Subwavelength Resonators Research Articles

Elastodynamical resonances and cloaking of negative material structures beyond quasistatic approximation

AbstractGiven the flexibility of choosing negative elastic parameters, we construct material structures that can induce two resonance phenomena, referred to as the elastodynamical resonances. They mimic the emerging plasmon/polariton resonance and anomalous localized resonance in optics for subwavelength particles. However, we study the peculiar resonance phenomena for linear elasticity beyond the subwavelength regime. It is shown that the resonance behaviors possess distinct characters, with some similar to the subwavelength resonances, but some sharply different due to the frequency effect. It is particularly noted that we construct a core–shell material structure that can induce anomalous localized resonance as well as cloaking phenomena beyond the quasistatic limit. The study is boiled down to analyzing the so‐called elastic Neumann–Poincaré (N‐P) operator in the frequency regime. We provide an in‐depth analysis of the spectral properties of the N‐P operator on a circular domain beyond the quasistatic approximation, and these results are of independent interest to the spectral theory of layer potential operators.

Dielectric Mie voids: confining light in air

Manipulating light on the nanoscale has become a central challenge in metadevices, resonant surfaces, nanoscale optical sensors, and many more, and it is largely based on resonant light confinement in dispersive and lossy metals and dielectrics. Here, we experimentally implement a novel strategy for dielectric nanophotonics: Resonant subwavelength localized confinement of light in air. We demonstrate that voids created in high-index dielectric host materials support localized resonant modes with exceptional optical properties. Due to the confinement in air, the modes do not suffer from the loss and dispersion of the dielectric host medium. We experimentally realize these resonant Mie voids by focused ion beam milling into bulk silicon wafers and experimentally demonstrate resonant light confinement down to the UV spectral range at 265 nm (4.68 eV). Furthermore, we utilize the bright, intense, and naturalistic colours for nanoscale colour printing. Mie voids will thus push the operation of functional high-index metasurfaces into the blue and UV spectral range. The combination of resonant dielectric Mie voids with dielectric nanoparticles will more than double the parameter space for the future design of metasurfaces and other micro- and nanoscale optical elements. In particular, this extension will enable novel antenna and structure designs which benefit from the full access to the modal field inside the void as well as the nearly free choice of the high-index material for novel sensing and active manipulation strategies.

Transmission properties of space‐time modulated metamaterials

AbstractWe prove the possibility of achieving exponentially growing wave propagation in space‐time modulated media and give an asymptotic analysis of the quasifrequencies in terms of the amplitude of the time modulation at the degenerate points of the folded band structure. Our analysis provides the first proof of existence of k‐gaps in the band structures of space‐time modulated systems of subwavelength resonators.

Magnetoelectric‐Field Electrodynamics: Search for Magnetoelectric Point Scatterers

AbstractResonant scattering of electromagnetic (EM) waves by small particles is considered as one of the basic problems in metamaterial science. At present, special subwavelength resonators are considered as structural elements in chiral and bianisotropic metamaterials. There is a general consensus that these small scatterers behave like “artificial atoms” with strong electrical and magnetic responses and an interconnection between these responses. However, the observed effect of magnetoelectric (ME) coupling in these meta‐atoms is not associated with the near‐field manipulation properties caused by intrinsic magnetoelectricity. This arises the question whether ME point scatterers of EM radiation really exist. In this paper, we show that there are mesoscopic structures with electric and magnetic dipole‐carrying excitations that behave like point scatterers with their inherent magnetoelectricity. In such subwavelength resonators, coherent oscillations of the electric polarization and magnetization can be considered as quasistatic oscillations described by electrostatic (ES) and magnetostatic (MS) scalar wave functions. The ME resonance effect arises from the coupling of two, ES and MS, oscillations. The near fields of these resonators, called the ME near fields, are characterized by simultaneous violation of time reversal and inversion symmetry. In study of ME fields and EM problems associated with these fields, we put forward the concept of ME‐field electrodynamics.

Subwavelength Terahertz Resonance Imaging (STRING) for Molecular Fingerprinting.

Subwavelength terahertz (THz) imaging methods are highly desirable for biochemical sensing as well as materials sciences, yet sensitive spectral fingerprinting is still challenging in the frequency domain due to weak light-matter interactions. Here, we demonstrate subwavelength THz resonance imaging (STRING) that overcomes this limitation to achieve ultrasensitive molecular fingerprinting. STRING combines individual ring-shaped coaxial single resonators with near-field spectroscopy, yielding considerable sensitivity gains from both local field enhancement and the near-field effect. As an initial demonstration, we obtained spectral fingerprints from isomers of α-lactose and maltose monohydrates, achieving sensitivity that was enhanced by up to 10 orders of magnitude compared to far-field THz measurements with pelletized samples. Our results show that the STRING platform could enable the development of THz spectroscopy as a practical and sensitive tool for the fingerprinting and spectral imaging of molecules and nanoparticles.

Controlling Asymmetric Retroreflection of Metasurfaces via Localized Loss Engineering

Metasurfaces based on subwavelength resonators enable novel ways to manipulate the flow of light at optical interfaces. In pursuit of multifunctional or reconfigurable metadevices, efficient tuning of macroscopic performance with little structural/material variation remains a challenge. Here, we propose the concept of localized loss engineering in electromagnetic (EM) metasurfaces, showing that an ordinary symmetric retroreflector can be switched to an extraordinary asymmetric one by introducing absorptive defects in local regions. The asymmetric performance begins with zero at the Hermitian state, gradually increases under non-Hermitian localized loss modulation, and reaches the maximum at the exceptional point (EP). The metasurface at the EP exhibits extremely asymmetric performance without the need of deeply discretized subwavelength elements. As a proof of concept, the localized loss-assisted asymmetric reflection is experimentally demonstrated at microwave frequencies via the observation of near-field distributions and far-field scatterings from a planar metasurface. Our methodology opens new opportunities for engineering EM systems with small perturbations and designing reconfigurable or multifunctional metadevices.

Exceptional Points in Parity--Time-Symmetric Subwavelength Metamaterials

When sources of energy gain and loss are introduced to a wave scattering system, the underlying mathematical formulation will be non-Hermitian. This paves the way for the existence of exceptional points, where eigenmodes are linearly dependent. The primary goal of this work is to study the existence of exceptional points in high-contrast subwavelength metamaterials. We begin by studying a parity--time-symmetric pair of subwavelength resonators and prove that this system supports asymptotic exceptional points. These are points at which the subwavelength eigenvalues and eigenvectors coincide at leading order in the asymptotic parameter. We then investigate the exotic scattering behavior of a metascreen composed of repeating parity-time-symmetric pairs of subwavelength resonators. We prove that the non-Hermitian nature of this structure means that it exhibits asymptotic unidirectional reflectionless transmission at certain frequencies and demonstrate extraordinary transmission close to these frequencies.

A resistance heating assisted free space method to measure temperature-dependent electromagnetic properties of carbon fiber reinforced polymer composites

Temperature-dependent electromagnetic properties are not only closely related to the manufacturing process of carbon fiber reinforced polymer (CFRP) components, e.g., curing of CFRP composites using microwaves, but also directly determines some key performance indicators of CFRP structures like electromagnetic stealth and anti-electromagnetic interference (EMI). In this paper, we proposed a resistance heating assisted free space method to measure the temperature-dependent electromagnetic properties of both CFRP materials and structures. On this basis, the temperature-dependent electromagnetic properties of a unidirectional carbon fiber/PEEK composite material were measured in the temperature range from room temperature to 400 °C. Moreover, the temperature-dependent electromagnetic properties of a CFRP structure, i.e., a multidirectional carbon fiber/PEEK laminate covered by subwavelength resonance structures, were also measured. The results indicated that temperature has a significant effect on the electromagnetic properties of both CFRP materials and structures. Our work provides a simple and practical solution to measure the temperature-dependent electromagnetic properties of CFRP materials and structures.

Control of Quasibound Waves in Spiral Metasurfaces

Resonant light scattering by a subwavelength dielectric resonator array can show narrow, highly tunable resonant lineshapes from the interaction among multiple resonances. Several approaches have been introduced to control the spectral response of resonant light scattering, including optimization of the aspect ratio of cylindrical resonators, breaking in-plane inverse symmetry of coupled resonators, and operating periodic structures at the Γ point in reciprocal space. In this paper, we report the experimental demonstration of the control of resonant light scattering by subwavelength spiral resonator arrays. The fabricated metasurfaces support two types of resonances: the first one characterized by a Lorentzian peak with a long lifetime and the second type with a short lifetime supported by the lattice. By controlling the coupling strength between these two resonances through tailored broken symmetries, the resonant line shape of the spiral metasurface can be precisely tuned. The measured spectra show an ultrawide frequency tuning (over 16 THz) of the resonant mode by changing the spiral parameter at a fixed lattice constant. A temporal couple-mode theory combined with frequency-domain finite element method simulations was used to describe the measured transmission spectra of the fabricated metasurfaces.

Mathematical analysis of electromagnetic scattering by dielectric nanoparticles with high refractive indices

In this work, we are concerned with the mathematical modeling of the electromagnetic (EM) scattering by arbitrarily shaped non-magnetic nanoparticles with high refractive indices. When illuminated by visible light, such particles can exhibit a very strong isotropic magnetic response, resulting from the coupling of the incident wave with the circular displacement currents of the EM fields. The main aim of this work is to mathematically illustrate this phenomenon. We shall first introduce the EM scattering resolvent and the concept of dielectric subwavelength resonances. Then we derive the a priori estimates for the subwavelength resonances and the associated resonant modes. We also show the existence of resonances and obtain their asymptotic expansions in terms of the small particle size and the high contrast parameter. After that, we investigate the enhancement of the scattering amplitude and the cross sections when the resonances occur. In doing so, we develop a novel multipole radiation framework that directly separates the electric and magnetic multipole moments and allows us to clearly see their orders of magnitude and blow-up rates. We prove that at the dielectric subwavelength resonant frequencies, the nanoparticles with high refractive indices behave like the sum of the electric dipole and the resonant magnetic dipole. Some explicit calculations and numerical experiments are also provided to validate our general results and formulas.

Broadband low-frequency sound absorption in open tunnels with deep sub-wavelength Mie resonators

We report both experimentally and numerically that near-perfect absorption of low-frequency sound is realized in an open tunnel embedded with two deep sub-wavelength (0.085 λ) Mie resonators. The resonators are composed of a multiple-cavity structure and an outer frame on three sides. In the eigenmode analysis, we obtain two types of monopolar Mie resonance modes (MMR I&II) in a single resonator around 250 Hz. The eigenfrequency of MMR I is mainly determined by the Helmholtz resonance of each cavity in the multiple-cavity structure, while that of MMR II is closely related to the coupling between the multiple-cavity structure and its outer frame, showing high performances of coupling and sound absorption. Based on the thermal viscous loss of sound energy in the channels created by the mutual coupling of MMR II of both Mie resonators with different diameters, the near-perfect sound absorption through the open tunnel is realized around 283 Hz. More interestingly, by increasing the number of Mie resonators in the tunnel, a broadband near-perfect sound absorption is observed, and the fractional bandwidth can reach about 0.25 and 0.46 for the tunnels with 6 and 13 resonators, The proposed deep sub-wavelength Mie resonator and its associated near-perfect sound absorptions have great potential applications in architectural acoustics and mechanical engineering.

A metasurface enabled dual band antenna exhibiting dual polarizations with reduced beam squint and cross polarization

A compact, dual-band metasurface-based microstrip patch antenna with both linear and circular polarization (CP) for multiband communication is reported in this work. The ingredients in the antenna design consist of a combination of metasurface and metamaterial inspired structures that are demonstrated as an effective technique to correct the beam squint problem and reduce the cross-polarized radiation, usually experienced by thick substrate-based broadband CP antennas. A fractal boundary metasurface is introduced to achieve CP over a small frequency band. Eight Electric-LC resonators are symmetrically placed in the same plane of the radiating patch to widen the CP bandwidth, correct the beam squint, reduce the cross-polarized radiation, and realize a subwavelength resonance of λ0/5.15 order. The measured impedance bandwidths (|S11| < −10 dB) of the proposed antenna are 7.8% (3.7–4.0 GHz) and 27.58% (5.05–6.65 GHz) in the lower and upper-frequency band, respectively. The measured 3-dB axial ratio bandwidth (ARBW) is 24.8% (5070–6510 MHz) at 5.8 GHz. The fabricated prototype has an overall size of 55 × 55 × 4.08 mm (0.696λ0 × 0.696λ0 × 0.051λ0 at 3.8 GHz or 1.063λ0 × 1.063λ0 × 0.078λ0 at 5.8 GHz) with a realized peak gain of 4 dBi in the lower band and 7.1 dBic in the upper band. The measured results are consistent with the simulation results.

Time-reversal in phononic crystal-based water filled pipe

Time-reversal based source localization in a conventional water-filled pipe using acoustic waves is hamstrung by the diffraction limit (λ/2). In this paper, time reversal in a novel metamaterial-based water pipeline is studied where a conventional rigid pipe is modified to have periodic square corrugations. Based on the choice of periodicity, such a novel waveguide can support subwavelength resonance in the low frequency regime while exhibiting Bragg resonance, avoided crossing of modes and forbidden bands in diffraction regime. This paper focuses on the latter such that a band structure is obtained. Full wave simulation performed on COMSOL verifies the phononic crystal-like properties in the diffraction regime. Time reversal refocusing is performed by placing a point source in the middle of the corrugated section while a receiver outside the said section records the response of the system. The receiver acts as a Time Reversal Mirror (TRM) such that it flips and reinjects the recorded signal into the system which retraces its path and focuses on its source. Time-reversal refocusing is found to be sensitive to the recorded frequency grains in addition to the energy propagated from the source such that if the source encompasses a band gap, the time reversal quality deteriorates.

Multimode resonances, intermode bound states, and bound states in the continuum in waveguides

We study analytically and numerically the formation of bound states in the continuum (BICs) in quantum-mechanical and optical spatially symmetric multimode waveguide systems. The widely used Friedrich-Wintgen model predicts a BIC for two (nearly) degenerate eigenstates of the resonator. However, numerical calculations typically show that BICs may appear away from the degeneracy point (or avoided crossing region) in the energy-parameter space. From the two-mode point of view based on the Friedrich-Wintgen model, such BICs can be considered as accidental. In this paper, we go beyond the two-mode approximation and, appealing to the notion of intermode bound states, provide an illustrative procedure for deriving conditions for BIC formation within an arbitrary finite-mode approximation. In particular, we show that a three-mode approximation allows the description of a continuous transition of a BIC between different pairs of modes, which can be associated with it within the two-mode model. Also, a manifestation of exceptional points as (anti)resonance coalescence is discussed. Analytical conclusions are verified by the results of numerical simulations of two-dimensional quantum-mechanical and optical stubbed waveguides with confining impurities in the stub. Moreover, numerical simulations confirm the existence of BICs in optical subwavelength resonators, which were earlier predicted in quantum-mechanical waveguides.

Tapered rainbow metabeam for wideband multimode acoustic blocking based on quadruple-mode resonators

We construct a rainbow metamaterial for multimode sound blocking over a broad range of sub-kHz frequencies in the form of a tapered rectangular cross section beam of machined cells based on elements that can, on average, simultaneously attenuate the majority of the possible elastic-wave polarizations. Using aluminum, we construct a five-cell structure containing sub-wavelength planar resonators with interconnected ribs, which couple to compressional, in-plane shear, flexural, and torsional vibrations. Backed up by numerical simulations, we verify that this tapered structure can, on average, strongly attenuate acoustic modes over the frequency range of the combined metamaterial bandgaps, that is over a frequency range representing ∼50% around ∼0.7 kHz. Applications include vibration isolation.

Broadband low-frequency bidimensional honeycomb lattice metastructure based on the coupling of subwavelength resonators

The development and application of noise control strategies have been recently the focus of attention by several researchers. In the present study, the authors develop a thin low frequency acoustic metamaterial, composed of critically coupled subwavelength Helmholtz resonators, axially arranged, with a focus on wideband perfect absorption and sound transmission problems, which are here denominated Acoustic Honeycomb Metasurface (AHS). Applying fluid equivalent models to describe the viscous thermal losses through the transfer matrix method and nonlinear optimization, it has been possible to investigate the critically coupling condition in local resonances to enhance quasi-perfect (α>0.8) and perfect sound absorption (α=1). The results demonstrate the proposed absorber system works on a subwavelength regime (λ/52), exhibiting high absorption efficiency (>80%) at low-frequency broadband absorption (approx. 300 Hz to 600 Hz). Finally, the present work opens new possibilities to develop subwavelength opened acoustic metamaterials, with potential application in ventilated systems in different engineering areas.

Experimental Realization of a Superdispersion-Enabled Ultrabroadband Terahertz Cloak.

Invisibility has been a topic of long-standing interest owing to the advent of metamaterials and transformation optics, but still faces open challenges after its tremendous development in recent decades. One of the big challenges is the narrow bandwidth, as the realization of an invisibility cloak is usually based on a metamaterial-an artificial composite material composed of subwavelength resonator structures that are always associated with dispersion. Different from previous works that have tried to eliminate the material dispersion to enhance the bandwidth of aninvisibility cloak, here, it is found that by judiciously harnessing the material dispersion, the bandwidth of the cloak can still be significantly increased. Interestingly, the material dispersion does not violate the law of causality. As a proof of concept, an ultrabroadband terahertz (THz) carpet cloak is experimentally demonstrated through an array of superdispersive microparticles, rendering the target object invisible to detection by both time- and frequency-domain wideband systems. The work presents a feasible invisibility strategy that is closer to practical applications and may pave a brand-new way for the development of dispersion-dominated ultrabroadband metadevices.

Toward a universal metasurface for optical imaging, communication, and computation

Abstract In recent years, active metasurfaces have emerged as a reconfigurable nanophotonic platform for the manipulation of light. Here, application of an external stimulus to resonant subwavelength scatterers enables dynamic control over the wavefront of reflected or transmitted light. In principle, active metasurfaces are capable of controlling key characteristic properties of an electromagnetic wave, such as its amplitude, phase, polarization, spectrum, and momentum. A ‘universal’ active metasurface should be able to provide independent and continuous control over all characteristic properties of light for deterministic wavefront shaping. In this article, we discuss strategies for the realization of this goal. Specifically, we describe approaches for high performance active metasurfaces, examine pathways for achieving two-dimensional control architectures, and discuss operating configurations for optical imaging, communication, and computation applications based on a universal active metasurface.

Active and tunable nanophotonic metamaterials

AbstractMetamaterials enable subwavelength tailoring of light–matter interactions, driving fundamental discoveries which fuel novel applications in areas ranging from compressed sensing to quantum engineering. Importantly, the metallic and dielectric resonators from which static metamaterials are comprised present an open architecture amenable to materials integration. Thus, incorporating responsive materials such as semiconductors, liquid crystals, phase-change materials, or quantum materials (e.g., superconductors, 2D materials, etc.) imbue metamaterials with dynamic properties, facilitating the development of active and tunable devices harboring enhanced or even entirely novel electromagnetic functionality. Ultimately, active control derives from the ability to craft the local electromagnetic fields; accomplished using a host of external stimuli to modify the electronic or optical properties of the responsive materials embedded into the active regions of the subwavelength resonators. We provide a broad overview of this frontier area of metamaterials research, introducing fundamental concepts and presenting control strategies that include electronic, optical, mechanical, thermal, and magnetic stimuli. The examples presented range from microwave to visible wavelengths, utilizing a wide range of materials to realize spatial light modulators, effective nonlinear media, on-demand optics, and polarimetric imaging as but a few examples. Often, active and tunable nanophotonic metamaterials yield an emergent electromagnetic response that is more than the sum of the parts, providing reconfigurable or real-time control of the amplitude, phase, wavevector, polarization, and frequency of light. The examples to date are impressive, setting the stage for future advances that are likely to impact holography, beyond 5G communications, imaging, and quantum sensing and transduction.

On the Validity of the Tight-Binding Method for Describing Systems of Subwavelength Resonators

The goal of this paper is to relate the capacitance matrix formalism to the tight-binding approximation. By doing so, we open the way to the use of mathematical techniques and tools from condensed matter theory in the mathematical and numerical analysis of metamaterials, in particular for the understanding of their topological properties. We first study how the capacitance matrix formalism, both when the material parameters are static and modulated, can be posed in a Hamiltonian form. Then, we use this result to compare this formalism to the tight-binding approximation. We prove that the correspondence between the capacitance formulation and the tight-binding approximation holds only in the case of dilute resonators. On the other hand, the tight-binding model is often coupled with a nearest-neighbor approximation, whereby long-range interactions are neglected. Even in the dilute case, we show that long-range interactions between subwavelength resonators are relatively strong and nearest-neighbor approximations are not generally appropriate.