Wire Metamaterials Research Articles

Tilted Wire Metamaterials Enabling Ultra-Broadband Absorption from Middle to Very Long Infrared Regimes

Tilted Wire Metamaterials Enabling Ultra-Broadband Absorption from Middle to Very Long Infrared Regimes

Oxygen Vacancy Controlled Hyperbolic Metamaterial Based on Indium Tin Oxide (ITO) Nanotubes with Switchable Optical Properties

AbstractNanostructured metamaterials can offer optical properties beyond what is achievable in conventional media, such as negative refraction or sub‐wavelength imaging. Due to their structural anisotropy, the class of high aspect ratio metamaterials is of interest for the possibility of achieving hyperbolic optical properties, i.e., both metallic and dielectric behavior based on the excitation direction. Although investigated numerically, the fabrication of tailor‐made metamaterials is complex or often beyond the reach of current technology. For wire metamaterials composed of aligned metallic nanowires in a dielectric matrix, since the free carrier concentration in metals is fixed, light‐matter interaction cannot be adjusted after fabrication. Here, metamaterials based on plasmonic ITO nanotubes with controllable hyperbolic response are introduced. The synthesis is achieved by a template‐based liquid‐phase technique. The tuning mechanism is based on controlling the carrier density in ITO via oxygen vacancy concentration. The process is reversible, the photonic features are activated by creating oxygen vacancies and can be switched off by filling them up again. Further, it is shown that the carrier concentration can also be controlled via a static electric field. Optical simulations support the experimental findings and highlight the parameters that determine the optical response of the metamaterial.

Controlling the dispersion of longitudinal waves via the affine deformation of the interlaced wire medium

We studied the dispersion properties of double interlaced wire metamaterials with geometry modified by affine transformation. That metamaterials were found to support eigenmodes with longitudinal polarization at low frequencies for all deformations. Due to spatial dispersion, the metamaterials’ isofrequency surfaces are centered at the Brillouin zone edges (rather than at Γ-point) and have the shape of ellipsoids. Their refractive indices corresponding to the ellipsoids were analyzed both analytically and numerically.

Emulating quantum photon-photon interactions in waveguides by double-wire media

Arrays of atoms coupled to photons propagating in a waveguide are now actively studied due to their prospects for generation and detection of quantum light. Quantum simulators based on waveguides with long-range couplings were also predicted to manifest unusual many-body quantum states. However, quantum tomography for large arrays with N ≳ 20 atoms remains elusive since it requires independent access to every atom. Here, we present a novel concept for analog emulation of such systems by unveiling an analogy between the setup of waveguide quantum electrodynamics and the classical problem of an electromagnetic wave propagating in a wire metamaterial. Experimentally measuring near electromagnetic fields, we emulate the two-particle localization arising from polariton-polariton interactions in the quantum problem. Our results demonstrate the potential of wire metamaterials to visualize quantum light-matter coupling in a table-top experiment and may be applied to emulate other exotic quantum effects, such as quantum chaos, and self-induced topological states.

Topological Engineering of the Iso-Frequency Contours in Connection-Type Metamaterials

The topology of isofrequency surface governs the electromagnetic wave propagation and light–matter interaction in metamaterials. For most metamaterials with local medium description, the low-frequency isosurfaces are typical spheres or ellipsoids centered at zero momentum, which, to some extent, limits our manipulation ability on low-frequency wave. In this work, based on connection-type wire metamaterials, we propose a scheme to engineer the shapes of isofrequency surfaces. An equivalent circuit model is developed to analyze the low-frequency dispersion of connection-type metamaterials. It implies that the shape of index ellipsoids at quasistatic limit is determined by the equivalent inductances and capacitances of the metallic meshes. By adjusting these equivalent circuit parameters, we can achieve the isotropic or anisotropic index ellipsoids at quasistatic limit and, hence, a cruciform or bowtie-shaped isofrequency contours for the lowest-frequency band. Our results demonstrate a feasible platform for topological engineering of isofrequency surfaces, which may pave the way to novel devices for manipulating long-wavelength electromagnetic wave.

First principles homogenization of periodic metamaterials and application to wire media

Here, we present an overview of a first principles homogenization theory of periodic metamaterials. It is shown that in a rather general context it is possible to formally introduce effective parameters that describe the time evolution of macroscopic (slowly-varying in space) initial states of the electromagnetic field using an effective medium formalism. The theory is applied to different types of characterized by a strong spatial dispersion in the long wavelength limit. It is highlighted that the spatial dispersion may tailor in unique ways the wave phenomena in wire metamaterials leading to exotic tunneling effects and broadband lossless anomalous dispersion.

Observation of normal mode splitting in THz Fabry–Pérot microcavity made of wire grid structures containing cut wire metamaterials

In this study, we realize normal mode splitting in Fabry–Pérot (FP) microcavity structures containing cut wire (CW) metamaterials inside the cavity layer in the terahertz (THz) region. We used a wire grid (WG) structure to mirror the microcavity. To verify this concept, we analyzed the reflection and transmission properties of WG, CW, WG cavity (WGC), and microcavity containing CW (CWWGC) using transmission line theory. Consequently, we discovered that when the resonance frequencies of the CW and WGC matched, normal mode splitting occurred and anti-crossing is observed in the dispersion relation in the CWWGC. The samples are designed by the finite difference time domain method and were fabricated via photolithography. We measured the transmission spectrum of the fabricated samples by THz time domain spectroscopy. Finally, we observed a large splitting of the transmission peak owing to mode coupling between the light mode of the FP microcavity and the electric dipole mode of a single CW in the CWWGC.

Enhancement of the Purcell Effect by the Wire Metamaterials Formed by the Hexagonal Unit Cells

The dependence of the Purcell factor on nanowire metamaterial geometry was analyzed. Calculations made about the Purcell factor in realistic composites, operating at an optical spectral range, are provided. We applied a metamaterial, aiming to mitigate the negative effects of absorption in metals on the Purcell effect in nanowire structures. A nanowire metamaterial was treated as an anisotropic composite in the long-wavelength limit. We investigated the mode patterns of the surface waves, propagating at the boundary separating such a structure and a dielectric material, along with the position of the peak in the local density of states, for the various filling factors of the periodic structure. By calculating the frequency dependence of the Purcell factor, we showed an increase in the peak value in comparison with the conventional plasmonic structure in the (1–100 THz) frequency range. Moreover, an optimal set of the parameters, needed to obtain the two topological transitions in the frequency range under investigation, is proposed.

Broadband subdiffraction imaging in highly anisotropic metamaterials using ultra-short super-Gaussian pulse

Metamaterials with large axial anisotropy posses a nearly flat dispersion profile in k (wave vector) space and thus offer an effective solution to overcome the diffraction limit by supporting the propagating high - k extraordinary modes. However, existing analytical models reveal that resonant high - k slab modes and the polarization dependent ordinary waves cause image distortion in metamaterial slabs. In this paper, we consider a two-dimensional (2D), local, highly anisotropic metamaterial slab as an imaging device and apply a standard transfer matrix approach to calculate the transmission properties of the slab at terahertz (THz) frequencies. Our simple analytical model reveals that resonances induced by the reflections are the main source of deteriorating the image quality, thus requires effective post-processing methods to remove them. For that, we apply an ultra-short super-Gaussian windowing function to minimize the resonant behavior of the metamaterial slabs, observing good imaging over the frequency band of interest. Our numerical method offers a pathway to mitigate observed image artefacts, and are applicable to a range of highly anisotropic metamaterial slabs, e.g., wire metamaterials, layered metamaterials and magnifying hyperlenses. Finally, finite element based software is used to model the 2D metamaterial slab to verify the analytical models.

Tunable Axion Plasma Haloscopes.

We propose a new strategy for searching for dark matter axions using tunable cryogenic plasmas. Unlike current experiments, which repair the mismatch between axion and photon masses by breaking translational invariance (cavity and dielectric haloscopes), a plasma haloscope enables resonant conversion by matching the axion mass to a plasma frequency. A key advantage is that the plasma frequency is unrelated to the physical size of the device, allowing large conversion volumes. We identify wire metamaterials as a promising candidate plasma, wherein the plasma frequency can be tuned by varying the interwire spacing. For realistic experimental sizes, we estimate competitive sensitivity for axion masses of 35-400 μeV, at least.

Numerical simulation study on bowtie antenna-based time reversal mirror for super-resolution target detection

Abstract Bowtie antenna-based time reversal mirror (TRM), incorporating with randomly distributed and arbitrarily shaped wire metamaterials medium, is proposed to realize super-resolution target detection. The achieved performance for standard and scatterer bowtie antenna TRM is compared and discussed. The dual-band bowtie antennas resonate at 2.45 GHz and 5.2 GHz and a super-resolution of 0.0817 of the free-space wavelength at 2.45 GHz has been achieved. For the first time, studies show that the TRM with microstructure perturbations (namely scatterers) can enhance the resolution in some cases. Proposing a method of super-resolving transmission of electromagnetic waves is very important to realize multi-independent channels in a compact space for the related applications.

Low-frequency nonlocal and hyperbolic modes in corrugated wire metamaterials.

Metamaterials based on arrays of aligned plasmonic nanowires have recently attracted significant attention due to their unique optical properties that combine tunable strong anisotropy and nonlocality. These optical responses provide a platform for implementation of novel sensing, imaging, and quantum optics applications. Basic building blocks, used for construction of those peculiar composites, are plasmonic metals, such as gold and silver, which have moderate negative values of permittivities at the optical spectral range. Scaling the plasmonic behavior to lower frequencies remains a longstanding challenge also owing to the emergence of strong spatial dispersion in homogenized artificial composites. At lower THz and GHz frequencies, the electromagnetic response of noble metals approaches that of perfect electric conductors, preventing straightforward scaling of visible-frequency plasmonics to the frequency domains that are important for a vast range of applications, including wireless communications, microwave technologies and many others. Here we demonstrate that both extreme anisotropy (so-called hyperbolicity) and nonlocality of artificial composites can be achieved and designed in arrays of corrugated perfectly conducting wires at relatively low GHz frequencies. The key concept is based on hybridization of spoof plasmon polariton modes that in turn emulate surface polariton waves in systems with corrugated interfaces. The method makes it possible to map the recent developments in the field of plasmonics and metamaterials to the domain of THz and RF photonics.

Flexible single-mode hollow-core terahertz fiber with metamaterial cladding

A key requirement for achieving high-density integration of terahertz (THz) systems is a strongly confining single-mode and low-loss waveguide. Several waveguide solutions based on technologies from both electronics and photonics have been proposed; among these, hollow-core waveguides are one of the best options for guiding THz radiation due to their very low material absorption of air. However, to minimize reflection losses, hollow-core waveguides typically have a core diameter larger than the operating wavelength, and as a consequence are multimode. Here, we report on a single-mode, single-polarization hollow-core THz fiber with a metamaterial cladding, consisting of subwavelength-diameter metal wires embedded in a dielectric host. The idea of using metal–dielectric hybrid cladding relies on the extreme anisotropy of wire metamaterials, which reflects transverse magnetic (TM) waves and transmits transverse electric waves, leading to a waveguide structure that only confines TM modes—thus halving the number of modes from the outset. Numerical simulations and experimental measurements confirm a wide single-mode single-polarization window ranging from 0.31 to 0.44 THz, with a wavelength-sized core (0.88 mm diameter). Our work overcomes a stumbling block for achieving compact and flexible single-mode THz waveguides, which may be important for future THz systems with high-density integration.

Double resonant wideband Purcell effect in wire metamaterials

In this paper, we theoretically show that a broadband resonant enhancement of emission may occur for infrared sources located in a polaritonic wire medium. The reason for this enhancement is the overlapping of two topological transitions of the wave dispersion in the medium. The first topological transition has been revealed as an effect inherent to polaritonic wire media at a certain frequency in the mid-infrared range. This work uncovers another topological transition for such wire media which holds at a higher frequency but still in the mid infrared. We show that the first transition frequency can be shifted towards the second one by variation of the design parameters. This shift enables a broadband resonant Purcell factor. We compare the results obtained for two orientations of a subwavelength electric dipole embedded into the wire medium—that along the optical axis and that perpendicular to it—and report on the resonant isotropic radiation enhancement. Also, we reveal the enhancement of radiation to the free space from a finite sample of the wire medium.

Phase matching in hyperbolic wire media for nonlinear frequency conversion.

Efficient nonlinear frequency conversion requires a phase matching condition to be satisfied. We analyze the dispersion of the modes of hyperbolic wire metamaterials and demonstrate that phase matching at infrared wavelengths can be achieved with a variety of constituent materials, such as GaAs, in which phase matching cannot easily be achieved by conventional means. Our finding promises access to many materials with attractive nonlinear properties.

Coherent Transition Radiation from Wire Metamaterials

The article discusses the problem of radiation generated during the interaction between the field of relativistic charged particles and metamaterials based on a one-dimensional periodic wire structure. The work includes the experimental study of the reverse coherent transition radiation emitted by these structures in the millimeter wavelength range for two orthogonal directions of radiation polarization. Under the same experimental conditions, authors also compared the intensity of this radiation with the reverse transition radiation from solid conductive targets. The study elucidates the asymmetrical polarization of radiation emitted by a metamaterial wire structure.

Effective-medium model of wire metamaterials in the problems of radiative heat transfer

In the present work, we check the applicability of the effective medium model (EMM) to the problems of radiative heat transfer (RHT) through so-called wire metamaterials (WMMs)—composites comprising parallel arrays of metal nanowires. It is explained why this problem is so important for the development of prospective thermophotovoltaic (TPV) systems. Previous studies of the applicability of EMM for WMMs were targeted by the imaging applications of WMMs. The analogous study referring to the transfer of radiative heat is a separate problem that deserves extended investigations. We show that WMMs with practically realizable design parameters transmit the radiative heat as effectively homogeneous media. Existing EMM is an adequate tool for qualitative prediction of the magnitude of transferred radiative heat and of its effective frequency band.

Wire metamaterial based on semiconductor matrices

AbstractIn this Letter, we have presented a new approach for the fabrication of nanowire media (wire metamaterials) by electrochemical methods. AIIIBV porous matrices (a host medium) were prepared by anodic electrochemical etching of industrial substrates. The host medium has been filled via electrochemical deposition with a metal and by means of annealing process. We have shown that this technique can be used to fabricate a nanowire medium with unique parameters (such as aspect ratio, high electric permittivity and strong χ(3) nonlinearity near the fundamental absorption edge of the host media). (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Purcell effect in wire metamaterials

We study theoretically the enhancement of spontaneous emission in wire metamaterials. We analyze the dependence of the Purcell factor dependence on wire dielectric constant for both electric and magnetic dipole sources, and find an optimal value of the dielectric constant for maximizing the Purcell factor for the electric dipole. We obtain analytical expressions for the Purcell factor and also provide estimates for the Purcell factor in realistic structures operating in both microwave and optical spectral range.

Fabricating Metamaterials Using the Fiber Drawing Method

Metamaterials are man-made composite materials, fabricated by assembling components much smaller than the wavelength at which they operate (1). They owe their electromagnetic properties to the structure of their constituents, instead of the atoms that compose them. For example, sub-wavelength metal wires can be arranged to possess an effective electric permittivity that is either positive or negative at a given frequency, in contrast to the metals themselves (2). This unprecedented control over the behaviour of light can potentially lead to a number of novel devices, such as invisibility cloaks (3), negative refractive index materials (4), and lenses that resolve objects below the diffraction limit (5). However, metamaterials operating at optical, mid-infrared and terahertz frequencies are conventionally made using nano- and micro-fabrication techniques that are expensive and produce samples that are at most a few centimetres in size (6-7). Here we present a fabrication method to produce hundreds of meters of metal wire metamaterials in fiber form, which exhibit a terahertz plasmonic response (8). We combine the stack-and-draw technique used to produce microstructured polymer optical fiber (9) with the Taylor-wire process (10), using indium wires inside polymethylmethacrylate (PMMA) tubes. PMMA is chosen because it is an easy to handle, drawable dielectric with suitable optical properties in the terahertz region; indium because it has a melting temperature of 156.6 °C which is appropriate for codrawing with PMMA. We include an indium wire of 1 mm diameter and 99.99% purity in a PMMA tube with 1 mm inner diameter (ID) and 12 mm outside diameter (OD) which is sealed at one end. The tube is evacuated and drawn down to an outer diameter of 1.2 mm. The resulting fiber is then cut into smaller pieces, and stacked into a larger PMMA tube. This stack is sealed at one end and fed into a furnace while being rapidly drawn, reducing the diameter of the structure by a factor of 10, and increasing the length by a factor of 100. Such fibers possess features on the micro- and nano- scale, are inherently flexible, mass-producible, and can be woven to exhibit electromagnetic properties that are not found in nature. They represent a promising platform for a number of novel devices from terahertz to optical frequencies, such as invisible fibers, woven negative refractive index cloths, and super-resolving lenses.