Near-field Coupling Research Articles

WiFi Energy-Harvesting Antenna Inspired by the Resonant Magnetic Dipole Metamaterial.

WiFi energy harvesting is a promising solution for powering microsensors and microsystems through collecting electromagnetic (EM) energies that exist everywhere in modern daily lives. In order to harvest EM energy, we proposed a metamaterial-inspired antenna (MIA) based on the resonant magnetic dipole operating in the WiFi bands. The MIA consists of two metallic split-ring resonators (SRRs), separated by an FR4 dielectric layer, in the broadside coupled configuration. The incident EM waves excite surface currents in the coupled SRRs, and the energy is oscillating between them due to near-field coupling. By varying the vertical distance of the two SRRs, we may achieve impedance matching without complicated matching networks. Collected EM energy can be converted to DC voltages via a rectifier circuit at the output of the coupling coil. Measured results demonstrate that the designed MIA may resonate at 2.4 GHz with a deep-subwavelength form factor (14 mm×14 mm×1.6 mm). The WiFi energy-harvesting capability of the proposed MIA with an embedded one-stage Dickson voltage multiplier has also been evaluated. A rectified DC voltage is approximately 500 mV when the MIA is placed at a distance of 2 cm from the WiFi transmit antenna with a 9 dBm transmitting power. The proposed compact MIA in this paper is of great importance for powering future distributed microsystems.

Giant terahertz pulling force within an evanescent field induced by asymmetric wave coupling into radiative and bound modes.

Manipulation of nano-scale objects by engineering the electromagnetic waves in the environment medium is pivotal for several particle handling techniques using optical resonators, waveguiding, and plasmonic devices. In this Letter, we theoretically demonstrate the possibility of engineering a compact and tunable plasmon-based terahertz (THz) tweezer using a graphene monolayer that is deposited on a high-index dielectric substrate. When a nanoparticle located in a vacuum in the vicinity of the graphene monolayer is illuminated under total internal reflection, as light is launched from the substrate, such a device is shown to be capable of inducing an enhanced rotating dipole in the nanoparticle thus enabling asymmetric, directional near-field coupling into the graphene plasmon mode and the radiative modes in the substrate. As a result of the total momentum conservation, the net force exerted on the particle points in a direction opposite to the pushing scattering force of the exciting evanescent field. Our results can contribute to novel realizations of photonic devices based on polarization-dependent interactions between nanoparticles and electromagnetic mode fields.

Strong Near-Field Coupling for Enhancing Plasmonic Chirality Toward Single-Molecule Sensing

Plasmonic chirality shows great potential in analytical chemistry, biomedicine, and life science due to the strong chiroptical response generated from metallic nanostructures. However, significant chiral effects are mainly realized in three-dimensional structures because of their high structural asymmetry and large plasmon mode volume. This paper describes planar plasmonic “τ”-shaped structure arrays that can obtain significantly enhanced chiroptical responses for single-molecule detection. The “τ”-shaped structure arrays based on a chiral–achiral coupling scheme are constructed to generate a large superchiral field and enhanced circular dichroism (CD). Numerical simulations show that the chiroptical responses can be efficiently manipulated by the near-field coupling strength of plasmons determined by the parameters of the arrays and the structural chirality of the unit cells. The planar “τ”-shaped structure arrays with a large CD signal and enhanced superchiral field enable ultrasensitive detection of single-molecule bovine serum albumin (BSA) protein.

Analytical analysis of inhomogeneous and anisotropic metamaterial cylindrical waveguides using transformation matrix method

Several applications of a cylindrical waveguide filled with inhomogeneous materials have been demonstrated. Analytical solutions for modes, an inhomogeneous cylindrical waveguide, are available only for a few material parameters and those are also mostly approximations. We have obtained exact analytical solutions for cylindrical waveguides filled with different inhomogeneous and anisotropic material parameters. Different sets of material parameters are obtained using the geometric transformation method. The analytical and numerical modal field distributions, the value of cutoff frequencies and effective mode indices for different propagating modes are well matched to each other. The inhomogeneity can be used to miniaturize the radial dimensions of the waveguide. This is highly desirable for many applications like sensing, near-field coupling and magnetic resonance imaging (MRI).

Plasmonics for advance single-molecule fluorescence spectroscopy and imaging in biology

The elucidation of complex biological processes often requires monitoring the dynamics and spatial organization of multiple distinct proteins organized on the sub-micron scale. This length scale is well below the diffraction limit of light, and as such not accessible by classical optical techniques. Further, the high molecular concentrations found in living cells, typically in the micro- to mili-molar range, preclude single-molecule detection in confocal volumes, essential to quantify affinity constants and protein-protein reaction rates in their physiological environment. To push the boundaries of the current state of the art in single-molecule fluorescence imaging and spectroscopy, plasmonic materials offer encouraging perspectives. From thin metallic films to complex nano-antenna structures, the near-field electromagnetic coupling between the electronic transitions of single emitters and plasmon resonances can be exploited to expand the toolbox of single-molecule based fluorescence imaging and spectroscopy approaches. Here, we review two of the most current and promising approaches to study biological processes with unattainable level of detail. On one side, we discuss how the reduction of the fluorescence lifetime of a molecule as it approaches a thin metallic film can be exploited to decode axial information with nanoscale precision. On the other, we review how the tremendous progress on the design of plasmonic antennas that can amplify and confine optical fields at the nanoscale, powered a revolution in fluorescence correlation spectroscopy. Besides method development, we also focus in describing the most interesting biological application of both technologies.

Dual-wavelength active and tunable modulation at telecommunication wavelengths using graphene-metal hybrid metamaterial based on plasmon induced transparency

Classic optical systems with a similar response to electromagnetically induced transparency (EIT) have received considerable attention. Plasmonically induced transparency (PIT) response is usually achieved by near-field coupling between resonators’ bright and dark modes. Most PIT structures are based on metamaterials and have a constant near-infrared spectral response; manipulating the PIT spectral response without changing the geometric structure and modifying the substrate or electrical biasing is impossible. A graphene-metal metamaterial structure is proposed to create an active tunable near-IR transparency window. In the proposed two-layer structure, the metal bars act as bright resonators in the upper layer. In the lower layer, the metal nanoribbons act as dark resonators and two sets of bilayer graphene are placed separately below the bright resonators and above the dark resonators. At first, the optimal induced transparency window is obtained by modifying the metal metamaterial’s geometrical parameters. Then, the properties of the generated induced window can be modified by varying the Fermi energies of the used graphene sheets in the hybrid metal-graphene metamaterial. The Fermi energies of the graphene sheets are adjusted by applying a voltage that causes the PIT phenomenon to be actively tunable. The proposed structure can be used as an active modulator in o and c communication bands. The designed modulator allows for 85% and 90% amplitude modulation depths (MD) at about 1307 nm and 1554 nm wavelengths.

Resonant Excitation Analysis of the Absorption Enhancement of L-Shaped Metasurfaces in Midinfrared Band

Surface plasmon waveguides have attracted a lot of attention in recent years due to their ability to conduct light in the subwavelength scale. A large number of metallic structures have been used as waveguides and applied in integrated photonic circuits. Among these structures, the metal-insulator-metal surface plasmon waveguide is considered to have unique advantages. Compared to other waveguide forms, it is more compact and thus easier to integrate into optical circuits. For these reasons, we investigated the transmission properties of surface plasmon waves in metal-insulator-metal waveguides and built a MIM absorber consisting of gold and dielectric. We developed an interference model for the MIM absorber and concluded that the interactions between the metal patch and the substrate are not negligible in the near field. In addition, magnetic resonance plays an equally important role in approaching uniform absorption. Absorption spectra with different structural parameters, incidence angles, and polarizations were investigated. A sharp absorption peak was also found to be caused by the Rayleigh anomaly. The study reported in this paper contributes to further understanding of the physical properties of the MIM absorber, and we expect to refine the theoretical model in the future to eliminate the bias caused by near-field coupling.

Spectral analysis of localized surface phonon polaritons in resonant silicon carbide hollow cylinder array

Manipulation of the amplitude and frequency of resonant optical surface waves in mid-infrared is of great interest for improvement of photonic devices and vibrational molecule sensing applications. Antennas supporting localized surface phonon polaritons (LSPhPs) fold the optical phonons into periodic pillar array to control the scattering process. Energy exchange, mode evolution and near-field coupling mechanism are investigated thoroughly, and it is demonstrated that the transverse dipole mode in the 6H-silicon carbide hollow cylinder array shows excellent absorption efficiency and tunable capability across a wide spectral range. Dependence of local field on structural parameters in the polarized sub-mode is explored to elucidate the optical properties. Near-field coupling is further evaluated by combining the values of current distribution with multipole decomposition. This study also provides a practical guide to establish a general framework for exploring the spectral tuning and coupling mechanisms of LSPhP modes.

Source Reconstruction of Electronic Circuits in Shielding Enclosures Based on Numerical Green’s Function and Application in Electromagnetic Interference Estimation

In this work, to characterize the radiated emission from electronic circuits in shielding enclosure, an improved electric dipole-based source reconstruction method (SRM) is developed. Moreover, by resorting to this reconstructed equivalent source, the estimation of electromagnetic interference (EMI) between different circuit modules in the enclosure can be conveniently and accurately evaluated. Different from the free-space SRM, the equivalent dipoles of the proposed SRM are directly placed over the shielding box enclosed circuit board, and the numerical Green’s function (NGF) is developed to bridge the connection between the equivalent dipoles and the planar scanned magnetic near-field. With the NGF strategy, the effects of surrounding environments (including the presence of substrate, ground plane, and shielding enclosure) are inclusively considered, which makes the proposed SRM valid for any complex EM environments in theory. Since the proposed SRM also has the capability of reproducing the radiated emission inside the shielding enclosure, it is critically helpful to evaluate the EMI between different circuit modules inside the shielding enclosure. To reach this aim, the Rayleigh-Carson Reciprocity theorem is referred, in which the original EM coupling problem is decomposed into the “forward problem” and “reverse problem.” A Huygens box enclosing the victim circuit module is built and the EM fields on the box are obtained using the reconstructed dipoles in both problems. Then the EMI can be calculated accordingly. Therefore, the novelty and merits of the proposed approach are threefold: 1) the physically-based equivalent dipole model considers the interactions between the radiation sources and the surrounding objects, leading to the accurate prediction of radiated emissions both outside and inside the enclosure; 2) due to the utilization of the NGF, there is no approximation involved in the solution of the dipole’s radiation, thus improving the calculation precision; and 3) the near-field coupling between multiple electronic circuit modules inside the shielding enclosure can be identified precisely, which is useful for diagnostics of radiation sources. The effectiveness of this algorithm is verified by representative numerical examples.

On the Correlation Between Near-Field Scan Immunity and Radiated Immunity at Printed Circuit Board Level – Part I

Near-field (NF) scan immunity is a powerful technique to identify the root-cause of failures produced during radiated immunity (RI) tests at the integrated circuit and printed circuit board (PCB) levels. However, a prediction method of the RI level from NF scan immunity results is still lacking. This type of method would help board designers to anticipate risks of RI noncompliance related to PCB directly after an NF scan campaign. This two-part article addresses this issue. In the first part, the equivalence between far-field (FF) and NF coupling on a microstrip line is discussed in order to derive an estimator of the FF induced voltage from NF scan results.

Effect of non-alignment on nearfield coupling, charge density and sensitivity to the host medium of a gold nanorod homodimer

Effect of non-alignment on nearfield coupling, charge density and sensitivity to the host medium of a gold nanorod homodimer

Crossover from renormalized to conventional diffusion near the three-dimensional Anderson localization transition for light

We report on anomalous light transport in the strong scattering regime. Using low-coherence interferometry, we measure the reflection matrix of titanium dioxide powders, revealing crucial features of strong optical scattering which can not be observed with transmission measurements: (i) a subdiffusive regime of transport at early times of flight that is a direct consequence of predominant recurrent scattering loops, and (ii) a crossover to a conventional, but extremely slow, diffusive regime at long times. These observations support previous predictions that near-field coupling between scatterers prohibits Anderson localization of light in three-dimensional disordered media.

Plasmonic coupling in transversely shifted symmetric Au nanocube trimer: New coupling mechanism and plasmonic scaling trend

Using FDTD simulation tool, the plasmon resonance and near-field couplings between three identical nanocubes made of gold material arranged in face-to-face configuration are investigated at a diverse range of transverse shifts and a constant gap separation. The dipole–dipole coupling approximation fails to explain the coupling features when the shift is smaller than the cube’s size and this due to the excitation of higher-order modes as the polarization pattern switched from facing facets into facing edges. The results of the simulations show the resonance wavelength is unexpectedly redshifted and contradicts the expected blue shift by the dipole–dipole model.

Constant frequency reconfigurable terahertz metasurface based on tunable electromagnetically induced transparency-like approach

A terahertz constant frequency reconfigurable metasurface based on tunable electromagnetically induced transparency (EIT)-like property was designed, whose transparency window frequency did not vary with Fermi energy. This structure was composed of two single-layer graphene resonators, namely, left double big rings and right double small rings. An evident transparency window (EIT-like phenomenon) was caused by the near-field coupling between bright modes of the two resonators in the transmission spectrum, in which amplitude over 80% was acquired at 1.98 THz. By individually reconfiguring the Fermi energy of each resonator, the EIT-like effects, transparency window amplitude, modulation speed and group delay could be actively controlled while the frequency of EIT-like window remained constant. Significantly, the transparency window was fully modulated without changing the frequency, and the maximum modulation depth reached 78%. Furthermore, the modulation speed also increased because the total graphene area A was effectively reduced in the proposed structure. Compared with other metasurface structures, the modulation properties of the proposed structure showed higher performance while the EIT-like window frequency remained static. This research provides an alternative method for developing constant frequency reconfigurable modulation terahertz devices (such as optical switches and modulators), as well as a potential approach for miniaturization of terahertz devices.

Silver Nanopillar Arrayed Thin Films with Highly Surface-Enhanced Raman Scattering for Ultrasensitive Detection.

Surface-enhanced Raman scattering (SERS) technique based on surface plasmon resonance has been considerably investigated in recent years due to its superior sensitivity in the detection of organic or biological molecules at trace levels. However, most research usually focuses on artificial architectures as SERS substrates that always have a complex and expensive micro-nanofabrication process. The high cost of masks for SERS substrates becomes a key obstacle for the widespread commercialization of SERS technology. In this paper, a biomimetic SERS substrate composed of silver-coated nanopillar arrays on the top of a cicada wing was advanced to overcome these challenges as both substrates and masks. Benefiting from the high near-field plasmon resonance coupling at the limited space among neighboring nanopillars, a dramatically increased SERS signal can be achieved using rhodamine 6G (R6G) as a model molecule. Encouragingly, the analytical enhancement factor of the order of more than 108 has been conveniently realized with a reliable detection concentration of R6G of about 100 pM or less. This work provides a promising route for designing cost-effective and highly sensitive SERS substrates and the related mask fabrication using our previously proposed template transfer nanoimprint.

Extracting optical absorption characteristics from semiconductor nanowire arrays

A method based on extraction of the effective absorption coefficient using Beer–Lambert’s law on simulated transmissions is used to understand the optical absorption characteristics of semiconductor nanowire arrays. Three different semiconductor nanowire arrays, viz. silicon (Si), gallium arsenide (GaAs) and amorphous silicon (a-Si), are evaluated using the method. These semiconductors were chosen since two of them have similar real parts of the refractive index in the visible range, while the other two have comparable imaginary parts of the refractive index in the visible range. We examine the roles of the real and imaginary parts of the refractive index in enhancing the absorption characteristics in the nanowire arrays due to the excitation of radial and photonic Bloch modes. We observe that high absorption peaks at modal resonances correspond to the resonance peaks in reflections from the nanowire–air interface. Further, the wavelengths of these two peak resonances are slightly detuned according to the Kramers–Kronig relation for an oscillator system. The study confirms that the resonance wavelengths of radial HE modes are diameter and refractive index dependent. The study extends the understanding to the absorption characteristics due to the excitation of the photonic Bloch modes caused by near-field coupling. Excitation of Bloch modes leads to increased absorption and quality factor as compared to only radial mode excitation. We also conclude that the imaginary part of the refractive index of the semiconductor, influence the diameters at which Bloch modes are excited for a given lattice spacing. We observe that semiconductors with a higher bulk value of absorption coefficient need to be ordered more densely in the nanowire array to be able to excite the photonic crystal modes within the array. Interestingly, we see that for Si, GaAs and a-Si arrays with an equal diameter of 80 nm and lattice spacing of 400 nm, the peak absorption is almost the same, even though GaAs and a-Si are highly absorptive materials compared to Si. Thus, both radial and Bloch mode excitations can be used to design absorption profiles in a semiconductor nanowire array.

Tailoring electromagnetic responses in a coupled-grating system with combined modulation of near-field and far-field couplings

Herein, we establish a hybrid coupling model (HCM) containing both near- and far-field couplings to describe the electromagnetic response of the coupled-grating system composed of two parallelly aligned subwavelength dielectric gratings. The HCM shows that the near-field coupling strength only contributes to the frequency splitting of two resonant modes, while the far-field one contributes to the frequency splitting and the linewidths of two resonant modes simultaneously. By changing the distance between two dielectric gratings, both the near- and far-field coupling strengths can be flexibly tuned, giving rise to rich electromagnetic responses. In addition, the formation of Fabry-Perot bound states in the continuum in coupled-grating systems can be clearly explained by the HCM. In this paper, we not only provide an all-dielectric platform for simultaneously manipulating near- and far-field couplings but also offer a viable approach to achieve reflectance/transmittance spectra with diverse shapes.

Mechanical model for the motion of RPV internals affecting neutron flux noise

In the decade after 2000, the amplitude of the neutron flux fluctuations in German 1300 MWe class PWR built by Kraftwerk Union (KWU) increased and later decreased significantly. To further investigate the hypothesis that changes in the mechanical properties of the fuel assemblies (FAs) and subsequent stronger mechanical oscillations of fuel assemblies and other RPV internals are responsible for the observations and to find possible excitation mechanisms, a simple mechanical model is developed. It describes the dynamic motion response of the mechanically coupled system of Reactor Pressure Vessel (RPV), core barrel and a row of fuel assemblies and takes reactive fluidic forces into account. To determine the components to be considered in this model, RPV internals with an effect on neutron flux are identified in a prior step.The dynamic answers of the model to generic excitation scenarios and parametric studies reveal distinct properties of the system. They indicate 1) that fluidic near-field coupling can equalize the fuel assemblies’ reaction amplitudes within a certain region regardless of their individual stiffnesses, 2) that the fluid must be part of the oscillation, 3) that a non-periodicity might stem from a superposition and interaction of several oscillators, 4) that a local excitation source does not spread or synchronize over the whole core, 5) that a neglection or separate consideration of the RPV and core barrel may be justified in some cases. The findings altogether affirm that core-wide oscillations of the fluid flow leading to a simultaneous oscillation of individual fuel assembly groups, possibly including bidirectional effects between fluid and structure, in combination with the changes of the fuel assemblies’ mechanical properties, might be responsible for the temporary increase of neutron flux fluctuations observed in PWR built by KWU.A variant of the mechanical model was studied in a full core investigation with coupled neutron kinetics simulations. These simulations qualitatively reproduce important features of measurement data, while being overall too small in amplitude to explain the observations quantitatively, which confirmed the necessity of considering further effects.

Space-Time-Modulated Reconfigurable Metamaterial Based on a Field-Focused Cavity for Nonreciprocal Transmission Control and Frequency Conversion.

Lorentz reciprocity is a fundamental physical property limiting advanced wave propagation control. Previously, special materials and magnetic bias were used to break the reciprocity; however, the approaches are limited by the bulky and costly implementation. To achieve nonreciprocity without magnetic bias, space-time-modulated metamaterials have been investigated for far-field wave propagation control. The metamaterial can also support wave propagation based on near-field coupling between the periodically arranged unit cells, i.e., magneto-inductive waves (MIWs). Near-field wave propagation control via the metamaterial has various significant applications; nevertheless, the potential for near-field wave propagation control has not been fully explored. Therefore, it is necessary to investigate the potential of the space-time-modulated near-field metamaterial. This paper demonstrates nonreciprocal MIW propagation control using a space-time-modulated metamaterial. To achieve field manipulation, we propose a tunable unit cell suitable for creating a cavity mode at a deep subwavelength scale (∼λ/103). Spatial field modulation, achieved by breaking the translational symmetry of the unit cells, allows for the creation of reconfigurable waveguides on the metamaterial. Temporal field modulation, achieved by breaking the capacitive symmetry of the varactor, allows for direction-dependent transmission in the waveguide. This spatiotemporal modulation successfully achieves nonreciprocal wave propagation and frequency conversion, investigated under various conditions. The proposed space-time-modulated metamaterial may provide significant advances for a wide range of systems that require dynamic, nonreciprocal, near-field wave propagation control.

Broadside-coupling–enabled insulator-to-metal transition in a terahertz metasurface

We theoretically demonstrate stacked-dipole-resonators–based (broadside near-field coupling configuration) multilayer metasurfaces separated by a vanadium dioxide film to achieve stronger field confinement in the spacer (VO2) region. Under relatively intense terahertz excitation (20 Vm−1) assisted by larger area electric field confinement, insulator-to-metal transition (IMT) in VO2 spacer is realized resulting in frequency (dipole mode) and amplitude (Fano mode) tunable metasurfaces. Enhancement in probing THz field triggers much stronger field confinement inside the spacer layer leading to increased VO2 conductivity (responsible for IMT) through the Poole-Frankel effect. Such broadside coupled IMT-based terahertz metamaterials can help in realizing active meta devices for THz domain.