Showing papers on "Metamaterial published in 2013"

Planar Photonics with Metasurfaces

Abstract: Metamaterials, or engineered materials with rationally designed, subwavelength-scale building blocks, allow us to control the behavior of physical fields in optical, microwave, radio, acoustic, heat transfer, and other applications with flexibility and performance that are unattainable with naturally available materials. In turn, metasurfaces-planar, ultrathin metamaterials-extend these capabilities even further. Optical metasurfaces offer the fascinating possibility of controlling light with surface-confined, flat components. In the planar photonics concept, it is the reduced dimensionality of the optical metasurfaces that enables new physics and, therefore, leads to functionalities and applications that are distinctly different from those achievable with bulk, multilayer metamaterials. Here, we review the progress in developing optical metasurfaces that has occurred over the past few years with an eye toward the promising future directions in the field.

Alternative Plasmonic Materials: Beyond Gold and Silver

Abstract: Materials research plays a vital role in transforming breakthrough scientific ideas into next-generation technology. Similar to the way silicon revolutionized the microelectronics industry, the proper materials can greatly impact the field of plasmonics and metamaterials. Currently, research in plasmonics and metamaterials lacks good material building blocks in order to realize useful devices. Such devices suffer from many drawbacks arising from the undesirable properties of their material building blocks, especially metals. There are many materials, other than conventional metallic components such as gold and silver, that exhibit metallic properties and provide advantages in device performance, design flexibility, fabrication, integration, and tunability. This review explores different material classes for plasmonic and metamaterial applications, such as conventional semiconductors, transparent conducting oxides, perovskite oxides, metal nitrides, silicides, germanides, and 2D materials such as graphene. This review provides a summary of the recent developments in the search for better plasmonic materials and an outlook of further research directions.

Terahertz Metamaterials for Linear Polarization Conversion and Anomalous Refraction

Abstract: Polarization is one of the basic properties of electromagnetic waves conveying valuable information in signal transmission and sensitive measurements. Conventional methods for advanced polarization control impose demanding requirements on material properties and attain only limited performance. We demonstrated ultrathin, broadband, and highly efficient metamaterial-based terahertz polarization converters that are capable of rotating a linear polarization state into its orthogonal one. On the basis of these results, we created metamaterial structures capable of realizing near-perfect anomalous refraction. Our work opens new opportunities for creating high-performance photonic devices and enables emergent metamaterial functionalities for applications in the technologically difficult terahertz-frequency regime.

Photonic topological insulators

Abstract: Recent progress in understanding the topological properties of condensed matter has led to the discovery of time-reversal-invariant topological insulators. A remarkable and useful property of these materials is that they support unidirectional spin-polarized propagation at their surfaces. Unfortunately topological insulators are rare among solid-state materials. Using suitably designed electromagnetic media (metamaterials) we theoretically demonstrate a photonic analogue of a topological insulator. We show that metacrystals-superlattices of metamaterials with judiciously designed properties-provide a platform for designing topologically non-trivial photonic states, similar to those that have been identified for condensed-matter topological insulators. The interfaces of the metacrystals support helical edge states that exhibit spin-polarized one-way propagation of photons, robust against disorder. Our results demonstrate the possibility of attaining one-way photon transport without application of external magnetic fields or breaking of time-reversal symmetry. Such spin-polarized one-way transport enables exotic spin-cloaked photon sources that do not obscure each other.

Metamaterial Huygens' surfaces: tailoring wave fronts with reflectionless sheets.

Abstract: Huygens' principle is a well-known concept in electromagnetics that dates back to 1690. Here, it is applied to develop designer surfaces that provide extreme control of electromagnetic wave fronts across electrically thin layers. These reflectionless surfaces, referred to as metamaterial Huygens' surfaces, provide new beam shaping, steering, and focusing capabilities. The metamaterial Huygens' surfaces are realized with two-dimensional arrays of polarizable particles that provide both electric and magnetic polarization currents to generate prescribed wave fronts. A straightforward design methodology is demonstrated and applied to develop a beam-refracting surface and a Gaussian-to-Bessel beam transformer. Metamaterial Huygens' surfaces could find a wide range of applications over the entire electromagnetic spectrum including single-surface lenses, polarization controlling devices, stealth technologies, and perfect absorbers.

Experimental demonstration of a unidirectional reflectionless parity-time metamaterial at optical frequencies

Abstract: Invisibility by metamaterials is of great interest, where optical properties are manipulated in the real permittivity– permeability plane. However, the most effective approach to achieving invisibility in various military applications is to absorb the electromagnetic waves emitted from radar to minimize the corresponding reflection and scattering, such that no signal gets bounced back. Here, we show the experimental realization of chip-scale unidirectional reflectionless optical metamaterials near the spontaneous parity-time symmetry phase transition point where reflection from one side is significantly suppressed. This is enabled by engineering the corresponding optical properties of the designed paritytime metamaterial in the complex dielectric permittivity plane. Numerical simulations and experimental verification consistently exhibit asymmetric reflection with high contrast ratios around a wavelength of of 1,550 nm. The demonstrated unidirectional phenomenon at the corresponding parity-time exceptional point on-a-chip confirms the feasibility of creating complicated on-chip parity-time metamaterials and optical devices based on their properties.

Directional visible light scattering by silicon nanoparticles

Abstract: Directional light scattering by spherical silicon nanoparticles in the visible spectral range is experimentally demonstrated for the first time. These unique optical properties arise because of simultaneous excitation and mutual interference of magnetic and electric dipole resonances inside a single nanosphere. Such behaviour is similar to Kerker's-type scattering by hypothetic magneto-dielectric particles predicted theoretically three decades ago. Here we show that directivity of the far-field radiation pattern of single silicon spheres can be strongly dependent on the light wavelength and the nanoparticle size. For nanoparticles with sizes ranging from 100 to 200 nm, forward-to-backward scattering ratio above six can be experimentally obtained, making them similar to 'Huygens' sources. Unique optical properties of silicon nanoparticles make them promising for design of novel low-loss visible- and telecom-range metamaterials and nanoantenna devices.

Hyperbolic metamaterials

Abstract: Metamaterials with hyperbolic dispersion (where two eigenvalues of the dielectric permittivity tensor have opposite signs) exhibit a broad bandwidth singularity in the photonic density of states, with resulting manifestations in a variety of phenomena, from spontaneous emission to light propagation and scattering. In this tutorial, I will review some of the recent developments in this field.

Geometry of Miura-folded metamaterials

Abstract: This paper describes two folded metamaterials based on the Miura-ori fold pattern. The structural mechanics of these metamaterials are dominated by the kinematics of the folding, which only depends on the geometry and therefore is scale-independent. First, a folded shell structure is introduced, where the fold pattern provides a negative Poisson's ratio for in-plane deformations and a positive Poisson's ratio for out-of-plane bending. Second, a cellular metamaterial is described based on a stacking of individual folded layers, where the folding kinematics are compatible between layers. Additional freedom in the design of the metamaterial can be achieved by varying the fold pattern within each layer.

Broadband focusing flat mirrors based on plasmonic gradient metasurfaces

Abstract: We demonstrate that metal–insulator–metal configurations, with the top metal layer consisting of a periodic arrangement of differently sized nanobricks, can be designed to function as broadband foc...

3D Soft Metamaterials with Negative Poisson's Ratio

Abstract: Buckling is exploited to design a new class of three-dimensional metamaterials with negative Poisson's ratio. A library of auxetic building blocks is identified and procedures are defined to guide their selection and assembly. The auxetic properties of these materials are demonstrated both through experiments and finite element simulations and exhibit excellent qualitative and quantitative agreement.

Acoustic metamaterials and phononic crystals

Abstract: Introduction to Phononic Crystals and Acoustic Metamaterials.- Discrete One-Dimensional Phononic and Resonant Crystals.- One-Dimensional Phononic Crystals.- 2d-3d Phononic Crystals.- Dynamic Mass Density and Acoustic Metamaterials.- Damped Phononic Crystals and Acoustic Metamaterials.- Nonlinear Periodic Phononic Structures and Granular Crystals.- Tunable Phononic Crystals and Metamaterials.- Nanoscale Phononic Crystals and Structures.- Phononic Band Structures and Transmission Coefficients: Methods and Approaches.

Metamaterials with Negative Parameters: Theory, Design, and Microwave Applications

Abstract: Dedicatory. Acknowledgements. Preface. 1. The electrodynamics of left-handed media. 1.1. Wave propagation in left-handed media. 1.2. Energy density and group velocity. 1.3. Negative refraction. 1.4. Fermat principle. 1.5. Other effects in left-handed media. 1.5.1. Inverse Doppler effect. 1.5.2. Backward Cerenkov radiation. 1.5.3. Negative Goos-Hanchen shift. 1.6. Waves at interfaces. 1.6.1. Transmission and reflection coefficients. 1.6.2. Surface waves. 1.7. Waves through left-handed slabs. 1.7.1. Transmission and reflection coefficients. 1.7.2. Guided waves. 1.7.3. Backward leaky and complex waves. 1.8. Slabs with epsilon/epsilon o -1 and / o -1. 1.8.1. Phase compensation and amplification of evanescent modes. 1.8.2. Perfect tunneling. 1.8.3. The perfect lens. 1.8.4. The perfect-lens as a tunneling/matching device. 1.9. Losses and dispersion. 1.10. Indefinite media. 1.11. Problems. References. 2. Synthesis of bulk metamaterials. 2.1. Scaling plasmas at microwave frequencies. 2.1.1. Metallic waveguides and plates as one- and two-dimensional plasmas. 2.1.2. Wire media. 2.1.3. Spatial dispersion in wire media. 2.2. Synthesis of negative magnetic permeability. 2.2.1. Analysis of the edge-coupled SRR. 2.2.2. Other SRR designs. The broadside-coupled SRR. The non-bianisotropic SRR. The double split SRR. Spirals. 2.2.3. Constitutive relationships for bulk SRR metamaterials. 2.2.4. Higher order resonances in SRRs. 2.2.5. Isotropic SRRs. 2.2.6. Scaling down SRRs to infrared and optical frequencies. 2.3. SRR-based left-handed metamaterials. 2.3.1. One-dimensional SRR-based left-handed metamaterials. 2.3.2. Two-dimensional and three-dimensional SRR-based lefthanded metamaterials. 2.3.3. On the application of the continuous medium approach to discrete SRR-based left-handed metamaterials. 2.3.4. The ?superposition? hypothesis. 2.3.5. On the numerical accuracy of the developed model for SRR-based metamaterials. 2.4. Other approaches to bulk metamaterial design. 2.4.1. Ferrite metamaterials. 2.4.2. Chiral metamaterials. 2.4.3. Other proposals. 2.5. Appendix. 2.6. Problems. References. 3. Synthesis of metamaterials in planar technology. 3.1. The dual (backward) transmission line concept. 3.2. Practical implementation of backward transmission lines. 3.3. Two-dimensional (2D) planar metamaterials. 3.4. Design of left handed transmission lines by means of SRRs: the resonant type approach. 3.4.1. Effective negative permeability transmission lines. 3.4.2. Left handed transmission lines in microstrip and CPW technologies. 3.4.3. Size reduction. 3.5. Equivalent circuit models for SRRs coupled to conventional transmission lines. 3.5.1. Dispersion diagrams. 3.5.2. Implications of the model. 3.6. Duality and complementary split rings resonators (CSRRs). 3.6.1. Electromagnetic properties of CSRRs. 3.6.2. Numerical calculation and experimental validation. 3.7. Synthesis of metamaterial transmission lines by using CSRRs. 3.7.1. Negative permittivity and left handed transmission lines. 3.7.2. Equivalent circuit models for CSRR loaded transmission lines. 3.7.3. Parameter extraction. 3.7.4. Effects of cell geometry on frequency response. 3.8. Comparison between the circuit models of resonant type and dual left handed lines. Problems. References. 4. Microwave applications of metamaterial concepts. 4.1. Filters and diplexers. 4.1.1. Stop band filters. 4.1.2. Planar filters with improved stop band. 4.1.3. Narrow band pass filter and diplexer design. 4.1.3.1. Band pass filters based on alternate right/left handed (ARLH) sections implemented by means of SRRs. 4.1.3.2. Band pass filters and diplexers based on alternate right/left handed (ARLH) sections implemented by means of CSRRs. 4.1.4. CSRR-based band pass filters with controllable characteristics. 4.1.4.1. Band pass filters based on the hybrid approach: design methodology and illustrative examples. 4.1.4.2. Other CSRR-based filters implemented by means of right handed sections. 4.1.5. High pass filters and ultra wide band pass filters (UWBPFs) implemented by means of resonant type balanced CRLH metamaterial transmission lines. 4.1.6. Tunable filters based on varactor-loaded split rings resonators (VLSRRs). 4.1.6.1. Topology of the VLSRR and equivalent circuit model. 4.1.6.2. Validation of the model. 4.1.6.3. Some illustrative results: tunable notch filters and stop band filters. 4.2. Synthesis of metamaterial transmission lines with controllable characteristics and applications. 4.2.1. Miniaturization of microwave components. 4.2.2. Compact broadband devices. 4.2.3. Dual band components. 4.2.4. Coupled line couplers. 4.3. Antenna applications. Problems. References. 5. Advanced and related topics. 5.1. SRR and CSRR based admittance surfaces. 5.1.1. Babinet principle for a single split rings resonator. 5.1.2. Surface admittance approach for SRR planar arrays. 5.1.3. Babinet principle for CSRR planar arrays. 5.1.4. Behavior at normal incidence. 5.1.5. Behavior at general incidence. 5.2. Magneto- and electro-inductive waves. 5.2.1. The magneto-inductive wave equation. 5.2.2. Magneto-inductive surfaces. 5.2.3. Electro-inductive waves in CSRR arrays. 5.2.4. Applications of magneto- and electro-inductive waves. 5.3. Sub-diffraction imaging devices. 5.3.1. Some universal features of sub-diffraction imaging devices. 5.3.2. Imaging in the quasi-electrostatic limit. Role of surface plasmons. 5.3.3. Imaging in the quasi-magnetostatic limit. Role of magnetostatic surface waves. 5.3.4. Imaging by resonant impedance surfaces. Magneto-inductive lenses. 5.3.5. Canalization devices. 5.4. Problems. References.

Realization of an all-dielectric zero-index optical metamaterial

Abstract: Previously demonstrated zero- or negative-refractive-index metamaterials at optical frequencies suffer from large ohmic losses because of the need to use metals. Metamaterials formed by stacked silicon rod unit cells allow the realization of all-dielectric impedance-matched zero-index metamaterials operating at optical frequencies, potentially benefiting the development of angular-selective optical devices.

Graphene metamaterials based tunable terahertz absorber: effective surface conductivity approach

Abstract: In this paper we present the efficient design of functional thin-film metamaterial devices with the effective surface conductivity approach. As an example, we demonstrate a graphene based perfect absorber. After formulating the requirements to the perfect absorber in terms of surface conductivity we investigate the properties of graphene wire medium and graphene fishnet metamaterials and demonstrate both narrowband and broadband tunable absorbers.

Demonstration of Zero Optical Backscattering from Single Nanoparticles

Abstract: We present the first experimental demonstration of zero backscattering from nanoparticles at optical frequencies as originally discussed by Kerker et al. [ Kerker , M. ; Wang , D. ; Giles , C. J. Opt. Soc. A 1983 , 73 , 765 ]. GaAs pillars were fabricated on a fused silica substrate and the spectrum of the backscattered radiation was measured in the wavelength range 600-1000 nm. Suppression of backscattering occurred at ~725 nm, agreeing with calculations based on the discrete dipole approximation. Particles with zero backscattering provide new functionality for metamaterials and optical antennas.

Liquid Crystal Tunable Metamaterial Absorber

Abstract: We present an experimental demonstration of electronically tunable metamaterial absorbers in the terahertz regime. By incorporation of active liquid crystal into strategic locations within the metamaterial unit cell, we are able to modify the absorption by 30% at 2.62 THz, as well as tune the resonant absorption over 4% in bandwidth. Numerical full-wave simulations match well to experiments and clarify the underlying mechanism, i.e., a simultaneous tuning of both the electric and magnetic response that allows for the preservation of the resonant absorption. These results show that fundamental light interactions of surfaces can be dynamically controlled by all-electronic means and provide a path forward for realization of novel applications.

Metamaterial Apertures for Computational Imaging

Abstract: By leveraging metamaterials and compressive imaging, a low-profile aperture capable of microwave imaging without lenses, moving parts, or phase shifters is demonstrated. This designer aperture allows image compression to be performed on the physical hardware layer rather than in the postprocessing stage, thus averting the detector, storage, and transmission costs associated with full diffraction-limited sampling of a scene. A guided-wave metamaterial aperture is used to perform compressive image reconstruction at 10 frames per second of two-dimensional (range and angle) sparse still and video scenes at K-band (18 to 26 gigahertz) frequencies, using frequency diversity to avoid mechanical scanning. Image acquisition is accomplished with a 40:1 compression ratio.

Spin-Optical Metamaterial Route to Spin-Controlled Photonics

Abstract: Spin optics provides a route to control light, whereby the photon helicity (spin angular momentum) degeneracy is removed due to a geometric gradient onto a metasurface. The alliance of spin optics and metamaterials offers the dispersion engineering of a structured matter in a polarization helicity-dependent manner. We show that polarization-controlled optical modes of metamaterials arise where the spatial inversion symmetry is violated. The emerged spin-split dispersion of spontaneous emission originates from the spin-orbit interaction of light, generating a selection rule based on symmetry restrictions in a spin-optical metamaterial. The inversion asymmetric metasurface is obtained via anisotropic optical antenna patterns. This type of metamaterial provides a route for spin-controlled nanophotonic applications based on the design of the metasurface symmetry properties.

Experimental realization of an epsilon-near-zero metamaterial at visible wavelengths

Abstract: Silver and silicon nitride metamaterial structures with dielectric permittivities close to zero are demonstrated at visible wavelengths. In such materials, the optical phase advance during propagation can be very small.

Hybrid nanocolloids with programmed three-dimensional shape and material composition

Abstract: Tuning the optical, electromagnetic and mechanical properties of a material requires simultaneous control over its composition and shape. This is particularly challenging for complex structures at the nanoscale because surface-energy minimization generally causes small structures to be highly symmetric. Here we combine low-temperature shadow deposition with nanoscale patterning to realize nanocolloids with anisotropic three-dimensional shapes, feature sizes down to 20 nm and a wide choice of materials. We demonstrate the versatility of the fabrication scheme by growing three-dimensional hybrid nanostructures that contain several functional materials with the lowest possible symmetry, and by fabricating hundreds of billions of plasmonic nanohelices, which we use as chiral metafluids with record circular dichroism and tunable chiroptical properties.

Giant non-reciprocity at the subwavelength scale using angular momentum-biased metamaterials

Abstract: Breaking time-reversal symmetry enables the realization of non-reciprocal devices, such as isolators and circulators, of fundamental importance in microwave and photonic communication systems. This effect is almost exclusively achieved today through magneto-optical phenomena, which are incompatible with integrated technology because of the required large magnetic bias. However, this is not the only option to break reciprocity. The Onsager-Casimir principle states that any odd vector under time reversal, such as electric current and linear momentum, can also produce a non-reciprocal response. These recently analysed alternatives typically work over a limited portion of the electromagnetic spectrum and/or are often characterized by weak effects, requiring large volumes of operation. Here we show that these limitations may be overcome by angular momentum-biased metamaterials, in which a properly tailored spatiotemporal modulation is azimuthally applied to subwavelength Fano-resonant inclusions, producing largely enhanced non-reciprocal response at the subwavelength scale, in principle applicable from radio to optical frequencies.

Recent progress in some composite materials and structures for specific electromagnetic applications

Abstract: This review aims to summarise the progress in some materials and structures for electromagnetic applications, such as microwave absorption, electric shielding and antenna designs, which have been developed in recent years. Composites with spherical powders for microwave absorption focus mainly on those based on ferrites (especially hexagonal), carbonyl iron and related alloys and various newly emerged nanosized materials. Composites with long conductive fibres as fillers will be summarised, with speical attentions to prediction, measurment and evaluation of their performances. Metamaterials include structures for microwave absorbing applications, tunable materials or structures with reflection or transmission coefficients that are tunable by external magnetic or electric fields, and specially designed structures for microwave absorbing applications, with thickness much smaller than that of conventional composite materials and performances that can be optimised by the physical properties of substra...

A full-parameter unidirectional metamaterial cloak for microwaves

Abstract: Invisibility is a notion that has long captivated the popular imagination. However, in 2006, invisibility became a practical matter for the scientific community as well, with the suggestion that artificially structured metamaterials could enable a new electromagnetic design paradigm, now termed transformation optics. Since the advent of transformation optics and subsequent initial demonstration of the microwave cloak, the field has grown rapidly. However, the complexity of the transformation optics material prescription has continually forced researchers to make simplifying approximations to achieve even a subset of the desired functionality. These approximations place profound limitations on the performance of transformation optics devices in general, and cloaks especially. Here, we design and experimentally characterize a two-dimensional, unidirectional cloak that makes no approximations to the underlying transformation optics formulation, yet is capable of reducing the scattering of an object ten wavelengths in size. We demonstrate that this approximation-free design regains the performance characteristics promised by transformation optics.

Metamaterials beyond electromagnetism

Abstract: Metamaterials are rationally designed man-made structures composed of functional building blocks that are densely packed into an effective (crystalline) material. While metamaterials are mostly associated with negative refractive indices and invisibility cloaking in electromagnetism or optics, the deceptively simple metamaterial concept also applies to rather different areas such as thermodynamics, classical mechanics (including elastostatics, acoustics, fluid dynamics and elastodynamics), and, in principle, also to quantum mechanics. We review the basic concepts, analogies and differences to electromagnetism, and give an overview on the current state of the art regarding theory and experiment-all from the viewpoint of an experimentalist. This review includes homogeneous metamaterials as well as intentionally inhomogeneous metamaterial architectures designed by coordinate-transformation-based approaches analogous to transformation optics. Examples are laminates, transient thermal cloaks, thermal concentrators and inverters, 'space-coiling' metamaterials, anisotropic acoustic metamaterials, acoustic free-space and carpet cloaks, cloaks for gravitational surface waves, auxetic mechanical metamaterials, pentamode metamaterials ('meta-liquids'), mechanical metamaterials with negative dynamic mass density, negative dynamic bulk modulus, or negative phase velocity, seismic metamaterials, cloaks for flexural waves in thin plates and three-dimensional elastostatic cloaks.

Metamaterial-based microfluidic sensor for dielectric characterization

Abstract: A microfluidic sensor is implemented from a single split-ring resonator (SRR), a fundamental building block of electromagnetic metamaterials. At resonance, an SRR establishes an intense electric field confined within a deeply subwavelength region. Liquid flowing in a micro-channel laid on this region can alter the local field distribution and hence affect the SRR resonance behavior. Specifically, the resonance frequency and bandwidth are influenced by the complex dielectric permittivity of the liquid sample. The empirical relation between the sensor resonance and the sample permittivity can be established, and from this relation, the complex permittivity of liquid samples can be estimated. The technique is capable of sensing liquid flowing in the channel with a cross-sectional area as small as (0.001 λ 0 ) 2 , where λ 0 denotes the free-space wavelength of the wave excitation. This work motivates the use of SRR-based microfluidic sensors for identification, classification, and characterization of chemical and biochemical analytes.

An electromechanically reconfigurable plasmonic metamaterial operating in the near-infrared.

Abstract: Current efforts in metamaterials research focus on dynamic functionalities such as tunability, switching and modulation of electromagnetic waves. To this end, various approaches have appeared, including embedded varactors, phase-change media, use of liquid crystals, electrical modulation with graphene and superconductors, and carrier injection or depletion in semiconductor substrates. However, tuning, switching and modulating metamaterial properties in the visible and near-infrared range remain major technological challenges: the existing microelectromechanical solutions for the subTHz and THz regimes cannot be shrunk by 2-3 orders of magnitude to enter the optical spectral range. Here we develop a new type of metamaterial operating in the optical part of the spectrum which is 3 orders of magnitude faster than previously reported electrically reconfigurable metamaterials. The metamaterial is actuated by electrostatic forces arising from the application of only a few volts to its nanoscale building blocks, the plasmonic metamolecules, which are supported by pairs of parallel strings cut from a nanoscale thickness flexible silicon nitride membrane. These strings of picogram mass can be synchronously driven to megahertz frequencies to electromechanically reconfigure the metamolecules and dramatically change the metamaterial’s transmission and reflection spectra. The metamaterial’s colossal electro-optical response allows for both fast continuous tuning of its optical properties (up to 8% optical signal modulation at up to megahertz rates) and high-contrast irreversible switching in a device of only 100 nm thickness without the need for external polarizers and analyzers.

A subwavelength plasmonic metamolecule exhibiting magnetic-based optical Fano resonance.

Abstract: The lack of symmetry between electric and magnetic charges, a fundamental consequence of the small value of the fine-structure constant, is directly related to the weakness of magnetic effects in optical materials. Properly tailored plasmonic nanoclusters have been proposed recently to induce artificial optical magnetism based on the principle that magnetic effects are indistinguishable from specific forms of spatial dispersion of permittivity at optical frequencies. In a different context, plasmonic Fano resonances have generated a great deal of interest, particularly for use in sensing applications that benefit from sharp spectral features and extreme field localization. In the absence of natural magnetism, optical Fano resonances have so far been based on purely electric effects. In this Letter, we demonstrate that a subwavelength plasmonic metamolecule consisting of four closely spaced gold nanoparticles supports a strong magnetic response coupled to a broad electric resonance. Small structural asymmetries in the assembled nanoring enable the interaction between electric and magnetic modes, leading to the first observation of a magnetic-based Fano scattering resonance at optical frequencies. Our findings are supported by excellent agreement with simulations and analytical calculations, and represent an important step towards the quest for artificial magnetism and negative refractive index metamaterials at optical frequencies.

Observation of Dirac plasmons in a topological insulator

Abstract: Plasmons are quantized collective oscillations of electrons and have been observed in metals and doped semiconductors. The plasmons of ordinary, massive electrons have been the basic ingredients of research in plasmonics and in optical metamaterials for a long time1. However, plasmons of massless Dirac electrons have only recently been observed in graphene, a purely two-dimensional electron system2. Their properties are promising for novel tunable plasmonic metamaterials in the terahertz and mid-infrared frequency range3. Dirac fermions also occur in the two-dimensional electron gas that forms at the surface of topological insulators as a result of the strong spin–orbit interaction existing in the insulating bulk phase4. One may therefore look for their collective excitations using infrared spectroscopy. Here we report the first experimental evidence of plasmonic excitations in a topological insulator (Bi2Se3). The material was prepared in thin micro-ribbon arrays of different widths W and periods 2W to select suitable values of the plasmon wavevector k. The linewidth of the plasmon was found to remain nearly constant at temperatures between 6 K and 300 K, as expected when exciting topological carriers. Moreover, by changing W and measuring the plasmon frequency in the terahertz range versus k we show, without using any fitting parameter, that the dispersion curve agrees quantitatively with that predicted for Dirac plasmons. Plasmonic excitation of massless electrons is observed in Bi2Se3.

Dynamical Casimir effect in a Josephson metamaterial

Abstract: The zero-point energy stored in the modes of an electromagnetic cavity has experimentally detectable effects, giving rise to an attractive interaction between the opposite walls, the static Casimir effect. A dynamical version of this effect was predicted to occur when the vacuum energy is changed either by moving the walls of the cavity or by changing the index of refraction, resulting in the conversion of vacuum fluctuations into real photons. Here, we demonstrate the dynamical Casimir effect using a Josephson metamaterial embedded in a microwave cavity at 5.4 GHz. We modulate the effective length of the cavity by flux-biasing the metamaterial based on superconducting quantum interference devices (SQUIDs), which results in variation of a few percentage points in the speed of light. We extract the full 4 × 4 covariance matrix of the emitted microwave radiation, demonstrating that photons at frequencies symmetrical with respect to half of the modulation frequency are generated in pairs. At large detunings of the cavity from half of the modulation frequency, we find power spectra that clearly show the theoretically predicted hallmark of the Casimir effect: a bimodal, “sparrow-tail” structure. The observed substantial photon flux cannot be assigned to parametric amplification of thermal fluctuations; its creation is a direct consequence of the noncommutativity structure of quantum field theory.