Wire metamaterials: physics and applications

Directional emission of a three-dimensional connection-type metamaterial

Directional emission of electromagnetic waves plays an essential role in laser radar and free-space communication. For most directional antennas, bandwidth and miniaturization are a pair of contradictions due to their underlying interference mechanism. Connection-type metamaterials exhibit exotic electromagnetic response near zero-frequency, which relies on the global topology of mesh connectivity rather than resonance and thus has a broad working bandwidth. In this Letter, we investigate the broadband orientation-dependent coupling effect of a 3D double mesh metamaterial. Based on this effect, we achieve a broadband directional emission (relative bandwidth of 37.72%) using a compact structure (compared to twice working wavelength). Our work provides a novel, to the best of our knowledge, scheme to manipulate a long-wavelength wave and may pave the way to a miniaturized directional antenna.

Omnidirectional broadband phase modulation by total internal reflection

Phase modulation plays a crucial role in shaping optical fields and physical optics. However, traditional phase modulation techniques are highly dependent on angles and wavelengths, limiting their applicability in smart optical systems. Here, we propose a first-principle theory for achieving constant phase modulation independent of incident angle and wavelength. By utilizing a hyperbolic metamaterial and engineering-specific optical parameters, different reflective phase jumps are achieved and tailored for both transverse electric (TE) and transverse magnetic (TM) waves. The aimed reflection phase difference between TE and TM waves can be thus achieved omnidirectionally and achromatically. As an example, we propose a perfect omnidirectional broadband reflection quarter wave plate. This work provides fundamental insights into manipulating optical phases through optical parameter engineering.

Quantitative polarization-sensitive super-resolution solid immersion microscopy reveals biological tissues' birefringence in the terahertz range

Terahertz (THz) technology offers a variety of applications in label-free medical diagnosis and therapy, majority of which rely on the effective medium theory that assumes biological tissues to be optically isotropic and homogeneous at the scale posed by the THz wavelengths. Meanwhile, most recent research discovered mesoscale ([Formula: see text]) heterogeneities of tissues; [Formula: see text] is a wavelength. This posed a problem of studying the related scattering and polarization effects of THz-wave-tissue interactions, while there is still a lack of appropriate tools and instruments for such studies. To address this challenge, in this paper, quantitative polarization-sensitive reflection-mode THz solid immersion (SI) microscope is developed, that comprises a silicon hemisphere-based SI lens, metal-wire-grid polarizer and analyzer, a continuous-wave 0.6 THz ([Formula: see text] µm) backward-wave oscillator (BWO), and a Golay detector. It makes possible the study of local polarization-dependent THz response of mesoscale tissue elements with the resolution as high as [Formula: see text]. It is applied to retrieve the refractive index distributions over the freshly-excised rat brain for the two orthogonal linear polarizations of the THz beam, aimed at uncovering the THz birefringence (structural optical anisotropy) of tissues. The most pronounced birefringence is observed for the Corpus callosum, formed by well-oriented and densely-packed axons bridging the cerebral hemispheres. The observed results are verified by the THz pulsed spectroscopy of the porcine brain, which confirms higher refractive index of the Corpus callosum when the THz beam is polarized along axons. Our findings highlight a potential of the quantitative polarization THz microscopy in biophotonics and medical imaging.

Exploring the Application of Multi-Resonant Bands Terahertz Metamaterials in the Field of Carbohydrate Films Sensing

Terahertz spectroscopy is a powerful tool for investigating the properties and states of biological matter. Here, a systematic investigation of the interaction of THz wave with "bright mode" resonators and "dark mode" resonators has been conducted, and a simple general principle of obtaining multiple resonant bands has been developed. By manipulating the number and positions of bright mode and dark mode resonant elements in metamaterials, we realized multi-resonant bands terahertz metamaterial structures with three electromagnetic-induced transparency in four-frequency bands. Different carbohydrates in the state of dried films were selected for detection, and the results showed that the multi-resonant bands metamaterial have high response sensitivity at the resonance frequency similar to the characteristic frequency of the biomolecule. Furthermore, by increasing the biomolecule mass in a specific frequency band, the frequency shift in glucose was found to be larger than that of maltose. The frequency shift in glucose in the fourth frequency band is larger than that of the second band, whereas maltose exhibits an opposing trend, thus enabling recognition of maltose and glucose. Our findings provide new insights into the design of functional multi-resonant bands metamaterials, as well as new strategies for developing multi-band metamaterial biosensing devices.

Broadband circular dichroism in chiral plasmonic woodpiles

The circular dichroism (CD) of a material is the difference in optical absorption under left- and right-circularly polarized illumination. It is crucial for a number of applications, from molecular sensing to the design of circularly polarized thermal light sources. The CD in natural materials is typically weak, leading to the exploitation of artificial chiral materials. Layered chiral woodpile structures are well known to boost chiro-optical effects when realized as a photonic crystal or an optical metamaterial. We here demonstrate that light scattering at a chiral plasmonic woodpile, which is structured on the order of the wavelength of the light, can be well understood by considering the fundamental evanescent Floquet states within the structure. In particular, we report a broadband circular polarization bandgap in the complex band structure of various plasmonic woodpiles that spans the optical transparency window of the atmosphere between 3 and 4 μ m and leads to an average CD of up to 90% within this spectral range. Our findings could pave the way for an ultra-broadband circularly polarized thermal source.

Quadraxial metamaterial

We study the dispersion of electromagnetic waves in a spatially dispersive metamaterial with Lorentz-like dependence of principal permittivity tensor components on the respective components of the wave vector performing the analysis of isofrequency contours. The considered permittivity tensor describes a triple non-connected wire medium. It is demonstrated that the metamaterial has four optic axes in the frequency range below the artificial plasma frequency. The directions of the optical axes do not depend on frequency and coincide with the diagonals of quadrants. The conical refraction effect is observed for all four optic axes.

Electromagnetic energy density in hyperbolic metamaterials

We present the theory of electromagnetic energy propagation through a dispersive and absorbing hyperbolic metamaterial (HMM). In this way, the permittivity tensor components of HMM (especially, nanowire HMM) may appear to be hopeless, but as a non-trivial step, we find that they can be cast into more transparent forms. We find under the influence of an electromagnetic wave, the responses of nanowire HMM (multilayer HMM) in the directions perpendicular to and parallel to the optical axis are similar to those of Lorentz (Drude) and Drude (Lorentz) media, respectively. We obtain simple expressions for the electromagnetic energy density formula of both typical structures of HMMs, i.e., nanowire and multilayer HMMs. Numerical examples reveal the general characteristics of the direction-dependent energy storage capacity of both nanowire and multilayer HMMs. The results of this study may shed more physical insight into the optical characteristics of HMMs.

Terahertz Metamaterial Sensor for Sensitive Detection of Citrate Salt Solutions

Citrate salts (CSs), as one type of organic salts, have been widely used in the food and pharmaceutical industries. Accurate and quantitative detection of CSs in food and medicine is very important for health and safety. In this study, an asymmetric double-opening ring metamaterial sensor is designed, fabricated, and used to detect citrate salts combined with THz spectroscopy. Factors that influence the sensitivity of the metamaterial sensor including the opening positions and the arrangement of the metal opening ring unit, the refraction index and the thickness of the analyte deposited on the metamaterial sensor were analyzed and discussed from electromagnetic simulations and THz spectroscopy measurements. Based on the high sensitivity of the metamaterial sensor to the refractive index of the analyte, six different citrate salt solutions with low concentrations were well identified. Therefore, THz spectroscopy combined with a metamaterials sensor can provide a new, rapid, and accurate detection of citrate salts.

THz generation by two-color laser air plasma coupled to antiresonance hollow-core sapphire waveguides: THz-wave delivery and angular distribution management

In this paper, hollow-core antiresonance sapphire waveguides were applied to guide the THz radiation emitted by the two-color laser air plasma, as well as to manage the THz source angular distribution. For this aim, three distinct waveguides were developed. Each of them is based on a cylindrical sapphire tube, either suspended in free space or coated by a polymer. The waveguides were first studied numerically, using the finite-difference eigenmode method, and experimentally, using the in-house THz pulsed spectrometer. The observed data uncovered the antiresonance regime of their operation, as well as their ability to guide broadband THz pulses over tens of centimeters with a high optical performance. The waveguides were then used to couple and guide (over the considerable distance) of THz radiation from the in-house two-color laser air plasma emitter, that exploits the mJ-energy-level femtosecond pulses of a Ti-sapphire laser. Small dispersion of a THz pulse and low-to-moderate propagation loss in the developed waveguide were observed, along with a considerable narrowing of the THz radiation angular distribution after passing the waveguide. Our findings revealed that such technologically-reliable hollow-core sapphire waveguides can boost the performance of laser air plasma-based THz emitters and make them more suitable for applications in the vigorously-explored THz sensing and exposure technologies.

3-D Metamaterials: Trends on Applied Designs, Computational Methods and Fabrication Techniques

Metamaterials are artificially engineered devices that go beyond the properties of conventional materials in nature. Metamaterials allow for the creation of negative refractive indexes; light trapping with epsilon-near-zero compounds; bandgap selection; superconductivity phenomena; non-Hermitian responses; and more generally, manipulation of the propagation of electromagnetic and acoustic waves. In the past, low computational resources and the lack of proper manufacturing techniques have limited attention towards 1-D and 2-D metamaterials. However, the true potential of metamaterials is ultimately reached in 3-D configurations, when the degrees of freedom associated with the propagating direction are fully exploited in design. This is expected to lead to a new era in the field of metamaterials, from which future high-speed and low-latency communication networks can benefit. Here, a comprehensive overview of the past, present, and future trends related to 3-D metamaterial devices is presented, focusing on efficient computational methods, innovative designs, and functional manufacturing techniques.

Adaptive Cylindrical Wireless Metasurfaces in Clinical Magnetic Resonance Imaging

The signal-to-noise ratio (SNR) is one of the most important criteria for evaluating the image quality in magnetic resonance imaging (MRI), and metasurfaces with unique electromagnetic properties provide a novel method for SNR improvement. However, their applications in clinical MRI are highly restricted by the inhomogeneous enhancement of the magnetic field and interference in the radio frequency (RF) transmitting field. In this study, an adaptive cylindrical wireless metasurface (ACWM) with homogeneous field enhancement and adaptive resonant modes is reported. The ACWM automatically switches its resonant modes between the partial (transmitting period) and whole (receiving period) resonance, which enables it to not only eliminate the interference in RF transmitting field, but also greatly enhance the SNR. Its adaptability also makes the ACWM applicable to all common clinical sequences without any modifications in the scan parameters. The SNR of MRI images of the human wrist, acquired with ACWM, is two to four times compared with the conventional coil. This work offers a practical control method to fill the scientific knowledge gaps between the preclinical research and medical applications for metasurfaces, and suggests a novel and powerful tool for diagnosing and evaluating human diseases.

Surface Nano-Patterning for the Bottom-Up Growth of III-V Semiconductor Nanowire Ordered Arrays

Ordered arrays of vertically aligned semiconductor nanowires are regarded as promising candidates for the realization of all-dielectric metamaterials, artificial electromagnetic materials, whose properties can be engineered to enable new functions and enhanced device performances with respect to naturally existing materials. In this review we account for the recent progresses in substrate nanopatterning methods, strategies and approaches that overall constitute the preliminary step towards the bottom-up growth of arrays of vertically aligned semiconductor nanowires with a controlled location, size and morphology of each nanowire. While we focus specifically on III-V semiconductor nanowires, several concepts, mechanisms and conclusions reported in the manuscript can be invoked and are valid also for different nanowire materials.

Extraction and control of permittivity of hyperbolic metamaterials with optical nonlocality

Metal nanorod arrays exhibit hyperbolic dispersion and optical nonlocality under certain conditions. Therefore, their optical behaviors can hardly be expressed by incident-angle-independent effective permittivity. Here we extract effective permittivity of silver nanorod arrays with diameters of 4 nm, 12 nm, and 20 nm by polarized transmission method in the visible range. The incident angles are chosen from 20° to 60° to study the influence of optical nonlocality on permittivity. We demonstrate how the diameter of the nanorods can control the effective permittivity beyond the effective medium theory. The results suggest that the effective permittivity gradually loses its accuracy as the diameter increases due to the optical nonlocality. Our experiment verifies that ultrathin nanorod arrays can resist the fluctuations caused by changes in incident angle. We also extract k-dependent effective permittivity of nanorods with larger diameters.

Nonreciprocal and Topological Plasmonics

Metals, semiconductors, metamaterials, and various two-dimensional materials with plasmonic dispersion exhibit numerous exotic physical effects in the presence of an external bias, for example an external static magnetic field or electric current. These physical phenomena range from Faraday rotation of light propagating in the bulk to strong confinement and directionality of guided modes on the surface and are a consequence of the breaking of Lorentz reciprocity in these systems. The recent introduction of relevant concepts of topological physics, translated from condensed-matter systems to photonics, has not only given a new perspective on some of these topics by relating certain bulk properties of plasmonic media to the surface phenomena, but has also led to the discovery of new regimes of truly unidirectional, backscattering-immune, surface-wave propagation. In this article, we briefly review the concepts of nonreciprocity and topology and describe their manifestation in plasmonic materials. Furthermore, we use these concepts to classify and discuss the different classes of guided surface modes existing on the interfaces of various plasmonic systems.

Control of the surface plasmon dispersion and Purcell effect at the metamaterial-dielectric interface

The use of metamaterial as a way to mitigate the negative effects of absorption in metals on the Purcell effect in metal-dielectric structures is investigated. A layered metal-dielectric structure is considered as an anisotropic medium in the long-wavelength limit. The dispersion of the surface plasmon appearing at the boundary between such a structure and a different dielectric material, as well as the position of the peak in the local density of states are studied for various combinations of materials and filling factors of the periodic structure. The calculated frequency dependence of the Purcell factor demonstrates an increase in peak value compared to the conventional plasmonic structure. The results obtained using effective media approach are compared to the results of numerical modelling.

3D Hybrid Trilayer Heterostructure: Tunable Au Nanorods and Optical Properties

Engineering plasmonic nanostructures from three dimensions (3D) is very attractive toward controllable and tunable nanophotonic components and devices. Herein, Au-based trilayer heterostructures composed of a dielectric spacer sandwiched by hybrid Au-TiN vertically aligned nanocomposite (VAN) nanoplasmonic claddings are demonstrated with a broad range of geometries and property tuning. Two types of spacer layers, that is, a pure dielectric BaTiO₃ layer and a hybrid plasmonic Au-BaTiO₃ VAN layer, contribute to the tuning of the Au nanorod dimension. Such geometrical variations of Au nanostructures originate from the surface energy and lattice strain tuned by the spacer layers. Optical measurements and numerical simulations suggest the change of the localized surface plasmon resonance which is strongly affected by the tailored Au nanorods as either separated or channeled. The uniaxial dielectric tensors suggest a tunable hyperbolic property affected by such a metal-insulator-metal trilayer stack. The complex 3D heterostructures offer additional tuning parameters and design flexibilities in hybrid plasmonic metamaterials toward potential applications in light harvesting, sensing, and nanophotonic devices.

Fabry-Pérot modes associated with hyperbolic-like dispersion in dielectric photonic crystals and demonstration of a bending angle sensor at microwave frequencies

The dispersion properties of metamaterials and photonic crystals (PhCs) lead to an intensive research in the development of cavity resonators for the confinement of electromagnetic (e-m) radiation. In this work, we investigate the formation of Fabry-Pérot (FP) modes associated with hyperbolic-like dispersion (HLD) regimes in two-dimensional dielectric PhCs. Conventionally, FP modes are formed using an optical etalon, in which electromagnetic (e-m) waves reflecting from a partially reflecting mirror separated by a distance can interfere constructively and form a resonating mode. The FP mode observed in dielectric PhCs is formed due to the interference of cylindrical wavefronts inside the PhC interface at HLD frequencies. The FP modes in PhCs are surface localized, in which maxima/minima of the electric field lies along the air-PhC interface as a standing wave pattern and decays in air medium. Projected bandstructure, Eigen Frequency Contours (EFC), phase and group index calculations are carried out to explain the formation of FP modes in PhCs under different coupling cases. By varying the PhC dimension, FP modes with different spatial profiles are witnessed and the role of source position in exciting specific mode is demonstrated. The observed FP modes in PhCs are compared with the FP mode in an ideal indefinite slab. Based on the FP resonance in PhCs, a sensing device capable of detecting a bending angle less than [Formula: see text] is demonstrated numerically. The FP modes in PhCs are scalable to other parts of e-m spectra so that the bending angle sensing can be extendable to terahertz and optical domains.

Creation of metasurface from vertically aligned carbon nanotubes as versatile platform for ultra-light THz components

Here a simple and reproducible method for obtaining terahertz metasurfaces formed from multiwall carbon nanotubes (MWCNTs) is presented. The metasurfaces were obtained from a vertically aligned array of MWCNTs using a laser engraving technique followed by polymer covering. The structures under study demonstrate frequency-selective reflection in terahertz range following the Huygens-Fresnel formalism. For a normal incidence of the electromagnetic wave, the model for numerical calculation of backscattering from the metasurfaces was proposed. Lightweight and compact MWCNT-based metasurfaces are capable to replace conventional pyramidal absorbers and could serve as a versatile platform for scalable cost-efficient production of ultra-light electromagnetic components for THz applications.

Achieving sub-wavelength imaging through a flat hyperlens in a modified anodic aluminum oxide template

Constrained by the diffraction limit, a lens can only resolve features larger than half of the incident wavelength owing to the decaying nature of evanescent waves. Several novel devices have been proposed, for example, superlenses and hyperlenses to break this limit. In this work, we present a flat hyperlens composed of silver nanowires embedded in a modified anodic aluminum oxide (AAO) template to demonstrate subwavelength imaging. Measurement conducted by the near-field scanning optical microscope at 633 nm suggests that our proposed flat hyperlens can indeed achieve sub-wavelength imaging with a resolution down to 0.34λ and 0.25λ along two orthogonal directions. Furthermore, to confirm the resolution limit of the flat hyperlens, numerical simulations were performed at the incident wavelengths of 633 and 365 nm, and the corresponding resolution were 0.19λ and 0.3λ, respectively, thus paving a route for sub-wavelength photolithography.

Tunable broadband hyperbolic light dispersion in metal diborides

The naturally hyperbolic materials that conquer the limitations of artificially structured hyperbolic metamaterials are promising candidates for the emerging devices based on light. However, the variety of natural hyperbolic materials and their hyperbolic frequency regime are presently limited. Here, on the basis of first-principles calculations, we demonstrated a family of natural hyperbolic materials, graphite-like metal diborides, with a broadband hyperbolic region from near-IR (∼2.5µm) to the ultraviolet regime (∼248 nm). The operating hyperbolic window and negative refraction can be effectively modulated by extracting electrons from the materials, offering a promising strategy for regulating the optical properties.

Multi-Segmented Nanowires: A High Tech Bright Future

In the last couple of decades, there has been a lot of progress in the synthesis methods of nano-structural materials, but still the field has a large number of puzzles to solve. Metal nanowires (NWs) and their alloys represent a sub category of the 1-D nano-materials and there is a large effort to study the microstructural, physical and chemical properties to use them for further industrial applications. Due to technical limitations of single component NWs, the hetero-structured materials gained attention recently. Among them, multi-segmented NWs are more diverse in applications, consisting of two or more segments that can perform multiple function at a time, which confer their unique properties. Recent advancement in characterization techniques has opened up new opportunities for understanding the physical properties of multi-segmented structures of 1-D nanomaterials. Since the multi-segmented NWs needs a reliable response from an external filed, numerous studies have been done on the synthesis of multi-segmented NWs to precisely control the physical properties of multi-segmented NWs. This paper highlights the electrochemical synthesis and physical properties of multi-segmented NWs, with a focus on the mechanical and magnetic properties by explaining the shape, microstructure, and composition of NWs.

Omnidirectional polarization beam splitter for white light

As a key element in optical systems, a broadband and omnidirectional polarization beam splitter has been long desired. Here, based on anisotropic metamaterials, a perfect polarizing beam splitter is theoretically designed for the extremely broad frequency and angle bands without energy loss. When an electromagnetic wave is incident on the beam splitter, the transverse magnetic-polarized component suffers total reflection, while the transverse electric-polarized component is completely transmitted within the incident angle range [-90°, 90°] for the white light. This study provides a new approach to design an efficient polarizing beam splitter and may promote the development and applications of anisotropic metamaterials.

Measuring Dirac Cones in a Subwavelength Metamaterial

The exciting discovery of bidimensional systems in condensed matter physics has triggered the search of their photonic analogues. In this Letter, we describe a general scheme to reproduce some of the systems ruled by a tight-binding Hamiltonian in a locally resonant metamaterial; by specifically controlling the structure and the composition it is possible to engineer the band structure at will. We numerically and experimentally demonstrate this assertion in the microwave domain by reproducing the band structure of graphene, the most famous example of those 2D systems, and by accurately extracting the Dirac cones. This is direct evidence that opting for a crystalline description of those subwavelength scaled systems, as opposed to the usual description in terms of effective parameters, makes them a really convenient tabletop platform to investigate the tantalizing challenges that solid-state physics offer.

Reciprocity and Babinet's principles applied to the enhancement of the electric and magnetic local density of states in integrated plasmonics on silicon photonics

Here, reciprocity and Babinet's principles were applied to the design of integrated plasmonic structures on silicon photonic waveguides. Numerical analyses and near-field optical microscopy observations show that one of the hybrid photonic-plasmonic structures exhibits high confinement and enhancement of the electric field, and, through Babinet's principle, the magnetic field of its complementary structure is confined and enhanced as well. Reciprocally, due to the modification of the electric and magnetic local density of states, enhanced emission of electric and magnetic dipoles by Purcell effect were obtained into specific silicon photonic modes. Such structures can be advantageously implemented for on-chip integrated single-photon sources.

Controlling Disorder by Electric-Field-Directed Reconfiguration of Nanowires To Tune Random Lasing

Top-down fabrication is commonly used to provide positioning control of optical structures; yet, it places stringent limitations on component materials, and oftentimes, dynamic reconfigurability is challenging to realize. Here, we present a reconfigurable nanoparticle platform that can integrate heterogeneous particle assembly of different shapes, sizes, and chemical compositions. We demonstrate dynamic control of disorder in this platform and use it to tune random laser emission characteristics for a suspension of titanium dioxide nanowires in a dye solution. Using an alternating current electric field, we control the nanowire orientation to dynamically engineer the collective scattering of the sample. Our theoretical model indicates that a change of up to 22% in scattering coefficient can be achieved for the experimentally determined nanowire length distribution upon alignment. Dependence of light confinement on anisotropic particle alignment provides a means to reversibly tune random laser characteristics; a nearly 20-fold increase in lasing intensity was observed with aligned particle orientation. We illustrate the generality of the approach by demonstrating enhanced lasing for aligned nanowires of other materials including gold, mixed gold/dielectric, and vanadium oxide.

Nano-Enabled Approaches to Chemical Imaging in Biosystems

Understanding and predicting how biosystems function require knowledge about the dynamic physicochemical environments with which they interact and alter by their presence. Yet, identifying specific components, tracking the dynamics of the system, and monitoring local environmental conditions without disrupting biosystem function present significant challenges for analytical measurements. Nanomaterials, by their very size and nature, can act as probes and interfaces to biosystems and offer solutions to some of these challenges. At the nanoscale, material properties emerge that can be exploited for localizing biomolecules and making chemical measurements at cellular and subcellular scales. Here, we review advances in chemical imaging enabled by nanoscale structures, in the use of nanoparticles as chemical and environmental probes, and in the development of micro- and nanoscale fluidic devices to define and manipulate local environments and facilitate chemical measurements of complex biosystems. Integration of these nano-enabled methods will lead to an unprecedented understanding of biosystem function.

A new quadrature annular resonator for 3 T MRI based on artificial-dielectrics

Dielectric resonators have previously been constructed for ultra-high frequency magnetic resonance imaging and microscopy. However, it is challenging to design these dielectric resonators at clinical field strengths due to their intrinsically large dimensions, especially when using materials with moderate permittivity. Here we propose and characterize a novel approach using artificial-dielectrics which reduces substantially the required outer diameter of the resonator. For a resonator designed to operate in a 3 Tesla scanner using water as the dielectric, a reduction in outer diameter of 37% was achieved. When used in an inductively-coupled wireless mode, the sensitivity of the artificial-dielectric resonator was measured to be slightly higher than that of a standard dielectric resonator operating in its degenerate circularly-polarized hybrid electromagnetic modes (HEM₁₁). This study demonstrates the first application of an artificial-dielectric approach to MR volume coil design.

Deep-Ultraviolet Hyperbolic Metacavity Laser

Given the high demand for miniaturized optoelectronic circuits, plasmonic devices with the capability of generating coherent radiation at deep subwavelength scales have attracted great interest for diverse applications such as nanoantennas, single photon sources, and nanosensors. However, the design of such lasing devices remains a challenging issue because of the long structure requirements for producing strong radiation feedback. Here, a plasmonic laser made by using a nanoscale hyperbolic metamaterial cube, called hyperbolic metacavity, on a multiple quantum-well (MQW), deep-ultraviolet emitter is presented. The specifically designed metacavity merges plasmon resonant modes within the cube and provides a unique resonant radiation feedback to the MQW. This unique plasmon field allows the dipoles of the MQW with various orientations into radiative emission, achieving enhancement of spontaneous emission rate by a factor of 33 and of quantum efficiency by a factor of 2.5, which is beneficial for coherent laser action. The hyperbolic metacavity laser shows a clear clamping of spontaneous emission above the threshold, which demonstrates a near complete radiation coupling of the MQW with the metacavity. This approach shown here can greatly simplify the requirements of plasmonic nanolaser with a long plasmonic structure, and the metacavity effect can be extended to many other material systems.

Electric-current-induced unidirectional propagation of surface plasmon-polaritons

Nonreciprocity and one-way propagation of optical signals are crucial for modern nanophotonic technology, and typically achieved using magneto-optical effects requiring large magnetic biases. Here we suggest a fundamentally novel approach to achieve unidirectional propagation of surface plasmon-polaritons (SPPs) at metal-dielectric interfaces. We employ a direct electric current in metals, which produces a Doppler frequency shift of SPPs due to the uniform drift of electrons. This tilts the SPP dispersion, enabling one-way propagation, as well as zero and negative group velocities. The results are demonstrated for planar interfaces and cylindrical nanowire waveguides.

Detailed correlations between SERS enhancement and plasmon resonances in subwavelength closely spaced Au nanorod arrays

Depending on the experimental conditions and plasmonic systems, the correlations between near-field surface enhanced Raman scattering (SERS) behaviors and far-field localized surface plasmon resonance (LSPR) responses have sometimes been accepted directly or argued or explored. In this work, we focus on the attractive subwavelength closely spaced metallic nanorod arrays and investigate in detail the complex relationship between their SERS behaviors and plasmon resonances. This is achieved utilizing a combination of array fabrication, conventional LSPR spectra, SERS measurements, electron microscopy and numerical modeling. Three key factors that may impact the correlations have been comprehensively analyzed: the intrinsic near-field to far-field red-shift is found to be rather small in the lattice; the surface roughness has actually little impact on the spectral alignment of the near- and far-field responses; the continuous dependence of individual SERS peak heights on the Stokes Raman shift has been visualized and further clarified. By 3D finite element method (FEM) plasmon mapping, the physical origin of the collective resonances in the lattice is verified directly to be the Fabry-Perot-like cavity mode. The strong near-field enhancement results from the coupling of surface plasmon polaritons (SPPs) propagating at the two sidewalls of neighbouring nanorods forming the resonant cavity. The physical principles demonstrated here benefit significantly the optimization of nano-optic devices based on closely spaced metallic nanorod arrays, as well as the fundamental understanding of the near- and far-field relationship.

Deep-subwavelength Decoupling for MIMO Antennas in Mobile Handsets with Singular Medium

Decreasing the mutual coupling between Multi-input Multi-output (MIMO) antenna elements in a mobile handset and achieving a high data rate is a challenging topic as the 5^th-generation (5G) communication age is coming. Conventional decoupling components for MIMO antennas have to be re-designed when the geometries or frequencies of antennas have any adjustment. In this paper, we report a novel metamaterial-based decoupling strategy for MIMO antennas in mobile handsets with wide applicability. The decoupling component is made of subwavelength metal/air layers, which can be treated as singular medium over a broad frequency band. The flexible applicable property of the decoupling strategy is verified with different antennas over different frequency bands with the same metamaterial decoupling element. Finally, 1/100-wavelength 10-dB isolation is demonstrated for a 24-element MIMO antenna in mobile handsets over the frequency band from 4.55 to 4.75 GHz.

Template-Free Growth of Well-Ordered Silver Nano Forest/Ceramic Metamaterial Films with Tunable Optical Responses

Currently, the limitations of conventional methods for fabricating metamaterials composed of well-aligned nanoscale inclusions either lack the necessary freedom to tune the structural geometry or are difficult for large-area synthesis. In this Communication, the authors propose a fabrication route to create well-ordered silver nano forest/ceramic composite single-layer or multi-layer vertically stacked structures, as a distinctive approach to make large-area nanoscale metamaterials. To take advantage of direct growth, the authors fabricate single-layer nanocomposite films with a well-defined sub-5 nm interwire gap and an average nanowire diameter of ≈3 nm. Further, artificially constructed multilayer metamaterial films are easily fabricated by vertical integration of different single-layer metamaterial films. Based upon the thermodynamics as well as thin film growth dynamics theory, the growth mechanism is presented to elucidate the formation of such structure. Intriguing steady and transient optical properties in these assemblies are demonstrated, owing to their nanoscale structural anisotropy. The studies suggest that the self-organized nanocomposites provide an extensible material platform to manipulate optical response in the region of sub-5 nm scale.

Single-mode tuning of the plasmon resonance in high-density pillar arrays

The Maxwell-Garnett (MG) effective medium model has a pure resonance controlled by volume fraction f, but is usually invalid at high density. I present special 2D structures that match quasistatic MG over the entire range 0  <  f  <  1, in several regular and semi-regular arrays, expanding the applicability of MG. Optimal contours depend on both lattice and fill-factor, transforming from circular at low f to nearly polygonal at high f. A key insight is the direct relationship between optimal surface polarization and surface position. Electrodynamic calculations underline the effect of constituent permittivity on spatial dispersion and required sizes for quasistatic response in various materials.

Metamaterial, plasmonic and nanophotonic devices

The field of metamaterials has opened landscapes of possibilities in basic science, and a paradigm shift in the way we think about and design emergent material properties. In many scenarios, metamaterial concepts have helped overcome long-held scientific challenges, such as the absence of optical magnetism and the limits imposed by diffraction in optical imaging. As the potential of metamaterials, as well as their limitations, become clearer, these advances in basic science have started to make an impact on several applications in different areas, with far-reaching implications for many scientific and engineering fields. At optical frequencies, the alliance of metamaterials with the fields of plasmonics and nanophotonics can further advance the possibility of controlling light propagation, radiation, localization and scattering in unprecedented ways. In this review article, we discuss the recent progress in the field of metamaterials, with particular focus on how fundamental advances in this field are enabling a new generation of metamaterial, plasmonic and nanophotonic devices. Relevant examples include optical nanocircuits and nanoantennas, invisibility cloaks, superscatterers and superabsorbers, metasurfaces for wavefront shaping and wave-based analog computing, as well as active, nonreciprocal and topological devices. Throughout the paper, we highlight the fundamental limitations and practical challenges associated with the realization of advanced functionalities, and we suggest potential directions to go beyond these limits. Over the next few years, as new scientific breakthroughs are translated into technological advances, the fields of metamaterials, plasmonics and nanophotonics are expected to have a broad impact on a variety of applications in areas of scientific, industrial and societal significance.

Optical properties of a one-dimensional photonic crystal containing a graphene-based hyperbolic metamaterial defect layer

The transmission properties of a one-dimensional defective photonic crystal have been investigated using the transfer matrix method. A layer of graphene-based hyperbolic metamaterial whose optical axis is tilted with respect to the interface is taken as a defect. It is shown that two kinds of the defect modes can be found in the band gaps of the structure for TM-polarized waves. One kind is created at the frequency range in which the principle elements of the effective permittivity tensor of the defect layer have the same signs. The frequency of this kind of defect mode is independent from the orientation of the optical axis of the defect layer. The other one is created at the hyperbolic dispersion frequency range. Such a defect mode appears due to the anisotropic behavior of the defect layer and its frequency strongly depends on the orientation of the optical axis. Unlike the conventional defect modes, the magnetic field of this defect mode is localized around the defect layer.

Strong light confinement and gradient force in a hexagonal boron nitride slot waveguide

In this Letter, we show that a hexagonal boron nitride (h-BN) slot waveguide can achieve strong field enhancement and light confinement in a slot region and a giant gradient force between h-BN slabs. Excellent agreement between simulations and results from an analytical model is observed. In a two-dimensional case, a field enhancement ratio near 60, a power confinement ratio of 80%, and a gradient force over -8.5  nN/μm×mW have been achieved, which are much higher than the slot waveguide based on artificial hyperbolic metamaterials. The gradient force and power confinement ratio in a three-dimensional slot waveguide structure are also studied. A gradient force of -1.2  nN/mW and a power confinement ratio of 50% have been obtained. The h-BN slot waveguide may have potential in particle manipulation.

Near-field coupling and resonant cavity modes in plasmonic nanorod metamaterials

Plasmonic resonant cavities are capable of confining light at the nanoscale, resulting in both enhanced local electromagnetic fields and lower mode volumes. However, conventional plasmonic resonant cavities possess large Ohmic losses at metal-dielectric interfaces. Plasmonic near-field coupling plays a key role in a design of photonic components based on the resonant cavities because of the possibility to reduce losses. Here, we study the plasmonic near-field coupling in the silver nanorod metamaterials treated as resonant nanostructured optical cavities. Reflectance measurements reveal the existence of multiple resonance modes of the nanorod metamaterials, which is consistent with our theoretical analysis. Furthermore, our numerical simulations show that the electric field at the longitudinal resonances forms standing waves in the nanocavities due to the near-field coupling between the adjacent nanorods, and a new hybrid mode emerges due to a coupling between nanorods and a gold-film substrate. We demonstrate that this coupling can be controlled by changing the gap between the silver nanorod array and gold substrate.

Controlling electromagnetic scattering with wire metamaterial resonators

Manipulation of radiation is required for enabling a span of electromagnetic applications. Since properties of antennas and scatterers are very sensitive to the surrounding environment, macroscopic artificially created materials are good candidates for shaping their characteristics. In particular, metamaterials enable controlling both dispersion and density of electromagnetic states, available for scattering from an object. As a result, properly designed electromagnetic environments could govern wave phenomena and tailor various characteristics. Here electromagnetic properties of scattering dipoles, situated inside a wire medium (metamaterial), are analyzed both numerically and experimentally. The effect of the metamaterial geometry, dipole arrangement inside the medium, and frequency of the incident radiation on the scattering phenomena is studied in detail. It is shown that the resonance of the dipole hybridizes with Fabry-Perot modes of the metamaterial, giving rise to a complete reshaping of electromagnetic properties. Regimes of controlled scattering suppression and super-scattering are experimentally observed. Numerical analysis is in agreement with the experiment, performed at the GHz spectral range. The reported approach to scattering control with metamaterials could be directly mapped into optical and infrared spectral ranges by employing scalability properties of Maxwell's equations.

Enhanced displacements in reflected beams at hyperbolic metamaterials

We examine the Goos-Hänchen (G-H) shift of a Gaussian beam reflected on a thin slab of Ag/TiO₂ hyperbolic multilayer metamaterial (HMM). The HMM is modeled using the effective medium theory which yields the anisotropic dielectric functions of the HMM. The G-H shifts can be very large on the surface of the HMM. It can be about 40 µm which are far bigger than the G-H shifts on the usual materials like metals and dielectrics. The enhancement is due to the excitation of the Brewster modes in HMM. Such Brewster modes in HMM have a well-defined frequency-dependent line shape. We relate the the half width at half maximum of the G-H shift to the imaginary part of the complex frequency of the Brewster mode. Moreover, we also present results for the Imbert-Fedorov shifts as well as the spin Hall effect of light on the surface of a thin HMM slab. We show that the spin Hall effect on the HMM slab is much more pronounced than that on the surface of metal. Thus a thin HMM slab can be used to enhance the lateral displacements, which can have many interesting applications for optical devices.

Topological and geometric measurements of force-chain structure

Developing quantitative methods for characterizing structural properties of force chains in densely packed granular media is an important step toward understanding or predicting large-scale physical properties of a packing. A promising framework in which to develop such methods is network science, which can be used to translate particle locations and force contacts into a graph in which particles are represented by nodes and forces between particles are represented by weighted edges. Recent work applying network-based community-detection techniques to extract force chains opens the door to developing statistics of force-chain structure, with the goal of identifying geometric and topological differences across packings, and providing a foundation on which to build predictions of bulk material properties from mesoscale network features. Here we discuss a trio of related but fundamentally distinct measurements of the mesoscale structure of force chains in two-dimensional (2D) packings, including a statistic derived using tools from algebraic topology, which together provide a tool set for the analysis of force chain architecture. We demonstrate the utility of this tool set by detecting variations in force-chain architecture with pressure. Collectively, these techniques can be generalized to 3D packings, and to the assessment of continuous deformations of packings under stress or strain.

Broadband giant-refractive-index material based on mesoscopic space-filling curves

The refractive index is the fundamental property of all optical materials and dictates Snell's law, propagation speed, wavelength, diffraction, energy density, absorption and emission of light in materials. Experimentally realized broadband refractive indices remain <40, even with intricately designed artificial media. Herein, we demonstrate a measured index >1,800 resulting from a mesoscopic crystal with a dielectric constant greater than three million. This gigantic enhancement effect originates from the space-filling curve concept from mathematics. The principle is inherently very broad band, the enhancement being nearly constant from zero up to the frequency of interest. This broadband giant-refractive-index medium promises not only enhanced resolution in imaging and raised fundamental absorption limits in solar energy devices, but also compact, power-efficient components for optical communication and increased performance in many other applications.

The refractive index of natural materials only covers a limited range. Here, Chang et al. use the principle of space-filling curves to construct a mesoscopic crystal with a refractive index greater than 1000 at GHz frequencies. The concept is inherently broadband and scalable.

Tailored loss discrimination in indefinite metamaterial-clad hollow-core fibers

We analyze the modal attenuation properties of silica hollow-core fibers with a gold-wire based indefinite metamaterial cladding at 10.6 µm. We find that by varying the metamaterial feature sizes and core diameter, the loss discrimination can be tailored such that either the HE₁₁, TE₀₁ or TM₀₁ mode has the lowest loss, which is particularly difficult to achieve for the radially polarized mode in commonly used hollow-core fibers. Furthermore, it is possible to tailor the HE₁₁ and TM₀₁ modes in the metamaterial-clad waveguide so that they possess attenuations lower than in hollow tubes composed of the individual constituent materials. We show that S-parameter retrieval techniques in combination with an anisotropic dispersion equation can be used to predict the loss discrimination properties of such fibers. These results pave the way for the design of metamaterial hollow-core fibers with novel guidance properties, in particular for applications demanding cylindrically polarized modes.

Tunable Lattice Coupling of Multipole Plasmon Modes and Near-Field Enhancement in Closely Spaced Gold Nanorod Arrays

Considering the nanogap and lattice effects, there is an attractive structure in plasmonics: closely spaced metallic nanoarrays. In this work, we demonstrate experimentally and theoretically the lattice coupling of multipole plasmon modes for closely spaced gold nanorod arrays, offering a new insight into the higher order cavity modes coupled with each other in the lattice. The resonances can be greatly tuned by changes in inter-rod gaps and nanorod heights while the influence of the nanorod diameter is relatively insignificant. Experimentally, pronounced suppressions of the reflectance are observed. Meanwhile, the near-field enhancement can be further enhanced, as demonstrated through surface enhanced Raman scattering (SERS). We then confirm the correlation between the near-field and far-field plasmonic responses, which is significantly important for maximizing the near-field enhancement at a specific excitation wavelength. This lattice coupling of multipole plasmon modes is of broad interest not only for SERS but also for other plasmonic applications, such as subwavelength imaging or metamaterials.

Enhancement of Magnetic Resonance Imaging with Metasurfaces

It is revealed that the unique properties of ultrathin metasurface resonators can improve magnetic resonance imaging dramatically. A metasurface formed when an array of metallic wires is placed inside a scanner under the studied object and a substantial enhancement of the radio-frequency magnetic field is achieved by means of subwavelength manipulation with the metasurface, also allowing improved image resolution.

Repulsion of polarised particles from anisotropic materials with a near-zero permittivity component

Reduction of adhesion and stiction is crucial for robust operation on nanomechanical and optofluidic devices as well as atom and molecule behaviour near surfaces. It can be achieved using electric charging, magnetic materials or light pressure and optical trapping. Here we show that a particle scattering or emitting in close proximity to an anisotropic substrate can experience a repulsive force if one of the diagonal components of the permittivity tensor is close to zero. We derive an analytic condition for the existence of such repulsive force depending on the optical properties of the substrate. We also demonstrate the effect using realistic anisotropic metamaterial implementations of a substrate. The anisotropic metamaterial approach using metal-dielectric and graphene-dielectric multilayers provides a tuneable spectral range and a very broad bandwidth of electromagnetic repulsion forces, in contrast to isotropic substrates.

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.

Tailored Height Gradients in Vertical Nanowire Arrays via Mechanical and Electronic Modulation of Metal-Assisted Chemical Etching

In current top-down nanofabrication methodologies the design freedom is generally constrained to the two lateral dimensions, and is only limited by the resolution of the employed nanolithographic technique. However, nanostructure height, which relies on certain mask-dependent material deposition or etching techniques, is usually uniform, and on-chip variation of this parameter is difficult and generally limited to very simple patterns. Herein, a novel nanofabrication methodology is presented, which enables the generation of high aspect-ratio nanostructure arrays with height gradients in arbitrary directions by a single and fast etching process. Based on metal-assisted chemical etching using a catalytic gold layer perforated with nanoholes, it is demonstrated how nanostructure arrays with directional height gradients can be accurately tailored by: (i) the control of the mass transport through the nanohole array, (ii) the mechanical properties of the perforated metal layer, and (iii) the conductive coupling to the surrounding gold film to accelerate the local electrochemical etching process. The proposed technique, enabling 20-fold on-chip variation of nanostructure height in a spatial range of a few micrometers, offers a new tool for the creation of novel types of nano-assemblies and metamaterials with interesting technological applications in fields such as nanophotonics, nanophononics, microfluidics or biomechanics.

Anisotropic metamaterial optical fibers

Internal physical structure can drastically modify the properties of waveguides: photonic crystal fibers are able to confine light inside a hollow air core by Bragg scattering from a periodic array of holes, while metamaterial loaded waveguides for microwaves can support propagation at frequencies well below cutoff. Anisotropic metamaterials assembled into cylindrically symmetric geometries constitute light-guiding structures that support new kinds of exotic modes. A microtube of anodized nanoporous alumina, with nanopores radially emanating from the inner wall to the outer surface, is a manifestation of such an anisotropic metamaterial optical fiber. The nanopores, when filled with a plasmonic metal such as silver or gold, greatly increase the electromagnetic anisotropy. The modal solutions in such anisotropic circular waveguides can be uncommon Bessel functions with imaginary orders.

New avenues for phase matching in nonlinear hyperbolic metamaterials

Nonlinear optical processes, which are of paramount importance in science and technology, involve the generation of new frequencies. This requires phase matching to avoid that light generated at different positions interferes destructively. Of the two original approaches to achieve this, one relies on birefringence in optical crystals, and is therefore limited by the dispersion of naturally occurring materials, whereas the other, quasi-phase-matching, requires direct modulation of material properties, which is not universally possible. To overcome these limitations, we propose to exploit the unique dispersion afforded by hyperbolic metamaterials, where the refractive index can be arbitrarily large. We systematically analyse the ensuing opportunities and demonstrate that hyperbolic phase matching can be achieved with a wide range of material parameters, offering access to the use of nonlinear media for which phase matching cannot be achieved by other means. With the rapid development in the fabrication of hyperbolic metamaterials, our approach is destined to bring significant advantages over conventional techniques for the phase matching of a variety of nonlinear processes.