Showing papers on "Metamaterial published in 2018"

Printing ferromagnetic domains for untethered fast-transforming soft materials

Abstract: Soft materials capable of transforming between three-dimensional (3D) shapes in response to stimuli such as light, heat, solvent, electric and magnetic fields have applications in diverse areas such as flexible electronics1,2, soft robotics3,4 and biomedicine5–7. In particular, magnetic fields offer a safe and effective manipulation method for biomedical applications, which typically require remote actuation in enclosed and confined spaces8–10. With advances in magnetic field control 11 , magnetically responsive soft materials have also evolved from embedding discrete magnets 12 or incorporating magnetic particles 13 into soft compounds to generating nonuniform magnetization profiles in polymeric sheets14,15. Here we report 3D printing of programmed ferromagnetic domains in soft materials that enable fast transformations between complex 3D shapes via magnetic actuation. Our approach is based on direct ink writing 16 of an elastomer composite containing ferromagnetic microparticles. By applying a magnetic field to the dispensing nozzle while printing 17 , we reorient particles along the applied field to impart patterned magnetic polarity to printed filaments. This method allows us to program ferromagnetic domains in complex 3D-printed soft materials, enabling a set of previously inaccessible modes of transformation, such as remotely controlled auxetic behaviours of mechanical metamaterials with negative Poisson’s ratios. The actuation speed and power density of our printed soft materials with programmed ferromagnetic domains are orders of magnitude greater than existing 3D-printed active materials. We further demonstrate diverse functions derived from complex shape changes, including reconfigurable soft electronics, a mechanical metamaterial that can jump and a soft robot that crawls, rolls, catches fast-moving objects and transports a pharmaceutical dose.

Observation of a phononic quadrupole topological insulator

Abstract: The modern theory of charge polarization in solids is based on a generalization of Berry’s phase. The possibility of the quantization of this phase arising from parallel transport in momentum space is essential to our understanding of systems with topological band structures. Although based on the concept of charge polarization, this same theory can also be used to characterize the Bloch bands of neutral bosonic systems such as photonic or phononic crystals. The theory of this quantized polarization has recently been extended from the dipole moment to higher multipole moments. In particular, a two-dimensional quantized quadrupole insulator is predicted to have gapped yet topological one-dimensional edge modes, which stabilize zero-dimensional in-gap corner states. However, such a state of matter has not previously been observed experimentally. Here we report measurements of a phononic quadrupole topological insulator. We experimentally characterize the bulk, edge and corner physics of a mechanical metamaterial (a material with tailored mechanical properties) and find the predicted gapped edge and in-gap corner states. We corroborate our findings by comparing the mechanical properties of a topologically non-trivial system to samples in other phases that are predicted by the quadrupole theory. These topological corner states are an important stepping stone to the experimental realization of topologically protected wave guides in higher dimensions, and thereby open up a new path for the design of metamaterials.

Deep-Learning-Enabled On-Demand Design of Chiral Metamaterials.

Abstract: Deep-learning framework has significantly impelled the development of modern machine learning technology by continuously pushing the limit of traditional recognition and processing of images, speech, and videos. In the meantime, it starts to penetrate other disciplines, such as biology, genetics, materials science, and physics. Here, we report a deep-learning-based model, comprising two bidirectional neural networks assembled by a partial stacking strategy, to automatically design and optimize three-dimensional chiral metamaterials with strong chiroptical responses at predesignated wavelengths. The model can help to discover the intricate, nonintuitive relationship between a metamaterial structure and its optical responses from a number of training examples, which circumvents the time-consuming, case-by-case numerical simulations in conventional metamaterial designs. This approach not only realizes the forward prediction of optical performance much more accurately and efficiently but also enables one to i...

Subwavelength integrated photonics.

Abstract: In the late nineteenth century, Heinrich Hertz demonstrated that the electromagnetic properties of materials are intimately related to their structure at the subwavelength scale by using wire grids with centimetre spacing to manipulate metre-long radio waves. More recently, the availability of nanometre-scale fabrication techniques has inspired scientists to investigate subwavelength-structured metamaterials with engineered optical properties at much shorter wavelengths, in the infrared and visible regions of the spectrum. Here we review how optical metamaterials are expected to enhance the performance of the next generation of integrated photonic devices, and explore some of the challenges encountered in the transition from concept demonstration to viable technology.

MEMS-tunable dielectric metasurface lens.

Abstract: Varifocal lenses, conventionally implemented by changing the axial distance between multiple optical elements, have a wide range of applications in imaging and optical beam scanning. The use of conventional bulky refractive elements makes these varifocal lenses large, slow, and limits their tunability. Metasurfaces, a new category of lithographically defined diffractive devices, enable thin and lightweight optical elements with precisely engineered phase profiles. Here we demonstrate tunable metasurface doublets, based on microelectromechanical systems (MEMS), with more than 60 diopters (about 4%) change in the optical power upon a 1-μm movement of one metasurface, and a scanning frequency that can potentially reach a few kHz. They can also be integrated with a third metasurface to make compact microscopes (~1 mm thick) with a large corrected field of view (~500 μm or 40 degrees) and fast axial scanning for 3D imaging. This paves the way towards MEMS-integrated metasurfaces as a platform for tunable and reconfigurable optics.

Broadband achromatic dielectric metalenses

Abstract: Metasurfaces offer a unique platform to precisely control optical wavefronts and enable the realization of flat lenses, or metalenses, which have the potential to substantially reduce the size and complexity of imaging systems and to realize new imaging modalities. However, it is a major challenge to create achromatic metalenses that produce a single focal length over a broad wavelength range because of the difficulty in simultaneously engineering phase profiles at distinct wavelengths on a single metasurface. For practical applications, there is a further challenge to create broadband achromatic metalenses that work in the transmission mode for incident light waves with any arbitrary polarization state. We developed a design methodology and created libraries of meta-units—building blocks of metasurfaces—with complex cross-sectional geometries to provide diverse phase dispersions (phase as a function of wavelength), which is crucial for creating broadband achromatic metalenses. We elucidated the fundamental limitations of achromatic metalens performance by deriving mathematical equations that govern the tradeoffs between phase dispersion and achievable lens parameters, including the lens diameter, numerical aperture (NA), and bandwidth of achromatic operation. We experimentally demonstrated several dielectric achromatic metalenses reaching the fundamental limitations. These metalenses work in the transmission mode with polarization-independent focusing efficiencies up to 50% and continuously provide a near-constant focal length over λ = 1200–1650 nm. These unprecedented properties represent a major advance compared to the state of the art and a major step toward practical implementations of metalenses. Small, high-performance imaging systems could be built using flat lenses made from specially arranged nanoscale pillars. Traditional lenses rely on the curvature and thickness of glass to focus light, but metalenses, which can be smaller, thinner, and more flexible, have surfaces comprised of thousands of nanoscale pillars whose geometries are carefully designed to control optical phase. However, problems still arise in maintaining the same focal length across a wide wavelength range, leading to image blurring. Now, Nanfang Yu at Columbia University in New York, USA, and co-workers have designed a library of meta-units—the nano-pillars used to create metalenses—with several different cross-sectional geometries. They have combined these meta-units in various patterns to build broadband metalenses, which exhibit consistent focal length across a broad near-infrared wavelength range, significantly improving the final image quality. Furthermore, such metalenses work in the transmission mode and can focus light of any arbitrary polarization state.

Metasurfaces and their applications

Abstract: Metasurfaces are a topic of significant research and are used in various applications due to their unique ability to manipulate electromagnetic waves in microwave and optical frequencies. These artificial sheet materials, which are usually composed of metallic patches or dielectric etchings in planar or multi-layer configurations with subwavelength thickness, have the advantages of light weight, ease of fabrication, and ability to control wave propagation both on the surface and in the surrounding free space. Recent progress in the field has been classified by application and reviewed in this article. Starting with the development of frequency-selective surfaces and metamaterials, the unique capabilities of different kinds of metasurfaces have been highlighted. Surface impedance can be varied and manipulated by patterning the metasurface unit cells, which has broad applications in surface wave absorbers and surface waveguides. They also enable beam shaping in both transmission and reflection. Another important application is to radiate in a leaky wave mode as an antenna. Other applications of metasurfaces include cloaking, polarizers, and modulators. The controllable surface refractive index provided by metasurfaces can also be applied to lenses. When active and non-linear components are added to traditional metasurfaces, exceptional tunability and switching ability are enabled. Finally, metasurfaces allow applications in new forms of imaging.

Quantum entanglement of the spin and orbital angular momentum of photons using metamaterials

Abstract: Metamaterials constructed from deep subwavelength building blocks have been used to demonstrate phenomena ranging from negative refractive index and e-near-zero to cloaking, emulations of general relativity, and superresolution imaging More recently, metamaterials have been suggested as a new platform for quantum optics We present the use of a dielectric metasurface to generate entanglement between the spin and orbital angular momentum of photons We demonstrate the generation of the four Bell states on a single photon by using the geometric phase that arises from the photonic spin-orbit interaction and subsequently show nonlocal correlations between two photons that interacted with the metasurface Our results show that metamaterials are suitable for the generation and manipulation of entangled photon states, introducing the area of quantum optics metamaterials

Tunable and reconfigurable metasurfaces and metadevices

Abstract: Metasurfaces, two-dimensional equivalents of metamaterials, are engineered surfaces consisting of deep subwavelength features that have full control of the electromagnetic waves. Metasurfaces are not only being applied to the current devices throughout the electromagnetic spectrum from microwave to optics but also inspiring many new thrilling applications such as programmable on-demand optics and photonics in future. In order to overcome the limits imposed by passive metasurfaces, extensive researches have been put on utilizing different materials and mechanisms to design active metasurfaces. In this paper, we review the recent progress in tunable and reconfigurable metasurfaces and metadevices through the different active materials deployed together with the different control mechanisms including electrical, thermal, optical, mechanical, and magnetic, and provide the perspective for their future development for applications.

Origami-inspired, on-demand deployable and collapsible mechanical metamaterials with tunable stiffness

Abstract: Origami has been employed to build deployable mechanical metamaterials through folding and unfolding along the crease lines. Deployable metamaterials are usually flexible, particularly along their deploying and collapsing directions, which unfortunately in many cases leads to an unstable deployed state, i.e., small perturbations may collapse the structure along the same deployment path. Here we create an origami-inspired mechanical metamaterial with on-demand deployability and selective collapsibility through energy analysis. This metamaterial has autonomous deployability from the collapsed state and can be selectively collapsed along two different paths, embodying low stiffness for one path and substantially high stiffness for another path. The created mechanical metamaterial yields load-bearing capability in the deployed direction while possessing great deployability and collapsibility. The principle in this work can be utilized to design and create versatile origami-inspired mechanical metamaterials that can find many applications.

3D Plate-Lattices: An Emerging Class of Low-Density Metamaterial Exhibiting Optimal Isotropic Stiffness.

Abstract: In lightweight engineering, there is a constant quest for low-density materials featuring high mass-specific stiffness and strength. Additively-manufactured metamaterials are particularly promising candidates as the controlled introduction of porosity allows for tailoring their density while activating strengthening size-effects at the nano- and microstructural level. Here, plate-lattices are conceived by placing plates along the closest-packed planes of crystal structures. Based on theoretical analysis, a general design map is developed for elastically isotropic plate-lattices of cubic symmetry. In addition to validating the design map, detailed computational analysis reveals that there even exist plate-lattice compositions that provide nearly isotropic yield strength together with elastic isotropy. The most striking feature of plate-lattices is that their stiffness and yield strength are within a few percent of the theoretical limits for isotropic porous solids. This implies that the stiffness of isotropic plate-lattices is up to three times higher than that of the stiffest truss-lattices of equal mass. This stiffness advantage is also confirmed by experiments on truss- and plate-lattice specimens fabricated through direct laser writing. Due to their porous internal structure, the potential impact of the new metamaterials reported here goes beyond lightweight engineering, including applications for heat-exchange, thermal insulation, acoustics, and biomedical engineering.

Ultra-broadband absorber from visible to near-infrared using plasmonic metamaterial.

Abstract: We propose a design of an ultra-broadband absorber based on a thin metamaterial nanostructure composed of a periodic array of titanium-silica (Ti-SiO2) cubes and an aluminum (Al) bottom film. The proposed structure can achieve nearly perfect absorption with an average absorbance of 97% spanning a broad range from visible to near-infrared (i.e., from 354 nm to 1066 nm), showing a 90% absorption bandwidth over 712 nm, and the peak absorption is up to 99.8%. The excitation of superior surface plasmon resonance combined with the resonance induced by the metal-insulator-metal Fabry-Perot (FP) cavity leads to this broadband perfect absorption. The polarization and angle insensitivity is demonstrated by analyzing the absorption performance with oblique incidences for both TE- and TM-polarized waves. In addition, we discuss the impact of various metal materials and geometry structure on absorption performance in detail. The proposed broadband metamaterial absorber shows a promising prospect in applications such as solar cell, infrared detection, and imaging. Moreover, the use of a thin titanium cap and an aluminum film instead of noble metals has the potential to reduce production cost in applications.

Broadband tunable terahertz absorber based on vanadium dioxide metamaterials.

Abstract: An active absorption device is proposed based on vanadium dioxide metamaterials. By controlling the conductivity of vanadium dioxide, resonant absorbers are designed to work at wide range of terahertz frequencies. Numerical results show that a broadband terahertz absorber with nearly 100% absorptance can be achieved, and its normalized bandwidth of 90% absorptance is 60% under normal incidence for both transverse-electric and transverse-magnetic polarizations when the conductivity of vanadium dioxide is equal to 2000 Ω−1cm−1. Absorptance at peak frequencies can be continuously tuned from 30% to 100% by changing the conductivity from 10 Ω−1cm−1 to 2000 Ω−1cm−1. Absorptance spectra analysis shows a clear independence of polarization and incident angle. The presented results may have tunable spectral applications in sensor, detector, and thermophotovoltaic device working at terahertz frequency bands.

Highly Broadband Absorber Using Plasmonic Titanium Carbide (MXene)

Abstract: Control of light transmission and reflection through nanostructured materials has led to demonstration of metamaterial absorbers that have augmented the performance of energy harvesting applications of several optoelectronic and nanophotonic systems. Here, for the first time, a broadband plasmonic metamaterial absorber is fabricated using two-dimensional titanium carbide (Ti3C2Tx) MXene. Arrays of nanodisks made of Ti3C2Tx exhibit strong localized surface plasmon resonances at near-infrared frequencies. By exploiting the scattering enhancement at the resonances and the optical losses inherent to Ti3C2Tx MXene, high-efficiency absorption (∼90%) for a wide wavelength window of incident illumination (∼1.55 μm) has been achieved.

Giant intrinsic chiro-optical activity in planar dielectric nanostructures

Abstract: The strong optical chirality arising from certain synthetic metamaterials has important and widespread applications in polarization optics, stereochemistry and spintronics. However, these intrinsically chiral metamaterials are restricted to a complicated three-dimensional (3D) geometry, which leads to significant fabrication challenges, particularly at visible wavelengths. Their planar two-dimensional (2D) counterparts are limited by symmetry considerations to operation at oblique angles (extrinsic chirality) and possess significantly weaker chiro-optical responses close to normal incidence. Here, we address the challenge of realizing strong intrinsic chirality from thin, planar dielectric nanostructures. Most notably, we experimentally achieve near-unity circular dichroism with ~90% of the light with the chosen helicity being transmitted at a wavelength of 540 nm. This is the highest value demonstrated to date for any geometry in the visible spectrum. We interpret this result within the charge-current multipole expansion framework and show that the excitation of higher-order multipoles is responsible for the giant circular dichroism. These experimental results enable the realization of high-performance miniaturized chiro-optical components in a scalable manner at optical frequencies. Giant optical chirality has been realized at visible wavelengths in planar engineered surfaces. Strong optical chirality is desired for various applications. Metamaterials are promising for realizing this, but three-dimensional ones are difficult to make, while planar ones impart low chirality. Now, Alexander Zhu of Harvard University and co-workers have made metasurfaces consisting of an array of miniature gammadion-cross-shaped dielectric structures that gave a circular dichroism in transmission of 80% for green light, while numerical simulations suggested that 95% should be possible. Furthermore, 600-namometer-thick surfaces can provide a circular birefringence as large 60 degrees of polarization rotation, equivalent to 100,000 degrees per millimeter thickness — much larger than that measured in other media, whether natural or engineered. The metasurfaces could lead to high-performance flat devices for controlling the polarization of light beams in applications such as telecommunications.

Room-temperature nine-µm-wavelength photodetectors and GHz-frequency heterodyne receivers

Abstract: Quantum-well photodetectors fabricated from photonic metamaterials show enhanced room-temperature sensitivity to long-wavelength infrared radiation and produce gigahertz-frequency heterodyne signals when pumped with quantum cascade lasers. Current technologies for detecting infrared radiation in the long-wavelength regime (8–12 micrometres) do not lend themselves to low-cost, compact implementations—usually because of the need for low-temperature operation—which limits their breadth of applicability. Daniele Palaferri et al. show how photonic metamaterial ideas can be combined with quantum-well infrared photodetectors to enhance room-temperature sensitivity to the level that potentially opens up a range of applications, from thermal imaging and environmental remote sensing to laser-based free-space communication. Room-temperature operation is essential for any optoelectronics technology that aims to provide low-cost, compact systems for widespread applications. A recent technological advance in this direction is bolometric detection for thermal imaging1, which has achieved relatively high sensitivity and video rates (about 60 hertz) at room temperature. However, owing to thermally induced dark current, room-temperature operation is still a great challenge for semiconductor photodetectors targeting the wavelength band between 8 and 12 micrometres2, and all relevant applications, such as imaging, environmental remote sensing and laser-based free-space communication3,4,5, have been realized at low temperatures. For these devices, high sensitivity and high speed have never been compatible with high-temperature operation6,7. Here we show that a long-wavelength (nine micrometres) infrared quantum-well photodetector8 fabricated from a metamaterial made of sub-wavelength metallic resonators9,10,11,12 exhibits strongly enhanced performance with respect to the state of the art up to room temperature. This occurs because the photonic collection area of each resonator is much larger than its electrical area, thus substantially reducing the dark current of the device13. Furthermore, we show that our photonic architecture overcomes intrinsic limitations of the material, such as the drop of the electronic drift velocity with temperature14,15, which constrains conventional geometries at cryogenic operation6. Finally, the reduced physical area of the device and its increased responsivity allow us to take advantage of the intrinsic high-frequency response of the quantum detector7 at room temperature. By mixing the frequencies of two quantum-cascade lasers16 on the detector, which acts as a heterodyne receiver, we have measured a high-frequency signal, above four gigahertz (GHz). Therefore, these wide-band uncooled detectors could benefit technologies such as high-speed (gigabits per second) multichannel coherent data transfer17 and high-precision molecular spectroscopy18.

Optical Anapole Metamaterial

Abstract: The toroidal dipole is a localized electromagnetic excitation independent from the familiar magnetic and electric dipoles. It corresponds to currents flowing along minor loops of a torus. Interference of radiating induced toroidal and electric dipoles leads to anapole, a nonradiating charge-current configuration. Interactions of induced toroidal dipoles with electromagnetic waves have recently been observed in artificial media at microwave, terahertz, and optical frequencies. Here, we demonstrate a quasi-planar plasmonic metamaterial, a combination of dumbbell aperture and vertical split-ring resonator, that exhibits transverse toroidal moment and resonant anapole behavior in the optical part of the spectrum upon excitation with a normally incident electromagnetic wave. Our results prove experimentally that toroidal modes and anapole modes can provide distinct and physically significant contributions to the absorption and dispersion of slabs of matter in the optical part of the spectrum in conventional tr...

An all-dielectric metasurface as a broadband optical frequency mixer

Abstract: A frequency mixer is a nonlinear device that combines electromagnetic waves to create waves at new frequencies. Mixers are ubiquitous components in modern radio-frequency technology and microwave signal processing. The development of versatile frequency mixers for optical frequencies remains challenging: such devices generally rely on weak nonlinear optical processes and, thus, must satisfy phase-matching conditions. Here we utilize a GaAs-based dielectric metasurface to demonstrate an optical frequency mixer that concurrently generates eleven new frequencies spanning the ultraviolet to near-infrared. The even and odd order nonlinearities of GaAs enable our observation of second-harmonic, third-harmonic, and fourth-harmonic generation, sum-frequency generation, two-photon absorption-induced photoluminescence, four-wave mixing and six-wave mixing. The simultaneous occurrence of these seven nonlinear processes is assisted by the combined effects of strong intrinsic material nonlinearities, enhanced electromagnetic fields, and relaxed phase-matching requirements. Such ultracompact optical mixers may enable a plethora of applications in biology, chemistry, sensing, communications, and quantum optics.

All-dielectric meta-optics and non-linear nanophotonics

Abstract: Most optical metamaterials fabricated and studied to date employ metallic components resulting in significant losses, heat and overall low efficiencies. A new era of metamaterial physics is associated with all-dielectric meta-optics, which employs electric and magnetic Mie resonances of subwavelength particles with high refractive index for an optically induced magnetic response, thus underpinning a new approach to design and fabricate functional and practical metadevices. Here we review the recent developments in meta-optics and subwavelength dielectric photonics and demonstrate that the Mie resonances can play a crucial role in the realization of the unique functionalities of meta-atoms, also driving novel effects in the fields of metamaterials and nanophotonics. We discuss the recent research frontiers in all-dielectric meta-optics and uncover how Mie resonances can be employed for a flexible control of light with full phase and amplitude engineering, including unidirectional metadevices, highly transparent metasurfaces, non-linear nanophotonics and topological photonics.

Programmable Self-Locking Origami Mechanical Metamaterials.

Abstract: Developing mechanical metamaterials with programmable properties is an emerging topic receiving wide attention. While the programmability mainly originates from structural multistability in previously designed metamaterials, here it is shown that nonflat-foldable origami provides a new platform to achieve programmability via its intrinsic self-locking and reconfiguration capabilities. Working with the single-collinear degree-4 vertex origami tessellation, it is found that each unit cell can self-lock at a nonflat configuration and, therefore, possesses wide design space to program its foldability and relative density. Experiments and numerical analyses are combined to demonstrate that by switching the deformation modes of the constituent cell from prelocking folding to postlocking pressing, its stiffness experiences a sudden jump, implying a limiting-stopper effect. Such a stiffness jump is generalized to a multisegment piecewise stiffness profile in a multilayer model. Furthermore, it is revealed that via strategically switching the constituent cells' deformation modes through passive or active means, the n-layer metamaterial's stiffness is controllable among 2n target stiffness values. Additionally, the piecewise stiffness can also trigger bistable responses dynamically under harmonic excitations, highlighting the metamaterial's rich dynamic performance. These unique characteristics of self-locking origami present new paths for creating programmable mechanical metamaterials with in situ controllable mechanical properties.

Reconfigurable MEMS Fano metasurfaces with multiple-input-output states for logic operations at terahertz frequencies.

Abstract: A broad range of dynamic metasurfaces has been developed for manipulating the intensity, phase and wavefront of electromagnetic radiation from microwaves to optical frequencies. However, most of these metasurfaces operate in single-input–output state. Here, we experimentally demonstrate a reconfigurable MEMS Fano resonant metasurface possessing multiple-input–output (MIO) states that performs logic operations with two independently controlled electrical inputs and an optical readout at terahertz frequencies. The far-field behaviour of Fano resonance exhibits XOR and XNOR operations, while the near-field resonant confinement enables the NAND operation. The MIO configuration resembling hysteresis-type closed-loop behaviour is realized through inducing electromechanically tuneable out-of-plane anisotropy in the near-field coupling of constituent resonator structures. The XOR metamaterial gate possesses potential applications in cryptographically secured terahertz wireless communication networks. Furthermore, the MIO features could lay the foundation for the realization of programmable and randomly accessible metamaterials with enhanced electro-optical performance across terahertz, infrared and optical frequencies. Here, the authors demonstrate excitation and active tuning of sharp Fano resonances in a MEMS reconfigurable metasurface possessing multiple-input-output states. They realize XOR, XNOR, NOT and NAND logic gate operations by using two independently controllable electrical inputs and an optical readout using terahertz beam.

Nonvolatile Reconfigurable Phase-Change Metadevices for Beam Steering in the Near Infrared

Abstract: C.D.W. acknowledges funding via the US Naval Research LaboratoriesONRG programme (#N62909-16-1-2174) and the EPSRC ChAMP and WAFT grants (EP/M015130/1 and EP/M015173/1). C.R.d.G. acknowledges funding via the EPSRC CDT in Metamaterials (EP/L015331/1). M.L.G. acknowledges funding via the EPSRC grant EP/M009033/1.

Electrically tunable slow light using graphene metamaterials

Abstract: Metamaterials with classical analogues of electromagnetically induced transparency open new avenues in photonics for realizing smaller, more efficient slow light devices without quantum approaches. However, most of the metamaterial-based slow light devices are passive, which limits their practical applications. Here, by combining diatomic metamaterials with a gated single-layer graphene, we demonstrate that the group delay of terahertz light can be dynamically controlled under a small gate voltage. Using a two coupled harmonic oscillators model, we show that this active control of group delay is made possible by an effective control of the dissipative loss of the radiative dark resonator by varying the graphene’s optical conductivity. Our work may provide opportunities in the design of various applications such as compact slow light devices and ultrasensitive sensors and switches.

Experimental realization of on-chip topological nanoelectromechanical metamaterials

Abstract: Guiding waves through a stable physical channel is essential for reliable information transport. However, energy transport in high-frequency mechanical systems, such as in signal-processing applications, is particularly sensitive to defects and sharp turns because of back-scattering and losses. Topological phenomena in condensed matter systems have shown immunity to defects and unidirectional energy propagation. Topological mechanical metamaterials translate these properties into classical systems for efficient phononic energy transport. Acoustic and mechanical topological metamaterials have so far been realized only in large-scale systems, such as arrays of pendulums, gyroscopic lattices, structured plates and arrays of rods, cans and other structures acting as acoustic scatterers9. To fulfil their potential in device applications, mechanical topological systems need to be scaled to the on-chip level for high-frequency transport. Here we report the experimental realization of topological nanoelectromechanical metamaterials, consisting of two-dimensional arrays of free-standing silicon nitride nanomembranes that operate at high frequencies (10–20 megahertz). We experimentally demonstrate the presence of edge states, and characterize their localization and Dirac-cone-like frequency dispersion. Our topological waveguides are also robust to waveguide distortions and pseudospin-dependent transport. The on-chip integrated acoustic components realized here could be used in unidirectional waveguides and compact delay lines for high-frequency signal-processing applications.

Strongly Enhanced Sensitivity in Planar Microwave Sensors Based on Metamaterial Coupling

Abstract: Limited sensitivity and sensing range are arguably the greatest challenges in microwave sensor design. Recent attempts to improve these properties have relied on metamaterial (MTM)-inspired open-loop resonators coupled to transmission lines (TLs). Although the strongly resonant properties of the resonator sensitively reflect small changes in the environment through a shift in its resonance frequency, the resulting sensitivities remain ultimately limited by the level of coupling between the resonator and the TL. This paper introduces a novel solution to this problem that employs negative-refractive-index TL MTMs to substantially improve this coupling so as to fully exploit its resonant properties. A MTM-infused planar microwave sensor is designed for operation at 2.5 GHz, and is shown to exhibit a significant improvement in sensitivity and linearity. A rigorous signal-flow analysis of the sensor is proposed and shown to provide a fully analytical description of all salient features of both the conventional and MTM-infused sensors. Full-wave simulations confirm the analytical predictions, and all data demonstrate excellent agreement with measurements of a fabricated prototype. The proposed device is shown to be especially useful in the characterization of commonly available high-permittivity liquids as well as in sensitively distinguishing concentrations of ethanol/methanol in water.

An overview of metamaterials and their achievements in wireless power transfer

Abstract: Metamaterials have been deployed for a wide range of fields including invisible cloak, superlens, electromagnetic wave absorption and magnetic resonance imaging, owing to their peculiar electromagnetic properties. However, few investigations on metamaterials were focused on wireless power transfer (WPT). WPT is the transmission of electrical energy from a power source to an electrical load without conductors like wires or cables. Metamaterials can enhance the transfer efficiency and enlarge the transfer distance due to their ability of focusing magnetic flux, which opens up a novel approach to promoting the development and application of WPT. This review paper aims to provide an overview of the fabrications, exotic properties, and their applications especially in the WPT field. Meanwhile, the perspective and future challenges of metamaterials and WPT are proposed.

Subwavelength-Grating Metamaterial Structures for Silicon Photonic Devices

Abstract: Segmenting silicon waveguides at the subwavelength scale produce an equivalent homogenous material. The geometry of the waveguide segments provides precise control over modal confinement, effective index, dispersion and birefringence, thereby opening up new approaches to design devices with unprecedented performance. Indeed, with ever-improving lithographic technologies offering sub-100-nm patterning resolution in the silicon photonics platform, many practical devices based on subwavelength structures have been demonstrated in recent years. Subwavelength engineering has thus become an integral design tool in silicon photonics, and both fundamental understanding and novel applications are advancing rapidly. Here, we provide a comprehensive review of the state of the art in this field. We first cover the basics of subwavelength structures, and discuss substrate leakage, fabrication jitter, reduced backscatter, and engineering of material anisotropy. We then review recent applications including broadband waveguide couplers, high-sensitivity evanescent field sensors, low-loss devices for mid-infrared photonics, polarization management structures, spectral filters, and highly efficient fiber-to-chip couplers. We finally discuss the future prospects for subwavelength silicon structures and their impact on advanced device design.