Plane Wave Excitation Research Articles

Dimers of gold-silver core-shell nanospheres: The effect of interparticle gap on the refractive index sensitivity and extinction spectrum

Gold-silver and silver-gold core-shell nanoparticle dimers were studied based on their extinction cross-section spectrum and bulk refractive index sensitivity. The simulations were performed by using the boundary element method (BEM) and the polarization direction of the used plane-wave excitation was parallel with the symmetry axis. The running parameters were the particles’ outer and inner radii and their (interparticle gap/full diameter). For different particle sizes and distances, the shape of the spectra and the refractive index sensitivities are presented. In the extinction spectra, the observable peaks originate from either the gold or silver components and the most intense peak position can be distinctly assigned to one of them. A sharp boundary separates these two regions in the plane of the core radius and shell thickness parameters. It was found that by decreasing the interparticle gap, the boundary line between these two regions shifts towards the thinner shells for Ag@Au dimers, while it shifts towards the smaller cores for Au@Ag dimers. Since the sensitivity of peaks corresponding to the Au and Ag components are significantly different, the presented data can help optimize interparticle gaps concerning the core/shell thicknesses to maximize the sensitivity of nanoparticle dimers.

Quasinormal mode theory for nanoscale electromagnetism informed by quantum surface response

We report a self-consistent quasinormal mode theory for nanometer scale electromagnetism where the possible nonlocal and quantum effects are treated through quantum surface responses. With Feibelman's frequency-dependent \textit{d} parameters to describe the quantum surface responses, we formulate the source-free Maxwell's equations into a generalized linear eigenvalue problem to define the quasinormal modes. We then construct an orthonormal relation for the modes and consequently unlock the powerful toolbox of modal analysis. The orthonormal relation is validated by the reconstruction of the full numerical results through modal contributions. Significant changes in the landscape of the modes are observed due to the incorporation of the quantum surface responses for a number of nanostructures. Our semi-analytical modal analysis enables transparent physical interpretation of the spontaneous emission enhancement of a dipolar emitter as well as the near-field and far-field responses of planewave excitations in the nanostructures.

Chiral discrimination by polarization singularities of a metal sphere

The practical applications of chiral discrimination are usually limited by the weak chiral response of enantiomers and the high complexity of detection methods. Here, we propose to use the C lines (i.e., lines of polarization singularities) emerged in light scattering by a metal sphere to detect small chiral particles. Using full-wave numerical simulations and analytical multipole expansions, we determined the absorption dissymmetry of deep-subwavelength helices (i.e., chiral particles) at different positions on the C lines. We find that the absorption dissymmetry can be much larger than that induced by circularly polarized plane wave excitation, and it can be attributed to the asymmetric absorption of the induced electric and magnetic dipoles. We also show that the absorption dissymmetry strongly depends on the anisotropy of the helices. The results may find applications in optical manipulations, optical sensing, and chiral quantum optics.

Physics-informed neural networks for imaging and parameter retrieval of photonic nanostructures from near-field data

In this paper, we develop a deep learning approach for the accurate solution of challenging problems of near-field microscopy that leverages the powerful framework of physics-informed neural networks (PINNs) for the inversion of the complex optical parameters of nanostructured environments. Specifically, we show that PINNs can be flexibly designed based on full-vector Maxwell’s equations to inversely retrieve the spatial distributions of the complex electric permittivity and magnetic permeability of unknown scattering objects in the resonance regime from near-field data. Moreover, we demonstrate that PINNs achieve excellent convergence to the true material parameters under both plane wave and point source (localized) excitations, enabling parameter retrieval in scanning near-field optical microscopy. Our method is computationally efficient compared to traditional data-driven deep learning approaches as it requires only a single dataset for training. Furthermore, we develop and successfully demonstrate adaptive PINNs with trainable loss weights that largely improve the accuracy of the inverse reconstruction for high-index materials compared to standard PINNs. Finally, we demonstrate the full potential of our approach by retrieving the space-dependent permittivity of a three-dimensional unknown object from near-field data. The presented framework paves the way to the development of a computationally driven, accurate, and non-invasive platform for the simultaneous retrieval of the electric and magnetic parameters of resonant nanostructures from measured optical images, with applications to biomedical imaging, optical remote sensing, and characterization of metamaterial devices.

Characterization of acoustic materials at arbitrary incidence angle using sound field synthesis

Standardized methods for measuring sound absorption such as the impedance tube and reverberation chamber methods are limited to normal or diffuse incidence, respectively. Two research axes have been generally followed in the literature to develop alternative techniques, the first one focusing on the measurement part, that is from the two-microphone technique to the use of microphone arrays or pressure-velocity sensors. The second axis focuses on the excitation part with for instance the use of sound field synthesis techniques. Since acoustic impedance and sound absorption coefficient of materials are classically defined under normal and oblique plane wave excitation, synthesizing an “ideal” plane wave using a loudspeaker array would allow measuring these acoustics quantities using a simple microphone pair. In this article, the effect of the different parameters of a loudspeaker array on acoustic plane waves reproduction on a material’s surface is first numerically studied. Then, numerical and experimental results for the estimation of both impedance and absorption coefficients are reported. These results show that sound field synthesis allows to characterize a material for arbitrary incidence angles over a wide frequency range, thus offering an alternative method to standard techniques and an improvement over existing works.

Computational modelling of dynamic soil-structure interaction in shear wall buildings with basements in medium stiffness sandy soils using a subdomain spectral element approach calibrated by micro-vibrations

This paper presents a strategy for modelling dynamic soil-structure interaction (DSSI) using the spectral element method (SEM) with a Discontinuous Galerkin approach, calibrated by micro-vibrations. The proposed methodology allows not only to adjust the vibration frequencies of the structure but also the observed vibration modes. First, models of two structural shear wall buildings with basements in medium dense sandy soils are developed to estimate empirical modal characteristics and calibrate the structural subdomain and low-strain site properties. Convenient 3D arrays of multiple seismic sensors are used to obtain the environmental vibrations measurements. Afterwards, an optimization process is conducted to calibrate volumetric models of structures. This optimization is performed by preserving the most relevant modal frequencies and shapes to achieve an equivalent dynamic response. Finally, structural models are placed into a neighbouring soil model (soil subdomain), approximating nonlinear soil behaviour by an equivalent linear strategy. Using this complete soil-structure interaction model, relevant engineering performance parameters are assessed via simulations of buildings subjected to a plane wave excitation. The results show the significant effect DSSI have in shear-wall buildings with basements and the importance of considering the flexibility of the foundation in the interpretation of the results. In general, results indicate that DSSI effects are strongly dependent on the input frequency content, which might cause a reduction of the inter-story drifts. Furthermore, a significant period lengthening of the studied structures up to 47% is found, as well as a considerable decrease in story shear up to 220% and a maximum lateral roof displacement reduction of 34% when compared against fixed base referential responses.

Bowtie nanoantenna driven by a Yagi-Uda nanoantenna: a device for plasmon-enhanced light matter interactions

We propose a plasmonic device, based on the combination of a Yagi-Uda nanoantenna and a bowtie nanoantenna, that can enable on-chip implementation of plasmon-enhanced light-matter interaction processes such as surface-enhanced Raman scattering (SERS), surface-enhanced infrared absorption spectroscopy, and plasmon-enhanced fluorescence. In this device, a localized source is employed to excite the Yagi-Uda nanoantenna, which in turn drives the bowtie nanoantenna. We employ finite difference time domain (FDTD) method to perform numerical simulations to obtain radiation characteristics of the Yagi-Uda nanoantenna as well as the electric field enhancements in the vicinity of the bowtie nanoantenna excited by the Yagi-Uda nanoantenna. We find that for a wavelength of 785 nm, an electric field enhancement of ∼ 196 can be achieved in between the arms of the bow-tie nanoantenna even when the minimum gap between nanostructures is as large as 10 nm. It is found that this electric field enhancement is significantly large when compared with the maximum electric field enhancement (∼ 11) obtained for direct excitation of the bowtie nanoantenna by a point source or with the maximum electric field enhancement (∼ 34) obtained for plane wave excitation of the bowtie nanoantenna. As the electromagnetic enhancement of SERS can be approximated to be the fourth power of the electric field enhancement, SERS electromagnetic enhancement of ∼ 1.5 × 109 is achieved for the bow-tie nanoantennas excited by the Yagi-Uda nanoantennas, even when the minimum gap between the arms of the bow-tie nanoantenna is as large as 10 nm. We also analyze the effect of various geometrical parameters of the nanoantennas and show that the maximum electric field enhancement at a given wavelength can only be obtained when both the Yagi-Uda nanoantenna and the bowtie nanoantenna are resonant at that wavelength. Moreover, we calculate the electric field enhancements at different near-infrared wavelengths. Employing the proposed device, an electric field enhancement of ∼ 945 is obtained at a wavelength of 1500 nm resulting in a SERS electromagnetic enhancement factor as high as ∼ 8 × 1011, even when the minimum gap between nanostructures is as large as 10 nm.

Modified Finite-Difference Time-Domain Method for Hertzian Dipole Source under Low-Frequency Band

In this paper, a modified finite-difference time-domain (FDTD) method is proposed for the rapid analysis of a Hertzian dipole source in the low-frequency band. The FDTD technique is one of the most widely used methods for interpreting high-resolution problems such as those associated with the human body. However, this method has been difficult to use in the low-frequency band as the required number of iterations has increased significantly in such cases. To avoid this problem, FDTD techniques using quasi-static assumptions in low-frequency bands were used. However, this method was applied only to plane wave excitation, making it difficult to apply to near-field problems. Therefore, a modified approach is proposed, involving the application of the FDTD technique with a quasi-static approximation to an electric and magnetic dipole problem. The results when using the proposed method are in good agreement with those from a theoretical solution. An example of comparison with the standard FDTD method is shown for illustrating the proposed method’s performance.

On the Determination of Acoustic Properties of Membrane Type Structural Skin Elements by Means of Surface Displacements

The article focuses on the determination of the acoustic properties (sound transmission loss, sound absorption and transmission coefficient under acoustic plane wave excitation) of membrane-type of specimens by means of a combination of incident plane wave sound pressure and membrane surface displacement information, measuring the sound pressure with a microphone and the membrane displacement by means of a laser Doppler vibrometer. An overview of known measurement methods and the theoretical background of the proposed so-called mobility-based method (MM) is presented. The proposed method was compared with the conventional methods for sound transmission loss and absorption measurement in the impedance tube, both numerically and experimentally. Finite element model (FEM) simulation results of two single layer membrane samples of different shape configurations were compared, amongst which six different variations of the backing wall termination. Four different approaches to determine the sound transmission loss and two methods to determine sound absorption properties of the membranes were compared. Subsequently, the proposed method was tested in a laboratory environment. The proposed MM method can be possibly used to measure the vibro-acoustic properties of building parts in situ.

SPP waveguide based on the Goos-Hänchen effect.

We propose and realize a single-mode surface plasmon polariton (SPP) waveguide formed by two parallel grooves on the surface of a high-quality silver film. In the waveguide, the SPP wave undergoes the Goos-Hänchen shift of up to 0.77⋅λSPP at each successive reflection, with the electric field of the SPP wave outside the metal, which significantly reduces the metal loss of the guided SPP wave. Using the waveguide, we demonstrate a complex on-chip optical circuit that includes (1) SPP plane wave excitation, (2) SPP plane wave focusing with a parabolic mirror, and (3) coupling of the focused SPP wave to the waveguide.

Torsional wave focusing in cylindrical structures with the conformal gradient-index phononic crystal lens

We present torsional wave focusing in cylindrical structures integrated with conformal gradient-index phononic crystal (GRIN-PC) lenses. The mechanical lens modifies the refractive characteristics of propagating waves along the axial direction such that the torsional wave energy focuses at a desired point with an increased amplitude. In this paper, our main goal is to exploit the GRIN-PC concept for amplification of the fundamental torsional wave mode, namely, T(0,1), which is especially favorable in the guided wave ultrasonic testing of pipelines and can extend important structural information over long-range pipelines. To this end, we first study d35 piezoelectric transducer arrays through multiphysics simulations coupling piezoelectric and solid mechanics models and determine the optimal number of transducers to achieve the excitation of torsional plane wave. Two-dimensional Fourier transform as well as the tangential displacement histories around the pipe circumference at various axial positions verify the pure T(0,1) excitation, which is validated with experiments. Next, we design, fabricate, and numerically and experimentally study the conformal GRIN-PC for T(0,1) mode focusing in a range of ultrasonic frequencies, from 30 to 50 kHz. Measurements show that the amplitude of the torsional wave mode is amplified by 39% at the focal point with the presence of the GRIN-PC lens.

Multilook phase difference distribution for slightly rough boundary layered ground

The authors derive the distribution of the phase difference between two multilook scattering signals for a multilayer stack with randomly rough surfaces under a plane wave excitation. First, for infinite slightly rough surfaces described by Gaussian centred stochastic processes, the authors show that the underlying complex scattering signals follow a Gaussian joint distribution. Also, it is demonstrated that this property is within the scope of the first-order perturbation theory. Secondly, the authors use this joint probability law to derive the closed-form expression for the probability density function of the phase difference. The theoretical formula is verified by comparison with Monte-Carlo simulations.

Impact of spatial variability of shear wave velocity on the lagged coherency of synthetic surface ground motions

The spatial incoherence of ground motion during an earthquake can have a significant effect on the dynamic response of engineering structures such as bridges, dams, nuclear power plants and lifeline facilities. The main objective of this paper is to study the effect of anisotropic heterogeneities in a soil layer overlying homogeneous bedrock on the lagged coherency of surface ground motion. A set of numerical experiments is performed based on 2D spatial variability of shear-wave velocities modeled as a homogeneous stationary random field and discretized by the EOLE method (Expansion Optimal Linear Estimation). Seismic ground motions were simulated using FLAC2D software in the 1–25 Hz band for a plane wave excitation with SV polarization. The soil is characterized by horizontal and vertical autocorrelation distances ranging between 5 and 20 m and 1 and 2 m, respectively, and a coefficient of variation of the shear-wave velocity varying between 5% and 40%. The synthetic seismograms calculated for 9 parameter sets (100 realizations each) clearly show seismic waves scattering and surface waves diffracted locally by the ground heterogeneities, generating large spatial variations in coherence mainly controlled by the coefficient of variation of shear-wave velocity. Consistently with existing models and experimental data, the numerical coherency curves decrease with frequency and receiver distance, however at a rate which is lower than that observed in the experimental data. This difference is probably due to intrinsic attenuation that is not accounted for in the simulations and/or to our 2D simulations that do not reproduce the complete wavefield. The numerical average coherency curves for each parameter set exhibit maxima within narrow frequency bands caused by the vertically trapped body waves and surface wave propagation properties within the average ground model. This interpretation is supported by experimental data recorded in the Koutavos-Argostoli valley (Greece).

On the Response of Multiconductor Superconducting Microstrip Lines Under Plane Wave Excitation

Superconducting microwave circuits are composed of components made from superconducting microstrip transmission lines (SMTLs). The spatial features of the superconductors provide the system operation at high speed and low noise floor. However, coupling the external electromagnetic field in these systems reduces or nullifies their performance. This article presents a general solution for evaluating the electromagnetic behavior of multiconductor SMTLs under plane wave excitation. The analysis is portrayed in the spectral domain. Using the proposed approach, the electromagnetic susceptibility of the lines is evaluated for two proximity SMTLs. It is then investigated in different scenarios where the effect of film thickness, film spacing, and operating temperature is investigated. The study reveals that by increasing the film thickness or film spacing or decreasing the operating temperature, the lines’ susceptibility is increased. Further, it is shown that for small film spacing, the susceptibility dependency on temperature or film thickness of one line can affect the susceptibility of the other line.

A Transfer Function Measurement Setup With an Electro-Optic Sensor for MR Safety Assessment in Cascaded Media

This article introduces a transfer function (TF) measurement setup for cascaded media (CM) and demonstrates that the full immersion of an implant in a lossy medium might result in an underestimation of the RF-induced heating compared to partial immersion. A CMTF measurement setup was constructed using medium-specific local electric field sources and electro-optical sensor (EOS). TFs of bare and insulated wire samples were measured in the CM air/saline. Finite-difference time-domain (FDTD) simulations were implemented using a reciprocal approach with a monopole excitation source as well as a piecewise plane wave excitation method. As a theoretical guideline for understanding the CM behavior of the devices, an analytical model based on the buried wire models was derived. CMTFs agree with good precision between FDTD simulations, analytical modeling, and the EOS measurements. Measured TFs were calibrated and validated similar to ISO/TS 10974:Ed.2. Calculated tip SAR values under uniform incident field show an increase up to a factor of 5.7 for partially immersed wires compared to the full immersion. On average, FDTD-based TFs overestimate SAR by 25% compared to measurement-based TFs. As safety assessments for fully immersed implant leads or devices might not represent the worst-case scenario, single-medium assessments might underestimate the RF-induced heating.

Design of a continuously tunable reflectarray element for 5G metrology in the k-band

Abstract. We introduce a new tunable reflectarray element for an operation frequency of 26 GHz in the k-band. It is shown that a 340∘ continuous tunning range of the reflected wave can be accomplished by using an aperture-coupled patch antenna with only one single varactor diode. The simplified design and the small needed space make it usable for k-band reflectarrays with many elements. The functionality of the reflectarray element is explained and the crucial parts are analyzed. The approach to get a full phase shift is discussed in detail. A bias-T is developed to provide the control voltage to the varactor diode without interfering with the high frequency path. The high frequency path and the DC-path are decoupled by 39 dB using a bias-T. A commercial off-the-shelf varactor diode is selected and its functionality at 26 GHz is verified. Therefore, a test printed circuit board with through, reflect, line standards is developed to de-embed the varactor diode and to evaluate it with a vector network analyzer. The reflectarray is simulated in a unit cell with plane wave excitation and periodic boundary condition using the simulation software package CST Microwave Studio™.

Manipulating the photonic Hall effect with hybrid Mie-exciton resonances

We examine the far-field optical response, under-plane wave excitation in the presence of a static magnetic field, of core-shell nanoparticles involving a gyroelectric component, either as the inner or the outer layer, through analytic calculations based on appropriately extended Mie theory. We focus on absorption and scattering of light by bismuth-substituted yttrium iron garnet (Bi:YIG) nanospheres and nanoshells, combined with excitonic materials such as organic-molecule aggregates or two-dimensional transition-metal dichalcogenides, and discuss the hybrid character of the modes emerging from the coupling of the two constituents. We observe the excitation of strong magneto-optic phenomena and explore, in particular, the response and tunability of a magneto-transverse light current, indicative of the photonic Hall effect. We show how interaction between the Bi:YIG and excitonic layers leads to a pair of narrow bands of highly directional scattering, emerging from the aforementioned hybridization, which can be tuned at will by adjusting the geometrical or optical parameters of the system. Our theoretical study introduces optically anisotropic media as promising templates for strong coupling in nanophotonics, offering a means to combine tunable magnetic and optical properties, with potential implications both in the design of all-dielectric photonic devices but also in novel clinical applications.

Asymmetric surface wave excitation through metasurface-edge diffraction.

The selective excitation of localized surface wave modes remains a challenge in the design of both leaky-wave and bound-wave devices. In this Letter, we show how the truncation of a metasurface can play an important role in breaking the spatial inversion symmetry in the excitation of surface waves supported by the structure. This is done by combining a large anisotropy in the dispersion relation and the presence of an edge that also serves as a coupling mechanism between the plane wave excitation and the induced surface waves. By resorting to the exact solution to the scattering problem based on a discrete Wiener-Hopf technique, we show that by inverting the component of the impinging wavevector parallel to the truncation, two distinct surface waves are excited.

A Low-Profile Quad-Beam Transmitarray

In this letter, a novel method is proposed to design a low-profile multibeam transmitarray. Instead of a conventional horn feed, a slot array (SA) feed is used to illuminate uniformly same size transmitarray aperture (TA) in near field with a plane wave excitation, and TA is designed to transform plane wave to high gain multibeam pattern. The separation between SA feed and TA is independent of aperture size, so a very small separation is selected to achieve a low-profile transmitarray. A quad layer 3-dipole unit cell with 360° phase shift range is used as a transmitarray element. A modified intersection approach is used to optimize distribution of transmitarray elements on aperture to synthesize multibeam pattern. Finally, a prototype of Ku-band quad-beam transmitarray antennais fabricated and tested. The designed transmitarray shows good quad-beam performance with reduced profile and good match has been observed between simulated and measured results.

Study of Eigenwaves of a Two-Dimensionally Periodic Lattice of Capacitive Metal Cylinders in Transparency Bands

Abstract—The article studies eigenwaves of an electromagnetic crystal in the form of a two-dimensionally periodic lattice of metal cylinders with capacitive gaps between two metal screens forming a plane waveguide. The standard system of electrodynamic modeling is used to study the dispersion characteristics of eigenwaves propagating in a structure in the first and second transparency bands in the self-oscillation mode. In the plane-wave excitation mode, the authors study the characteristics of an electromagnetic crystal layer infinite in one coordinate in its transparency bands. It is shown that the fundamental eigenwave of the structure in the first transparency band is a slow wave, the deceleration coefficient of which increases at the boundary of the first band gap. It has been established that in the second transparency band, under certain conditions, backward waves can occur with anomalous frequency dispersion of the propagation constant. The relationship between the conditions of the occurrence of backward waves and the frequency of the series resonance of the capacitive cylinder is shown.