Response Of Metamaterials Research Articles

Magnetic monopole-like response in metamaterials

In this paper, we explored magnetic monopole-like responses in metamaterials. We designed a sub-wavelength metamolecule that is composed of two dielectric-spaced split-ring resonators. In response to incident waves, the induced magnetic field in the metamolecule resembles that of a two-dimensional magnetic monopole. The magnetic monopole-like response is resulted from electric resonance of the metamolecule, so an electric dipole is always attached. By combining two mirror-symmetric metamolecules with inward and outward radial magnetic fields, magnetic dipole-like responses can be produced just as an electric dipole is formed by separating two opposite-signed electric charges.

Optically controllable THz chiral metamaterials.

Switchable and tunable chiral metamaterial response is numerically demonstrated here in different uniaxial chiral metamaterial structures operating in the THz regime. The structures are based on the bi-layer conductor design and the tunable/switchable response is achieved by replacing parts of the metallic components of the structures by photoconducting Si, which can be transformed from an insulating to an almost conducting state through photoexcitation, achievable under external optical pumping. All the structures proposed and discussed here exhibit frequency regions with giant tunable circular dichroism, as well as regions with giant tunable optical activity, showing unique potential in the achievement of active THz polarization components, like tunable polarizers and polarization filters.

Conductive coupling of split ring resonators: a path to THz metamaterials with ultrasharp resonances.

We report on a novel metamaterial structure that sustains extremely sharp resonances in the terahertz domain. This system involves two conductively coupled split ring resonators that together exhibit a novel resonance, in broad analogy to the antiphase mode of the so-called Huygens coupled pendulum. Even though this resonance is in principle forbidden in each individual symmetric split ring, our experiments show that this new coupled mode can sustain quality factors that are more than one order of magnitude larger than those of conventional split ring arrangements. Because of the universality of the metamaterial response, the design principle we present here can be applied across the entire electromagnetic spectrum and to various metamaterial resonators.

Equivalence of Magnetic Metamaterials and Composites in the View of Effective Medium Theories

The electromagnetic response of metamaterials is governed by the collective behavior of engineered electric and magnetic dipoles. Therefore metamaterials may be replaced by hypothetical composites of spherical particles embedded in a host material. The effective electric permittivity and magnetic permeability of such systems can be computed with high-frequency extension of the Maxwell-Garnett mixing rule. The validity of this assumption is discussed and the features of the model are presented by calculating the effective electromagnetic material parameters of a deep subwavelength spherical composite in three different ways: with the Maxwell-Garnett mixing rule, high-frequency mixing rule and directly extracted from transmission reflection data. Afterwards the developed theory is applied to find the parameters of a composite with similar magnetic response as a metamaterial built of split ring resonators or fishnet structures.

Tunable negative permeability in a quantum plasmonic metamaterial

We consider the integration of quantum emitters into a negative permeability metamaterial design in order to introduce tunability as well as nonlinear behavior. The unit cell of our metamaterial is a ring of metamolecules, each consisting of a metal nanoparticle and a two-level semiconductor quantum dot (QD). Without the QDs, the ring of the unit cell is known to act as an artificial optical magnetic resonator. By adding the QDs we show that a Fano interference profile is introduced into the magnetic field scattered from the ring. This induced interference is shown to cause an appreciable effect in the collective magnetic resonance of the unit cell. We find that the interference provides a means to tune the response of the negative permeability metamaterial. The exploitation of the QD's inherent nonlinearity is proposed to modulate the metamaterial's magnetic response with a separate control field.

Nonlocal optics of plasmonic nanowire metamaterials

We present an analytical description of the nonlocal optical response of plasmonic nanowire metamaterials that enable negative refraction, subwavelength light manipulation, and emission lifetime engineering. We show that dispersion of optical waves propagating in nanowire media results from coupling of transverse and longitudinal electromagnetic modes supported by the composite and derive the nonlocal effective medium approximation for this dispersion. We derive the profiles of electric field across the unit cell, and use these expressions to solve the long-standing problem of additional boundary conditions in calculations of transmission and reflection of waves by nonlocal nanowire media. We verify our analytical results with numerical solutions of Maxwell's equations and discuss generalization of the developed formalism to other uniaxial metamaterials.

Screw split ring resonator as building block of three-dimensional chiral metamaterials

We proposed and numerically investigated the influence of spatial topology on the infrared frequency region response of chiral metamaterials based on discrete deformed split ring resonators. Compared with the well studied continuous helix, the proposed metamaterials with discrete topology exhibit broad band chiral electromagnetic response. It is shown that the conversion between left and right circular polarization waves for our model is much broader than the continuous helix model. The observed cross-coupling between electric and magnetic fields results from the chiral electric currents on the resonators due to the broken mirror symmetry. The findings are useful for the design of future real three-dimensional chiral metamaterials with tunable optical response.

Effect of local field enhancement on the nonlinear terahertz response of a silicon-based metamaterial

We demonstrate the strong effect of the local field enhancement on the nonlinear terahertz response of a hybrid photoexcited silicon/double concentric ring metamaterial structure. The ring resonators enhance the local terahertz electric field by more than a factor of ten, pushing the terahertz-semiconductor interaction into the high-field regime even for moderate-strength incident terahertz pulses. In this regime, terahertz field-induced intervalley scattering in the photoexcited silicon substrate dynamically alters the substrate conductivity, which in turn strongly modifies the pulse transmission. The spatial distribution of the local field enhancement within the resonator structure results in a modified bandwidth, amplitude, and central frequency of the transmission resonance occurring on a subcycle time scale. These results demonstrate an enhancement of the nonlinear terahertz response of silicon-based metamaterials that must be accounted for in the design of terahertz nonlinear devices.

On the use of a hierarchical multi-level building block basis function scheme in periodic plasmonic structures

A Volumetric Method of Moments algorithm is applied to predict the plasmonic responses of chiral metamaterials. This algorithm is based on the use of a multi-level building block basis function scheme, in combination with a dedicated Kummer transformation in the calculation of periodic Green’s functions. The validity of the algorithm is demonstrated by analyzing a Ninja Star periodic structure. A good agreement can be found between simulation and experiment.

Temperature Control of Terahertz Metamaterials With Liquid Crystals

We design and experimentally characterize terahertz (THz) electric split-ring-resonator metamaterials infiltrated with nematic liquid crystals (LCs). We demonstrate that the resonant response of such metamaterials can be tuned by temperature due to the change of the LC arrangement from nematic to isotropic phase. In particular, we show that the frequency tuning is strongly dependent on the initial LC orientation, with resonant frequency shifting towards lower frequencies for LC director perpendicular to the gap and towards higher frequencies for LC director parallel to the gap of the electric split ring resonators. Our results open new opportunities for the design of tunable THz filters based on LCs.

Optically Tunable Terahertz Metamaterials on Highly Flexible Substrates

We present optically tunable metamaterials (MMs) on flexible polymer sheets operating at terahertz (THz) frequencies. The flexible MMs, consisting of electric split-ring resonators (eSRRs) on patterned GaAs patches, were fabricated on a thin polyimide layer using a transfer technique. Optical excitation of the GaAs patches modifies the metamaterial response. Our experimental results revealed that, with increasing fluence, a transmission modulation depth of ~ 60% was achieved at the LC resonant frequency of 0.98 THz. In addition, a similar modulation depth was obtained over a broad range from 1.1 to 1.8 THz. Numerical simulations agree with experiment and indicate efficient tuning of the effective permittivity of the MMs. Our flexible tunable device paves the way to create multilayer nonplanar tunable electromagnetic composites for nonlinear and multifunctional applications, including sensing, modulation, and energy harvesting.

Near-infrared trapped mode magnetic resonance in an all-dielectric metamaterial

Optical responses in conventional metamaterials based on plasmonic metal nanostructures are inevitably accompanied by Joule losses, which obstruct practical applications by limiting resonance quality factors and compromising the efficiency of metamaterial devices. Here we experimentally demonstrate a fully-dielectric metamaterial that exhibits a 'trapped mode' resonance at optical frequencies, founded upon the excitation by incident light of anti-parallel displacement currents in meta-molecules comprising pairs of parallel, geometrically dissimilar dielectric nano-bars. The phenomenon is demonstrated in the near-infrared part of the spectrum using silicon, showing that in principle strong, lossless resonant responses are possible anywhere in the optical spectral range.

Signature of bulk longitudinal plasmon-polaritons in the transmission/reflection spectra of one-dimensional metamaterial heterostructures

A systematic theoretical study of the transmission and reflection spectra of a 1D metamaterial heterostructure consisting of the finite repetition of a double layer of two different building blocks A (normal non-dispersive layer) and B (dispersive metamaterial) is presented, with the multi-layered system sandwiched between two semi-infinite layers of the A material. For the metamaterial slabs B we have studied two different dielectric/magnetic responses: a first one with both the electric permittivity and magnetic permeability given by a Drude-like metamaterial dispersion response, and a second one with the electric permittivity still given by a Drude-like response, whereas the magnetic permeability is given by a more realistic split-ring resonator metamaterial response. Theoretical calculations are performed within the transfer-matrix formalism, and transmission and reflection coefficients of the metamaterial heterostructure are straightforwardly evaluated. For oblique incidence, a finite projection along the growth direction of the electric or magnetic field of the incident wave associated with the TM or TE modes, respectively, leads to a coupling in each layer of the metamaterial heterostructure of the photon modes with the bulk electric or magnetic metamaterial plasmons, respectively. Such photon–plasmon coupling results in the formation of plasmon-polariton modes and gives rise to signatures of the electric or magnetic longitudinal bulk-plasmon polariton modes in the transmission and reflection spectra of the metamaterial heterostructure. Such features survive even in the case of a single AB double-layer metamaterial heterostructure, and experimental observation should be, therefore, easily achieved.

Investigating the Impact of Microwave Breakdown on the Responses of High-Power Microwave Metamaterials

We investigate the effect of microwave-induced breakdown on the frequency responses of a class of metamaterials composed of planar sub-wavelength periodic structures. When breakdown occurs in such a structure, its frequency response changes based on the nature of the plasma created within its unit cell. We examine how the frequency responses of such periodic structures change as a result of creation of microwave-induced discharges within their unit cells. To do this, we examine single-layer metasurfaces composed of miniature LC resonators arranged in a 2-D periodic lattice. These metasurfaces are engineered to be opaque at microwave frequencies when operated at low power levels but can be made transparent if a localized discharge is created within the LC resonators. By measuring their transmission and reflection coefficients under high-power excitation in different conditions, the impact of breakdown on the frequency responses of these devices is determined. Several prototypes of such structures are examined both theoretically and experimentally. It is demonstrated that when breakdown occurs in air and at atmospheric pressure levels, the responses of such periodic structures can be predicted with a reasonable degree of accuracy. Additionally, when the unit cell of the metasurface is composed of two different resonators, breakdown is always observed to occur in both resonators despite their different topologies and local field enhancement factors. In such structures, the discharge in one resonator appears to be mediated by the one in the other.

Superpositions of Lorentzians as the class of causal functions

We prove that all functions obeying the Kramers-Kronig relations can be approximated as superpositions of Lorentzian functions, to any precision. As a result, the typical text-book analysis of dielectric dispersion response functions in terms of Lorentzians may be viewed as encompassing the whole class of causal functions. A further consequence is that Lorentzian resonances may be viewed as possible building blocks for engineering any desired metamaterial response, for example by use of split ring resonators of different parameters. Two example functions, far from typical Lorentzian resonance behavior, are expressed in terms of Lorentzian superpositions: A steep dispersion medium that achieves large negative susceptibility with arbitrarily low loss/gain, and an optimal realization of a perfect lens over a bandwidth. Error bounds are derived for the approximation.

Electromagnetic dipole coupling mechanism in layered terahertz metamaterials

Interplay between adjacent dipoles is an experimental priori for designing artificially-engineered structure because the dipole coupling is one critical factor for determining the electromagnetic response in metamaterials. Although numerous investigations have been performed to study the coupling effect of the split-ring resonator (SRR), the interlayer dipole coupling of its complementary SRR, called C-SRR, has been largely unexplored. Here, we present experimental and theoretical investigations on the electromagnetic coupling effect in the two stacks of layered C-SRR structures. By adjusting the relative lateral distance between the two-dimensionally stacked meta-structures, we observe that the confined magnetic dipole plays an important role in determining the resonance frequency and the bandwidth broadening of the C-SRR, exhibiting an exactly opposite behavior to the SRR structure. Our investigation provides experimental basis for developing frequency tunable three-dimensional metamaterial devices.

The magnetic response of graphene split-ring metamaterials

Graphene has emerged as a promising platform for THz plasmonics, allowing high confinement, long lifetimes and fast electrical tunability. Here, we predict a strong magnetic dipole response by graphene split nanorings at THz frequencies, allowing the attainment of metamaterials with a high degree of field confinement (approximately one hundredth of the excitation wavelength) that is not reachable with conventional noble metals. The magnetic response of highly doped graphene split-rings in the far-infrared is much stronger than that displayed by gold structures of similar thicknesses. We further explored stacked graphene layers as a practical way of producing high-frequency magnetism in thin, electrically tunable metamaterials. Our results support the great potential of using graphene to achieve electrically tunable magnetic metamaterials.

A metamaterial terahertz modulator based on complementary planar double-split-ring resonator

A metamaterial based on complementary planar double-split-ring resonator (DSRR) structure is presented and demonstrated, which can optically tune the transmission of the terahertz (THz) wave. Unlike the traditional DSRR metamaterials, the DSRR discussed in this paper consists of two split rings connected by two bridges. Numerical simulations with the finite-difference time-domain (FDTD) method reveal that the transmission spectra of the original and the complementary metamaterials are both in good agreement with Babinet’s principle. Then by increasing the carrier density of the intrinsic GaAs substrate, the magnetic response of the complementary special DSRR metamaterial can be weakened or even turned off. This metamaterial structure is promised to be a narrow-band THz modulator with response time of several nanoseconds.

Hyperbolic metamaterial lens with hydrodynamic nonlocal response

We investigate the effects of hydrodynamic nonlocal response in hyperbolic metamaterials (HMMs), focusing on the experimentally realizable parameter regime where unit cells are much smaller than an optical wavelength but much larger than the wavelengths of the longitudinal pressure waves of the free-electron plasma in the metal constituents. We derive the nonlocal corrections to the effective material parameters analytically, and illustrate the noticeable nonlocal effects on the dispersion curves numerically. As an application, we find that the focusing characteristics of a HMM lens in the local-response approximation and in the hydrodynamic Drude model can differ considerably. In particular, the optimal frequency for imaging in the nonlocal theory is blueshifted with respect to that in the local theory. Thus, to detect whether nonlocal response is at work in a hyperbolic metamaterial, we propose to measure the near-field distribution of a hyperbolic metamaterial lens.

Three-Dimensional Plasmonic Metamaterials and Their Fabrication Techniques

We present our research progress on the development of 3-D plasmonic metamaterials. Specifically, we describe the theoretical investigation of optical responses of metamaterials and their fabrication techniques. Based on an LC resonant circuit model considering the dispersive properties of metals, we show the comprehensive design principle of the 3-D metamaterials that exhibit strong magnetic responses at high frequencies. Based on the theoretical result, we propose and demonstrate top-down and bottom-up approaches for the fabrication of 3-D plasmonic metamaterials: two-photon fabrication of 3-D metallic nanostructures, DNA-templating mass production of gold trimer rings, and magnetic-field-induced self-assembly of gold necklace structures.