Response Of Metamaterials Research Articles

Role of loss in all-dielectric metasurfaces.

Arrays of dielectric cylinders support two fundamental dipole active eigenmodes, which can be manipulated to elicit a variety of electromagnetic responses in all-dielectric metamaterials. Dissipation is a critical parameter in determining functionality; the present work varies material loss to explore the rich electromagnetic response of this class of metasurface. Four experimental cases are investigated which span electromagnetic response ranging from Huygens surfaces with transmissivity T = 94%, and phase ϕS21 = 235°, to metasurfaces which absorb 99.96% of incident energy. We find perfect absorption to be analogous to the driven damped harmonic oscillator, with critical damping occurring at resonance. With high phase contrast, transmission, and absorption all accessible from a single system, we present a uniquely diverse all-dielectric system.

Electromagnetically induced transparency in sinusoidal modulated ring resonator

In the present work, we demonstrate controlling the excitation of bright mode (continuum mode) resonance and dark mode (discrete mode) resonance in a planar metasurface made of sinusoidal modulation inside a closed rectangular metallic ring placed over a dielectric substrate. Unlike asymmetrical breaking of a meta-atom (often referred to as the unit cell) to achieve the dark mode response in regular metamaterials, in the present structure, the bright or dark mode resonance is achieved using even or odd half cycle modulation. The achieved dark-mode shows a sharp resonance for a particular polarization of the incident electric field, which results in an electromagnetically induced transparency like spectrum. The electromagnetic behavior of the proposed meta-atom has been investigated in the frequency domain using commercially available software and validated through experiments in the gigahertz regime.

Optically controlled redshift switching effects in hybrid fishscale metamaterials

We numerically demonstrate optically controlled THz response in a hybrid fishscale metamaterial with embedded photoconductive silicon at oblique incidence of TE wave. The oblique incidence allows excitation of Fano-type trapped mode resonance in a 2-fold rotational symmetric metamaterial. The hybrid fishscale metamaterial exhibits an optically controlled redshift switching effect in the THz range. The switching effect is dominated by the conductivity of the silicon instead of mechanically adjusting angles of incidence. The tuning frequency range is up to 0.3THz with a large modulation depth and high transmission in the “ON” state. The fishscale metamaterial-based switching has been experimentally verified by its microwave counterpart integrated by variable resistors. Our work provides an alternative route to realize tunable Fano-type response in metamaterials and is of importance to active manipulation, sensing and switching of THz waves in practical applications.

Predicting double negativity using transmitted phase in space coiling metamaterials.

Metamaterials are engineered materials that offer the flexibility to manipulate the incident waves leading to exotic applications such as cloaking, extraordinary transmission, sub-wavelength imaging and negative refraction. These concepts have largely been explored in the context of electromagnetic waves. Acoustic metamaterials, similar to their optical counterparts, demonstrate anomalous effective elastic properties. Recent developments have shown that coiling up the propagation path of acoustic wave results in effective elastic response of the metamaterial beyond the natural response of its constituent materials. The effective response of metamaterials is generally evaluated using the ‘S’ parameter retrieval method based on amplitude of the waves. The phase of acoustic waves contains information of wave pressure and particle velocity. Here, we show using finite-element methods that phase reversal of transmitted waves may be used to predict extreme acoustic properties in space coiling metamaterials. This change is the difference in the phase of the transmitted wave with respect to the incident wave. This method is simpler when compared with the more rigorous ‘S’ parameter retrieval method. The inferences drawn using this method have been verified experimentally for labyrinthine metamaterials by showing negative refraction for the predicted band of frequencies.

Realization of a topological phase transition in a gyroscopic lattice

Topological metamaterials exhibit unusual behaviors at their boundaries, such as unidirectional chiral waves, that are protected by a topological feature of their band structure. The ability to tune such a material through a topological phase transition in real time could enable the use of protected waves for information storage and readout. Here we dynamically tune through a topological phase transition by breaking inversion symmetry in a metamaterial composed of interacting gyroscopes. Through the transition, we track the divergence of the edge modes' localization length and the change in Chern number characterizing the topology of the material's band structure. This work provides a new axis with which to tune the response of mechanical topological metamaterials.

Graphene-induced modulation effects on magnetic plasmon in multilayer metal-dielectric-metal metamaterial

Motivated by the increasing interest in active control of the optical response in magnetic metamaterials, we theoretically demonstrate that monolayer graphene, even only a single atom thick, can provide an efficient modulation on the magnetic plasmon (MP) resonance, including over 10 meV resonance shift and over 25% modulation of resonance absorption intensity. We show that the resonance shift is distinctly different from the graphene-induced change in electrically excited surface plasmon resonances in plasmonic systems. Our analysis based on the equivalent nanocircuit method reveals that the MP resonance shift is governed by both the real and imaginary parts of graphene permittivity. Importantly, we find that an additional dissipation channel relevant to the graphene-induced resistance governs the MP absorption and that even the dissipation channel of interband transition is blocked. The interplay between both channels results in a pronounced modification of MP absorption intensities. The findings will offer a promising way to realize the dynamic control of the magnetic response, which holds great potential applications in graphene-based active metamaterials.

Multiresonance response in hyperbolic metamaterials.

In this paper we demonstrate a new class of anisotropic 1D hyperbolic metamaterials (HMMs) possessing multiresonant dispersion characteristics. With the help of an EMT-based model, we analyze HMMs with unit cells composed of layers characterized by various plasma frequencies, revealing multiple resonance transitions corresponding to the critical absorptions points. In particular, we show that relative locations of plasma frequencies of constituent materials and the unit cell's geometry determine the type of dispersion characteristics as well as spectral locations of critical absorption points. It is shown that a multispectral low-loss highly dispersive medium is achieved in a structure comprising layers of closely located plasma resonances. Moreover, we present that pure metallic multilayer structure can exhibit hyperbolic dispersion. The obtained results possess a significant potential in applications where a multispectral character is required, including phase matching, multiple-point perfect absorption, as well as diffractionless imaging and focusing.

Active Control Transmission of Terahertz Metamaterials Based on Ion Implantation and Light Pumping

A method (ion-implantation) that can control surface carrier density of silicon wafer in silicon fabrication industry is combined with metamaterial to demonstrate a new way to control response of metamaterial transmission operating at terahertz frequency. Ion-implantation which is the most exact and easy way to control carriers of silicon wafer is used to fabricate a stable response of transmission metamaterial. Therefore, a split-ring resonators (SRRs) metamaterial is designed and fabricated to investigate the relationship between carrier density, power of pumping light, and transmission. Meanwhile, the numerical simulation was carried out to verify the experimental results. The relationship between the simulation results and experiments results was confirmed. This method could be a potential way to make stable response of metamaterial, which could be switcher, filter, and terahertz detectors.

Predicting Observable Quantities of Self-Assembled Metamaterials from the T-Matrix of Its Constituting Meta-Atom.

Self-assembled metamaterials attract considerable interest as they promise to make isotropic bulk metamaterials available at low costs. The optical response of self-assembled metamaterials is derived predominantly from the response of its individual constituents, i.e., the meta-atoms. Beyond effective properties, primary experimentally observable quantities, such as specific cross-sections, are at the focus of interest as they are frequently considered when exploiting metamaterials in specific applications. This posses the challenge of predicting these observable quantities for a diluted ensemble of randomly oriented meta-atoms. Thus far, this has been achieved by either averaging the optical response of the meta-atom across all possible incident fields or by restricting the consideration to only an electric and magnetic dipolar response. This, however, is either time-consuming or imposes an unnecessary limitation. Here, we solve this problem by deriving and presenting explicit expressions for experimentally observable quantities of metamaterials made from randomly arranged and oriented meta-atoms characterized by their T-matrix.

Magnetic Response of Metamaterials

We present a general numerical approach for the calculation of the non‐local macroscopic dielectric response of a binary metamaterial made of arbitrary non‐magnetic materials with any geometry. From the spatial dispersion of the permittivity, characterized by its dependence on the wavevector, we obtain the corresponding effective local macroscopic permeability of the metamaterial. We illustrate our computational approach by calculating the permeability of two‐dimensional split‐ring resonators and validate the results through comparison with those of simple models. Our scheme may be further applied to arbitrary geometries. We discuss the limitations of the local permeability approach.

Intrinsic rippling enhances static non-reciprocity in a graphene metamaterial.

In mechanical systems, Maxwell-Betti reciprocity means that the displacement at point B in response to a force at point A is the same as the displacement at point A in response to the same force applied at point B. Because the notion of reciprocity is general, fundamental, and is operant for other physical systems like electromagnetics, acoustics, and optics, there is significant interest in understanding systems that are not reciprocal, or exhibit non-reciprocity. However, most studies on non-reciprocity have occurred in bulk-scale structures for dynamic problems involving time reversal symmetry. As a result, little is known about the mechanisms governing static non-reciprocal responses, particularly in atomically-thin two-dimensional materials like graphene. Here, we use classical atomistic simulations to demonstrate that out-of-plane ripples, which are intrinsic to graphene, enable significant, multiple orders of magnitude enhancements in the statically non-reciprocal response of graphene metamaterials. Specifically, we find that a striking interplay between the ripples and the stress fields that are induced in the metamaterials due to their geometry impacts the displacements that are transmitted by the metamaterial, thus leading to a significantly enhanced static non-reciprocal response. This study thus demonstrates the potential of two-dimensional mechanical metamaterials for symmetry-breaking applications.

Controlling Metamaterial Transparency with Superchiral Fields

The advent of metamaterials has heralded a period of unprecedented control of light. The optical responses of metamaterials are determined by the properties of constituent nanostructures. The current design philosophy for tailoring metamaterial functionality is to use geometry to control the nearfield coupling of the elements of the nanostructures. A drawback of this geometry-focused strategy is that the functionality of a metamaterial is predetermined and cannot be manipulated easily postfabrication. Here we present a new design paradigm for metamaterials, in which the coupling between chiral elements of a nanostructure is controlled by the chiral asymmetries of the nearfield, which can be externally manipulated. We call this mechanism dichroic coupling. This phenomenon is used to control the electromagnetic induced transparency displayed by a chiral metamaterial by tuning the chirality of the near fields. This “non-geometric” paradigm for controlling optical properties offers the opportunity to optimall...

Waves in hyperbolic and double negative metamaterials including rogues and solitons

The topics here deal with some current progress in electromagnetic wave propagation in a family of substances known as metamaterials. To begin with, it is discussed how a pulse can develop a leading edge that steepens and it is emphasised that such self-steepening is an important inclusion within a metamaterial environment together with Raman scattering and third-order dispersion whenever very short pulses are being investigated. It is emphasised that the self-steepening parameter is highly metamaterial-driven compared to Raman scattering, which is associated with a coefficient of the same form whether a normal positive phase, or a metamaterial waveguide is the vehicle for any soliton propagation. It is also shown that the influence of magnetooptics provides a beautiful and important control mechanism for metamaterial devices and that, in the future, this feature will have a significant impact upon the design of data control systems for optical computing. A major objective is fulfiled by the investigations of the fascinating properties of hyperbolic media that exhibit asymmetry of supported modes due to the tilt of optical axes. This is a topic that really merits elaboration because structural and optical asymmetry in optical components that end up manipulating electromagnetic waves is now the foundation of how to operate some of the most successful devices in photonics and electronics. It is pointed out, in this context, that graphene is one of the most famous plasmonic media with very low losses. It is a two-dimensional material that makes the implementation of an effective-medium approximation more feasible. Nonlinear non-stationary diffraction in active planar anisotropic hyperbolic metamaterials is discussed in detail and two approaches are compared. One of them is based on the averaging over a unit cell, while the other one does not include sort of averaging. The formation and propagation of optical spatial solitons in hyperbolic metamaterials is also considered with a model of the response of hyperbolic metamaterials in terms of the homogenisation (‘effective medium’) approach. The model has a macroscopic dielectric tensor encompassing at least one negative eigenvalue. It is shown that light propagating in the presence of hyperbolic dispersion undergoes negative (anomalous) diffraction. The theory is ten broadened out to include the influence of the orientation of the optical axis with respect to the propagation wave vector. Optical rogue waves are discussed in terms of how they are influenced, but not suppressed, by a metamaterial background. It is strongly discussed that metamaterials and optical rogue waves have both been making headlines in recent years and that they are, separately, large areas of research to study. A brief background of the inevitable linkage of them is considered and important new possibilities are discussed. After this background is revealed some new rogue wave configurations combining the two areas are presented alongside a discussion of the way forward for the future.

Deconvolution methods to obtain the impulse response of acoustic metamaterial samples of finite extent

One complication of characterizing the response of acoustic metamaterials is that models often assume the medium is of infinite extent. On the other hand, material property measurement can only be done with samples of finite size. The result is that acoustic field measurements include the effects of edge diffraction and scattering from fixtures. This work presents deconvolution methods to extract the frequency-dependent reflection and transmission behavior of acoustic metamaterial samples. Measurements using broadband chirp signals are obtained and subsequently post-processed using deconvolution techniques to obtain high-time-resolution impulse responses. Additionally, the chirp signals can be modified to compensate for the frequency response of the transducers being used. Examples will be shown for the transmission behavior through a two-dimensional pentamode gradient index lens and the reflection response of a flat square plate. [Work supported by ONR.]

Nonlinear terahertz metamaterials with active electrical control

We present a study of an electrically modulated nonlinear metamaterial consisting of an array of split-ring resonators fabricated on n-type gallium arsenide. The resonant metamaterial nonlinearity appears as an intensity-dependent transmission minimum at terahertz frequencies and arises from the interaction between local electric fields in the split-ring resonator (SRR) capacitive gaps and charge carriers in the n-type substrate. We investigate the active tuning range of the metamaterial device as the incident terahertz field intensity is increased and conversely the effect of an applied DC bias on the terahertz field-induced nonlinear modulation of the metamaterial response. Applying a DC bias to the metamaterial sample alters the nonlinear response and reduces the net nonlinear modulation. Similarly, increasing the incident terahertz field intensity decreases the net modulation induced by an applied DC bias. We interpret these results in terms of DC and terahertz-field-assisted carrier acceleration, scattering, and multiplication processes, highlighting the unique nature of this DC-field modulated terahertz nonlinearity.

Nonlocal response of tunable photonic metamaterials with semiconductor inclusions

Effective parameters of homogenized photonic crystals with a semiconductor bar in the unit cell are calculated and analyzed. The applied homogenization theory is based on the Fourier formalism within the form-factor division approach. The calculated effective parameters, namely, the components of the effective permittivity tensor, are nonlocal since they depend on not only frequency but also the wave vector. Using the nonlocal effective parameters, the complex dispersion relation for photonics modes propagating in a periodic semiconductor–dielectric array can be described even beyond the long-wavelength limit. We have analyzed the temperature dependence of the nonlocal effective parameters for photonic metamaterials composed of InSb square bars in a silica–glass host matrix. In the case of continuous (infinitely long) bars, the metamaterial shows plasma-like behavior with a temperature-dependent effective plasma frequency in the THz range. For the case of cut semiconductor bars, the metamaterial behaves as a dielectric in the lowest frequency band, whose width can also be tuned by varying the temperature. Using the nonlocal effective permittivity, the optical spectra (reflectivity and transmittance) for a slab of InSb square bars in a glass matrix were calculated and analyzed.

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.