribution, as is in agreement with the experimental results.

Gap controlled plasmon-dielectric coupling effects investigated with single nanoparticle-terminated atomic force microscope probes.

Precise positioning of a plasmonic nanoparticle (NP) near a small dielectric surface is not only necessary for understanding gap-dependent interactions between a metal and dielectric but it is also a critical component in building ultrasensitive molecular rulers and force sensing devices. In this study we investigate the gap-dependent scattering of gold and silver NPs by controllably depositing them on an atomic force microscope (AFM) tip and monitoring their scattering within the evanescent field of a tin dioxide nanofiber waveguide. The enhanced distance-dependent scattering profiles due to plasmon-dielectric coupling effects show similar decays for both gold and silver NPs given the strong dependence of the coupling on the decaying power in the near-field. Experiments and simulations also demonstrate that the NPs attached to the AFM tips act as free NPs, eliminating optical interference typically observed from secondary dielectric substrates. With the ability to reproducibly place individual plasmonic NPs on an AFM tip, and optically monitor near-field plasmon-dielectric coupling effects, this approach allows a wide-variety of light-matter interactions studies to be carried out on other low-dimensional nanomaterials.

Ultimate Limit of Light Extinction by Nanophotonic Structures.

Nanophotonic structures make it possible to precisely engineer the optical response at deep subwavelength scales. However, a fundamental understanding of the general performance limits remains a challenge. Here we use extensive electrodynamics simulations to demonstrate that the so-called f-sum rule sets a strict upper bound to the light extinction by nanostructures regardless their internal interactions and retardation effects. In particular, we show that the f-sum rule applies to arbitrarily complex plasmonic metal structures that exhibit an extraordinary spectral sensitivity to size, shape, near-field coupling effects, and incident polarization. The results may be used for benchmarking light scattering and absorption efficiencies, thus imposing fundamental limits on solar light harvesting, biomedical photonics, and optical communications.

Absorbance amplification using chromophore-nanoplasmon coupling for ultrasensitive protein quantification.

A plasmonic nanoscale Lycurgus cup array (nanoLCA), via near-field interaction with chromophores in commercial colorimetric biochemical assays, can drastically enhance assay sensitivity by over 2 orders of magnitude. A 96-microwell plate modified by placing the plasmonic nanoLCA on the well-bottom was used with the commercial Bradford protein quantification assay. Plasmons on the nanoLCA serve as an energy donor to matched resonance chromophores, and the near-field plasmonic energy coupling effect results in an increase in absorbance value at the plasmonic resonance wavelength. Even with a 5.1-fold reduced sample volume, a limit of detection enhancement factor of 200 is accomplished using the nanoLCA compared to using the conventional Bradford assay without plasmon aid. The nanoLCA-microplate sensing platform is readily scalable to 384- or 1536-microwell plates, which further reduces the sensing volume and boosts detection throughput with the enhanced sensitivity.

Concurrent Scanning Double-Tip-Enhanced Raman Scattering by Near-Field Coupling Effect

Near-field coupling effect in the narrow gaps between two metallic probes strongly enhances the electromagnetic field. In this paper, we design and implement a concurrent scanning double-tip-enhanced Raman scattering (CS-TERS) system, which can strongly enhance the tip-enhanced Raman scattering (TERS) enhancement factor without the restriction on the sample height. We use finite-difference time domain (FDTD) simulations to study the enhancement properties of double-tip-enhanced Raman scattering and then discuss the tip taper angle dependence of the TERS enhancement. The obtained results show that the proposed CS-TERS system can be implemented without the near-field coupling effect between the metal tip and the metal substrate; thus, the height of the measured sample will be greatly enlarged and the application of TERS method will be expanded.

Multispectral spatial and frequency selective sensing with ultra-compact cross-shaped antenna plasmonic crystals

We predict a novel multispectral spatial and frequency selective sensing based on a plasmonic sub-diffraction-limited (<λ/4) nanostructure. Via combining the strong plasmon near-field coupling effects, particle plasmon resonances (PPRs) and their hybridization by the metallic cross-shaped antennas (CSAs), a total of seven resonances with the minimum bandwidth<7nm are obtained in the visible and near-infrared region. Remarkably distinct biosensing behaviors at different spatial locations and different resonant wavelengths are simultaneously achieved, suggesting a new impressive sensing motif. High-quality biosensing with the maximal sensitivity (S=1134nm/RIU), figure of merit (FoM∼71.4), and high contrast ratio of spectral intensity difference (ΔR=44.2%) can be attained by detection a slight refractive index change of a thin biomolecular layer. These unique features of the proposed sensing motif could provide a powerful approach to develop desirable plasmon sensors with simultaneous multispectral spatial and frequency selective sensing, and hold potential applications in the high-integrated components for the high-performance plasmonic biosensing, detection and imaging.

Macroscopic Layers of Chiral Plasmonic Nanoparticle Oligomers from Colloidal Lithography

Optical near-field coupling between closely spaced plasmonic metal nanoparticles is important to a range of nanophotonic applications of high contemporary interest, including surface-enhanced molecular spectroscopy, nanooptical sensing, and various novel light-harvesting concepts. Here we report on monolayers of chiral heterotrimers and heterotetramers composed of closely spaced silver and/or gold nanodisks of different heights fabricated through facile hole-mask colloidal lithography. These quasi-three-dimensional oligomers are interesting for applications because they exhibit hot gaps and crevices of nanometric dimensions, a pronounced circular dichroism, and optical chirality in the visible to near-infrared wavelength range, and they can be produced in large ensembles (>109) of identical orientation. We analyze the optical properties of the samples based on simulation results and find that the circular dichroism is due to strong near-field coupling and intricate phase retardation effects originating in the three-dimensional character of the individual oligomers.