Response Of Metamaterials Research Articles

Toroidal response in all-dielectric metamaterials based on water

We experimentally demonstrate for the first time the toroidal dipolar response in metamaterials based on clusters of cylindrical dielectric particles in microwave frequency range. Instead of expensive ceramic elements we used distilled water with permittivity at room temperature is about 75, while the dielectric loss tangent is not large at frequencies up to 4 GHz. Moreover, we show all-dielectric metamaterial consisting of water box with hollow tubes which is more practical for future applications. Our findings also demonstrate that the proposed ideas can be applicable in optics with low-index dielectrics.

Magneto-optical response in bimetallic metamaterials

Abstract We demonstrate resonant Faraday polarization rotation in plasmonic arrays of bimetallic nano-ring resonators consisting of Au and Ni sections. This metamaterial design allows the optimization of the trade-off between the enhancement of magneto-optical effects and plasmonic dissipation. Nickel sections corresponding to as little as ~6% of the total surface of the metamaterial result in magneto-optically induced polarization rotation equal to that of a continuous nickel film. Such bimetallic metamaterials can be used in compact magnetic sensors, active plasmonic components, and integrated photonic circuits.

Geometry Defines Ultrafast Hot‐Carrier Dynamics and Kerr Nonlinearity in Plasmonic Metamaterial Waveguides and Cavities

Hot carrier dynamics in plasmonic nanorod metamaterials and its influence on the metamaterial's optical Kerr nonlinearity is studied. The electron temperature distribution induced by an optical pump in the metallic component of the plasmonic metamaterial leads to geometry‐dependent variations of the optical response and its dynamics as observed in both the transmission and reflection properties of the metamaterial slab. Thus, the ultrafast dynamics of a metamaterial's optical response can be controlled via modal engineering. Both the transient response relaxation time and magnitude of the nonlinearity are shown to depend on the modal‐induced spatial profile of the electron temperature distribution and the hot‐electron diffusion in nanorods. The nonlocal effects, depending on the excitation‐induced losses in the metal, are shown to dictate the modal structure of the metamaterial slab and the associated dynamics of its nonlinear response. The opportunity of controlling the electron temperature profile induced in the plasmonic nanorods by changing the metamaterial's geometry and/or excitation conditions paves the way to achieve controllable dynamics of the nonlinear optical response for free‐space as well as integrated nanophotonic applications involving nonequilibrium electrons.

Bistability in mushroom-type metamaterials

Here, we study the electromagnetic response of asymmetric mushroom-type metamaterials loaded with nonlinear elements. It is shown that near a Fano resonance, these structures may have a strong tunable, bistable, and switchable response and enable giant nonlinear effects. By using an effective medium theory and full wave simulations, it is proven that the nonlinear elements may allow the reflection and transmission coefficients to follow hysteresis loops, and to switch the metamaterial between “go” and “no-go” states similar to an ideal electromagnetic switch.

Self-assembled nanostructured metamaterials(a)

The concept of metamaterials emerged in the years 2000 with the achievement of artificial structures enabling nonconventional propagation of electromagnetic waves, such as negative phase velocity or negative refraction. The electromagnetic response of metamaterials is generally based on the presence of optically resonant elements —or meta-atoms— of sub-wavelength size and well-designed morphology so as to provide the desired electric and magnetic optical properties. Top-down technologies based on lithography techniques have been intensively used to fabricate a variety of efficient electric and magnetic resonators operating from microwave to visible light frequencies. However, the technological limits of the top-down approach are reached in visible light where a huge number of nanometre-sized elements is required. We show here that the bottom-up fabrication route based on the combination of nanochemistry and the self-assembly methods of colloidal physics provide an excellent alternative for the large-scale synthesis of complex meta-atoms, as well as for the fabrication of 2D and 3D samples exhibiting meta-properties in visible light.

Photonic metadevices: introduction

The research field of metamaterials is reaching its maturity, encouraging one to actively search for real-life applications of the metamaterial concept. This issue features works covering various aspects of metadevices—a natural progression of the paradigm that aims to accommodate and dynamically control metamaterial response in miniature photonic and optoelectronic devices.

Large deformations of soft metamaterials fabricated by 3D printing

The aim of this paper is to explore large-deformation responses of hyper-elastic porous metamaterials made by three-dimensional (3D) printing technology. They are designed as a repeating arrangement of unit-cells in parallelogram and hexagonal shapes. Fused deposition modeling is implemented to fabricate metamaterial structures from soft poly-lactic acid. 3D printed metamaterials are tested under in-plane tension-compression in axial and transverse directions. Experiments reveal that unit-cell shape, direction, type and magnitude of mechanical loading have significant effects on metamaterial anisotropic response and its instability characteristics. To replicate experimental observations, a finite element solution is developed adopting the hyper-elastic Mooney-Rivlin constitutive equations and non-linear Green-Lagrange strains. Iterative Newton-Raphson approach is implemented to solve governing equations with material-geometric non-linearities. The accuracy of the computational tool is verified by capturing main features observed in the experiments. It is found that modeling of hyper-elasticity and large strain is essential to accurately predict non-linear responses of the 3D printed soft metamaterials. Due to the absence of similar results in the specialized literature, this paper is likely to advance the state of the art of metamaterial printing, and provide pertinent results that are instrumental in the design of hyper-elastic metamaterial structures and infill patterns for printing purpose.

Design and analysis of a new composite double negative metamaterial for multi-band communication

In this paper, a double C-shaped structure of double negative composite metamaterial is designed and depicts in configurations that can capable in a multi-band microwave frequency band. The design has achieved relative negative permeability, relative negative permittivity and relative negative refractive index. Analysis and comparison were done by using four configurations of composite metamaterial such as horizontal 1 × 1 array and vertical 1 × 1 array structures and the horizontal 1 × 1 and vertical 1 × 1 unit-cell configurations. Multi-band operating frequencies namely, S-band, C-band and X-band have been achieved using all configurations. The proposed metamaterial size is 1.2 cm × 1.2 cm × 0.16 cm which includes all geometrical parameters to fit the design inside the substrate area. Computer Simulation Technology (CST) is adopted to investigate this design where an incident electromagnetic wave travelling along the positive z-axis with an E-field polarized along the y-axis. The results of the proposed metamaterial depict multi-band metamaterial response over the frequency span from 1 to 15 GHz. The effective medium ratio of the metamaterial unit-cell is 7.44. Moreover, the results clearly seen that the single-negative and double-negative metamaterial characteristics of the unit-cell and arrays over the multi-band. The dimensions and scattering parameters of the proposed double C-shaped metamaterial are suitable for the S -band, C-band and X-band applications.

Three-dimensional modeling of the wave dynamics of tensegrity lattices

This paper develops effective numerical models to study the wave dynamics of highly nonlinear tensegrity metamaterials. Recent studies have highlighted the geometrically nonlinear response of structural lattices based on tensegrity prisms, which may gradually change their elastic response from stiffening to softening through the modification of mechanical, geometrical, and prestress variables. We here study the nonlinear dynamics of columns of tensegrity prisms subject to impulsive compressive loading. An effective nonlinear rigid body dynamics is employed to simulate the dynamic response of such metamaterials. We illustrate how to pass from the matrix to the vector form of the equations of motions, on accounting for a rigid response of the compressive members (bars). Numerical simulations show that the wave dynamics of the examined metamaterials supports compression solitary pulses with profile dependent on the elastic properties of the tensile members (strings), the given impact velocity, and the applied prestress. We conclude that tensegrity columns can be effectively used as tunable acoustic lenses, which are able to generate acoustic solitary waves with adjustable profile in a host medium.

Large deformation response of additively-manufactured FCC metamaterials: From octet truss lattices towards continuous shell mesostructures

Starting with the statically-determinate solid octet truss lattice, the large strain compression response of different metamaterial architectures is analyzed through finite element analysis and compression experiments on additively-manufactured stainless steel specimens. Simulation results clearly demonstrate that for a relative density of 20% the conventional solid octet-truss lattice has a lower specific energy absorption capability than some simple periodic shell structures. The analysis of the closed-packed hollow sphere assembly subject to hydrostatic compression reveals that the constituent hollow spheres initially transform into rhombic dodecahedra, which results in significant strengthening at the macroscopic level. Unlike the octet-truss with solid struts, hollow octet truss structures remain stable for hydrostatic, confined and uniaxial compression at the lowest relative density considered. Its strength, in particular at small plastic strains, is substantially improved when substituting the geometric hollow truss joints for hollow spheres. The continuous shell mesostructures defined by the Hybrid (hollow) Truss-Sphere (HTS) and the hollow octet truss assembly exhibited the highest strength and energy absorption potential at 20% relative density for the high and low strain-hardening materials considered, respectively. Moreover, it is found that the HTS metamaterial exhibits the highest relative Young's and bulk moduli among all four architectures considered. It is also shown computationally that hollow rhombic dodecahedral mesostructures could provide nearly twice the strength and energy absorption of the conventional octet truss.

Experimental evidence of directivity-enhancing mechanisms in nonlinear lattices

In this letter, we experimentally investigate the directional characteristics of propagating, finite-amplitude wave packets in lattice materials, with an emphasis on the functionality enhancement due to the nonlinearly generated higher harmonics. To this end, we subject a thin, periodically perforated sheet to out-of-plane harmonic excitations, and we design a systematic measurement and data processing routine that leverages the full-wavefield reconstruction capabilities of a laser vibrometer to precisely delineate the effects of nonlinearity. We demonstrate experimentally that the interplay of dispersion, nonlinearity, and modal complexity which is involved in the generation and propagation of higher harmonics gives rise to secondary wave packets with characteristics that conform to the dispersion relation of the corresponding linear structure. Furthermore, these nonlinearly generated wave features display modal and directional characteristics that are complementary to those exhibited by the fundamental harmonic, thus resulting in an augmentation of the functionality landscape of the lattice. These results provide a proof of concept for the possibility to engineer the nonlinear wave response of mechanical metamaterials through a geometric and topological design of the unit cell.

Dynamical thermal metamaterial response at terahertz frequencies

ABSTRACTUtilizing terahertz time domain spectroscopy, we have characterized the electromagnetic response of a planar array of complementary square metal frames fabricated upon a high resistivity Si substrate, and between the two layers existed an important ferroelectric layer strontium titanate(STO). The resonance frequency dependent magnetic and electric resonances are in excellent agreement with theory. We demonstrate, it has two distinctive reflection dips through reconfigures the metamaterial by etched a rectangular stripe, and the former metamaterial has only one resonance frequency. Cooling the metamaterial from 400 K to 200 K causes excellent blue shift. The resonance shift is owing to the variation of the dielectric constant of the low-temperature co-fired ceramic (LTCC) substrate. Because of the universality of metamaterial response over many decades of frequency, these results have implications for other regions of the electromagnetic spectrum.

Designing Multipolar Resonances in Dielectric Metamaterials.

Dielectric resonators form the building blocks of nano-scale optical antennas and metamaterials. Due to their multipolar resonant response and low intrinsic losses they offer design flexibility and high-efficiency performance. These resonators are typically described in terms of a spherical harmonic decomposition with Mie theory. In experimental realizations however, a departure from spherical symmetry and the use of high-index substrates leads to new features appearing in the multipolar response. To clarify this behavior, we present a systematic experimental and numerical characterization of Silicon disk resonators. We demonstrate that for disk resonators on low-index quartz substrates, the electric and magnetic dipole modes are easily identifiable across a wide range of aspect-ratios, but that higher order peaks cannot be unambiguously associated with any specific multipolar mode. On high-index Silicon substrates, even the fundamental dipole modes do not have a clear association. When arranged into arrays, resonances are shifted and pronounced preferential forward and backward scattering conditions appear, which are not as apparent in individual resonators and may be associated with interference between multipolar modes. These findings present new opportunities for engineering the multipolar scattering response of dielectric optical antennas and metamaterials, and provide a strategy for designing nano-optical components with unique functionalities.

Intermodulation in nonlinear SQUID metamaterials: Experiment and theory

The response of nonlinear metamaterials and superconducting electronics to two-tone excitation is critical for understanding their use as low-noise amplifiers and tunable filters. A new setting for such studies is that of metamaterials made of radio frequency superconducting quantum interference devices (rf-SQUIDs). The two-tone response of self-resonant rf-SQUID meta-atoms and metamaterials is studied here via intermodulation (IM) measurement over a broad range of tone frequencies and tone powers. A sharp onset followed by a surprising strongly suppressed IM region near the resonance is observed. Using a two time scale analysis technique, we present an analytical theory that successfully explains our experimental observations. The theory predicts that the IM can be manipulated with tone power, center frequency, frequency difference between the two tones, and temperature. This quantitative understanding potentially allows for the design of rf-SQUID metamaterials with either very low or very high IM response.

Nonlinear Multiconductor Transmission Line Analysis of Broadband Switching Metamaterials

In this paper, we present a nonlinear multiconductor transmission line model to analyze the time-domain and nonlinear responses of broadband switching metamaterials. This analysis employs coupled transmission lines loaded with nonlinear elements to model the response of the metamaterial structures to a vertically polarized free-space excitation. Using this analysis we first examine the time-domain response of a broadband switching metamaterial structure loaded with high-speed Schottky diodes. We then show that this structure’s response to incident power can be effectively inverted by replacing these Schottky diodes with varactor diodes. Experimental results are presented for both structures in WR-284 waveguide, which show good agreement with results generated from full-wave and transmission line analysis. We also use the circuit model to simulate power-dependent material parameters and both the input and output third-order intercept point for both the Schottky-loaded and varactor-loaded metamaterials.

On the origin of pure optical rotation in twisted-cross metamaterials.

We present an experimental and computational study of the response of twisted-cross metamaterials that provide near dispersionless optical rotation across a broad band of frequencies from 19 GHz to 37 GHz. We compare two distinct geometries: firstly, a bilayer structure comprised of arrays of metallic crosses where the crosses in the second layer are twisted about the layer normal; and secondly where the second layer is replaced by the complementary to the original, i.e. an array of cross-shaped holes. Through numerical modelling we determine the origin of rotatory effects in these two structures. In both, pure optical rotation occurs in a frequency band between two transmission minima, where alignment of electric and magnetic dipole moments occurs. In the cross/cross metamaterial, the transmission minima occur at the symmetric and antisymmetric resonances of the coupled crosses. By contrast, in the cross/complementary-cross structure the transmission minima are associated with the dipole and quadrupole modes of the cross, the frequencies of which appear intrinsic to the cross layer alone. Hence the bandwidth of optical rotation is found to be relatively independent of layer separation.

Multipolar Third-Harmonic Generation in Fishnet Metamaterials

We study the third-harmonic generation from metal–dielectric–metal layered fishnet metamaterials and identify experimentally the multipolar contributions to the generated nonlinear harmonic fields by analyzing the radiation patterns of the emission. We observe that the third harmonic radiated from the fishnet structure is a result of the interference of the electric and magnetic dipoles and the electric quadrupole modes. Our results provide direct evidence of the importance of higher order multipoles in the nonlinear response of fishnet metamaterials, opening new opportunities for enhanced nonlinearities and controlled directionality of nonlinear processes in metamaterials.

Ultrafast control of third-order optical nonlinearities in fishnet metamaterials

Nonlinear photonic nanostructures that allow efficient all-optical switching are considered to be a prospective platform for novel building blocks in photonics. We performed time-resolved measurements of the photoinduced transient third-order nonlinear optical response of a fishnet metamaterial. The mutual influence of two non-collinear pulses exciting the magnetic resonance of the metamaterial was probed by detecting the third-harmonic radiation as a function of the time delay between pulses. Subpicosecond-scale dynamics of the metamaterial’s χ(3) was observed; the all-optical χ(3) modulation depth was found to be approximately 70% at a pump fluence of only 20 μJ/cm2.

Numerical investigation of amplitude-dependent dynamic response in acoustic metamaterials with nonlinear oscillators.

The amplitude-dependent dynamic response in acoustic metamaterials having nonlinear local oscillator microstructures is studied using numerical simulations on representative discrete mass-spring models. Both cubically nonlinear hardening and softening local oscillator cases are considered. Single frequency, bi-frequency, and wave packet excitations at low and high amplitude levels were used to interrogate the models. The propagation and attenuation characteristics of harmonic waves in a tunable frequency range is found to correspond to the amplitude and nonlinearity-dependent shifts in the local resonance bandgap for such nonlinear acoustic metamaterials. A predominant shift in the propagated wave spectrum towards lower frequencies is observed. Moreover, the feasibility of amplitude and frequency-dependent selective filtering of composite signals consisting of individual frequency components which fall within propagating or attenuating regimes is demonstrated. Further enrichment of these wave manipulation mechanisms in acoustic metamaterials using different combinations of nonlinear microstructures presents device implications for acoustic filters and waveguides.

Toroidal Dipolar Response in Metamaterials Composed of Metal–Dielectric–Metal Sandwich Magnetic Resonators

Toroidal metamaterials have been drawing increasing interest recently because of their unusual electromagnetic properties and a variety of potential applications. In this work, we have investigated numerically toroidal dipolar response at optical frequency in metamaterials whose unit cell includes three magnetic resonators. The magnetic resonators are metal–dielectric–metal sandwich nanostructures, which are composed of two Ag rods and a SiO 2 spacer. They have the same shape and dimension, but they are placed at different positions to break the space-inversion symmetry. The near-field plasmon coupling between magnetic resonators leads to the excitation of a toroidal dipolar mode, which is characterized by a head-to-tail distribution of magnetic dipoles within magnetic resonators. In our designed toroidal metamaterials, space-inversion symmetry breaking is needed only in the polarization direction of incident light, and light can be normally incident on the toroidal metamaterials.