Showing papers on "Transformation optics published in 2016"

Acoustic metamaterials: From local resonances to broad horizons.

Abstract: Within a time span of 15 years, acoustic metamaterials have emerged from academic curiosity to become an active field driven by scientific discoveries and diverse application potentials. This review traces the development of acoustic metamaterials from the initial findings of mass density and bulk modulus frequency dispersions in locally resonant structures to the diverse functionalities afforded by the perspective of negative constitutive parameter values, and their implications for acoustic wave behaviors. We survey the more recent developments, which include compact phase manipulation structures, superabsorption, and actively controllable metamaterials as well as the new directions on acoustic wave transport in moving fluid, elastic, and mechanical metamaterials, graphene-inspired metamaterials, and structures whose characteristics are best delineated by non-Hermitian Hamiltonians. Many of the novel acoustic metamaterial structures have transcended the original definition of metamaterials as arising from the collective manifestations of constituent resonating units, but they continue to extend wave manipulation functionalities beyond those found in nature.

Implementation of dispersion-free slow acoustic wave propagation and phase engineering with helical-structured metamaterials.

Abstract: The ability to slow down wave propagation in materials has attracted significant research interest. A successful solution will give rise to manageable enhanced wave-matter interaction, freewheeling phase engineering and spatial compression of wave signals. The existing methods are typically associated with constructing dispersive materials or structures with local resonators, thus resulting in unavoidable distortion of waveforms. Here we show that, with helical-structured acoustic metamaterials, it is now possible to implement dispersion-free sound deceleration. The helical-structured metamaterials present a non-dispersive high effective refractive index that is tunable through adjusting the helicity of structures, while the wavefront revolution plays a dominant role in reducing the group velocity. Finally, we numerically and experimentally demonstrate that the helical-structured metamaterials with designed inhomogeneous unit cells can turn a normally incident plane wave into a self-accelerating beam on the prescribed parabolic trajectory. The helical-structured metamaterials will have profound impact to applications in explorations of slow wave physics.

Invisibility exposed: physical bounds on passive cloaking

Abstract: Invisibility cloaks have been one of the major breakthroughs in the field of metamaterials, and several techniques are currently available to suppress the electromagnetic scattering from different objects. So far, however, theoretical and experimental results have consistently shown fundamental challenges in terms of the bandwidth and size of the object to be hidden. Understanding the bandwidth limitations of cloaking therefore becomes important to assess the applicability and potential of cloaking devices in practical scenarios. While it is generally accepted that cloaking is difficult to achieve, no work to date has quantified this issue for general invisibility devices. Here, by applying the Bode–Fano theory of broadband matching, we derive fundamental bounds on bandwidth and performance for general passive cloaking schemes applied to planar objects, determined by the properties of the object to be hidden, and then explore their implications in three-dimensional scenarios. Our results define a general framework to estimate the ultimate performance of passive cloaks and suggest that fundamentally new directions, involving nonlinearities and active metamaterials, become necessary to realize broadband cloaking, opening a new phase in the quest for invisibility.

A design of active elastic metamaterials for control of flexural waves using the transformation method

Abstract: The ability to control flexural wave propagation is of fundamental interest in many areas of structural engineering and physics. Metamaterials have shown a great potential in subwavelength wave propagation control due to their inherent local resonance mechanism. In this study, we propose a transformation method to derive the material properties of a flexural waveguide and implement the functionality based on a design of active elastic metamaterials. The numerically demonstrated flexural waveguide can not only steer an elastic wave beam as predicted from the transformation method but also exhibit various unique properties including extraordinary wave beam deflection and tunabilities over a broad frequency range and various steering directions. The waveguide is equipped with an array of active elastic metamaterials composed of the electrorheological elastomer subjected to adjustable electric fields. Such metamaterial-based waveguides provide a new design methodology for guided wave signal modulation devices...

Programmable Extreme Chirality in the Visible by Helix-Shaped Metamaterial Platform

Abstract: The capability to fully control the chiro-optical properties of metamaterials in the visible range enables a number of applications from integrated photonics to life science. To achieve this goal, a simultaneous control over complex spatial and localized structuring as well as material composition at the nanoscale is required. Here, we demonstrate how circular dichroic bands and optical rotation can be effectively and independently tailored throughout the visible regime as a function of the fundamental meta-atoms properties and of their three dimensional architecture in a the helix-shaped metamaterials. The record chiro-optical effects obtained in the visible range are accompanied by an additional control over optical efficiency, even in the plasmonic context. These achievements pave the way toward fully integrated chiral photonic devices.

Ultratransparent Media and Transformation Optics with Shifted Spatial Dispersions.

Abstract: A photonic crystal metamaterial is realized with perfect transmission of light at all incident angles. The material can be used to create aberration-free virtual images.

Transformation Laplacian metamaterials: recent advances in manipulating thermal and dc fields

Abstract: The full control of single or even multiple physical fields has attracted intensive research attention in the past decade, thanks to the development of metamaterials and transformation optics. Significant progress has been made in vector fields (e.g., optics, electromagnetics, and acoustics), leading to a host of strikingly functional metamaterials, such as invisibility cloaks, illusion devices, concentrators, and rotators. However, metamaterials in vector fields, designed through coordinate transformation of Maxwell's equations, usually require extreme parameters and impose challenges on the actual realization. In this context, metamaterials in scalar fields (e.g., thermal and dc fields), which are mostly governed by the Laplace equation, lead to more plausible and facile implementations, since there are native insulators and excellent conductors (serving as two extreme cases). This paper therefore is particularly dedicated to reviewing the most recent advances in Laplacian metamaterials in manipulating thermal (both transient and steady states) and dc fields, separately and (or) simultaneously. We focus on the theory, design, and realization of thermal/dc functional metamaterials that can be used to control heat flux and electric current at will. We also provide an outlook toward the challenges and future directions in this fascinating area.

Surface Wave Cloak from Graded Refractive Index Nanocomposites.

Abstract: Recently, a great deal of interest has been re-emerged on the possibility to manipulate surface waves, in particular, towards the THz and optical regime. Both concepts of Transformation Optics (TO) and metamaterials have been regarded as one of key enablers for such applications in applied electromagnetics. In this paper, we experimentally demonstrate for the first time a dielectric surface wave cloak from engineered gradient index materials to illustrate the possibility of using nanocomposites to control surface wave propagation through advanced additive manufacturing. The device is designed analytically and validated through numerical simulations and measurements, showing good agreement and performance as an effective surface wave cloak. The underlying design approach has much wider applications, which span from microwave to optics for the control of surface plasmon polaritons (SPPs) and radiation of nanoantennas.

Coherent beam control with an all-dielectric transformation optics based lens

Abstract: Transformation optics (TO) concept well known for its huge possibility in patterning the path of electromagnetic waves is exploited to design a beam steering lens. The broadband directive in-phase emission in a desired off-normal direction from an array of equally fed radiators is numerically and experimentally reported. Such manipulation is achieved without the use of complex and bulky phase shifters as it is the case in classical phased array antennas. The all-dielectric compact low-cost lens prototype presenting a graded permittivity profile is fabricated through three-dimensional (3D) polyjet printing technology. The array of radiators is composed of four planar microstrip antennas realized using standard lithography techniques and is used as excitation source for the lens. To validate the proposed lens, we experimentally demonstrate the broadband focusing properties and in-phase directive emissions deflected from the normal direction. Both the far-field radiation patterns and the near-field distributions are measured and reported. Measurements agree quantitatively and qualitatively with numerical full-wave simulations and confirm the corresponding steering properties. Such experimental validation paves the way to inexpensive easy-made all-dielectric microwave lenses for beam forming and collimation.

Flat Optics for Surface Waves

Abstract: The name flat optics (FO) has been introduced in a recent paper by Capasso’s group for denoting light-wave manipulations through a general type of penetrable or impenetrable metasurfaces (MTSs). There, the attention was focused on plane waves, whereas here we treat surface waves (SWs) excited on impenetrable impedance surfaces. Space variability of the boundary conditions imposes a deformation of the SW wavefront, which addresses the local wavector along not-rectilinear paths. The ray paths are subjected to an eikonal equation analogous to the one for geometrical optics (GO) rays in graded index materials. The basic relations among ray paths, ray velocity, and transport of energy for both isotropic and anisotropic boundary conditions are presented for the first time. This leads to an elegant formulation which allows for closed form analysis of flat operational devices (lenses or beam formers), giving a new guise to old concepts. It is shown that when an appropriate transformation is found, the ray paths can be conveniently controlled without the use of ray tracing, thus simplifying the problem and leading to a flat version of transformation optics, which is framed here in the general FO theory.

Experimental validation of ultra-thin metalenses for N-beam emissions based on transformation optics

Abstract: A general design of metalenses for N-beam emissions is proposed based on transformation optics. A linear mapping function is adopted to achieve the homogeneous characterization of the transforming medium, which is therefore easy to be achieved compared with previous designs limited by inhomogeneity based on transformation optics. To verify the theoretical design, a four-beam antenna constructed with ultrathin, homogenous, and uniaxial anisotropic metalens is designed, fabricated, and measured. It is shown that the realized gain of the four-beam antenna is increased by 6 dB compared with the single dipole source, while working frequency and relative bandwidth are kept unchanged. The measured far-field pattern verifies theoretical design procedure.

Bidirectional invisibility in Kramers-Kronig optical media.

Abstract: A broad class of planar dielectric media with complex permittivity profiles that are fully invisible, for both left and right incidence sides, is introduced. Such optical media are locally isotropic, non-magnetic, and belong to the recently discovered class of Kramers-Kronig media [Nat. Photonics9, 436 (2015)], i.e., the spatial profiles of the real and imaginary parts of the dielectric permittivity are related each other by a Hilbert transform. The transition from unidirectional to bidirectional invisibility, and the possibility to realize sharp reflection above a cut-off incidence angle, are also discussed.

Transformation seismology: composite soil lenses for steering surface elastic Rayleigh waves

Abstract: Metamaterials are artificially structured media that exibit properties beyond those usually encountered in nature. Typically they are developed for electromagnetic waves at millimetric down to nanometric scales, or for acoustics, at centimeter scales. By applying ideas from transformation optics we can steer Rayleigh-surface waves that are solutions of the vector Navier equations of elastodynamics. As a paradigm of the conformal geophysics that we are creating, we design a square arrangement of Luneburg lenses to reroute Rayleigh waves around a building with the dual aim of protection and minimizing the effect on the wavefront (cloaking). To show that this is practically realisable we deliberately choose to use material parameters readily available and this metalens consists of a composite soil structured with buried pillars made of softer material. The regular lattice of inclusions is homogenized to give an effective material with a radially varying velocity profile and hence varying the refractive index of the lens. We develop the theory and then use full 3D numerical simulations to conclusively demonstrate, at frequencies of seismological relevance 3–10 Hz, and for low-speed sedimentary soil (vs: 300–500 m/s), that the vibration of a structure is reduced by up to 6 dB at its resonance frequency.

Efficient Thermal-Light Interconversions Based on Optical Topological Transition in the Metal-Dielectric Multilayered Metamaterials.

Abstract: Metal-dielectric multilayered metamaterials are proposed to work as wideband spectral-selective emitters/absorbers due to the topological change in isofrequency contour around the epsilon-near-zero point. By setting the transition at the border between the visible and IR ranges, the metal-dielectric multilayered metamaterials become good absorbers/emitters for visible light and good reflectors for IR light, which are desirable for efficient thermal-light interconversions.

Loss-induced topological transition of dispersion in metamaterials

Abstract: Topological transition of dispersion in anisotropic metamaterials, in which isofrequency contour changes from a closed ellipsoid to an open hyperboloid, is usually realized by changing the sign of one component of permittivity (e) or permeability (μ) from positive to negative. However, we show that topological transition of dispersion can occur by tuning the imaginary part of e(μ) while fixing the real part of e(μ). By adding different lumped resistors into two-dimensional transmission-line-based metamaterials, we just tune the imaginary part of μ at a fixed frequency. With the increase of loss, we measure the different emission patterns from a point source in the metamaterials to observe the changing process of isofrequency contours.

Single-material semiconductor hyperbolic metamaterials

Abstract: Layered semiconductor hyperbolic metamaterials for the mid-infrared are grown by molecular beam epitaxy using a single material system, doped and undoped InAs. The onset wavelength for metamaterial behavior can be tuned from 5.8μm to beyond 10μm, while the fill factor ranges from 0.25 to 0.75, resulting in designer optical behavior. The reflection and transmission behavior were studied by Fourier transform spectroscopy and modeled using effective medium theory. We also conducted a geometric optics experiment to demonstrate negative refraction of our materials.

Optically induced metal-to-dielectric transition in Epsilon-Near-Zero metamaterials.

Abstract: Epsilon-Near-Zero materials exhibit a transition in the real part of the dielectric permittivity from positive to negative value as a function of wavelength. Here we study metal-dielectric layered metamaterials in the homogenised regime (each layer has strongly subwavelength thickness) with zero real part of the permittivity in the near-infrared region. By optically pumping the metamaterial we experimentally show that close to the Epsilon-Near-Zero (ENZ) wavelength the permittivity exhibits a marked transition from metallic (negative permittivity) to dielectric (positive permittivity) as a function of the optical power. Remarkably, this transition is linear as a function of pump power and occurs on time scales of the order of the 100 fs pump pulse that need not be tuned to a specific wavelength. The linearity of the permittivity increase allows us to express the response of the metamaterial in terms of a standard third order optical nonlinearity: this shows a clear inversion of the roles of the real and imaginary parts in crossing the ENZ wavelength, further supporting an optically induced change in the physical behaviour of the metamaterial.

Invisibility Dips of Near-Field Energy Transport in a Spoof Plasmonic Metadimer

Abstract: Invisibility dips, minima in scattering spectrum associated with asymmetric Fano-like line-shapes, have been predicted with transformation optics in studying strong coupling between two plasmonic nanoparticles. This feature of strongly coupled plasmonic nanoparticles holds promise for sensor cloaking. It requires an extremely narrow gap between the two nanoparticles, though, preventing its experimental observation at optical frequencies. Here, the concept of spoof surface plasmons is utilized to facilitate the strong coupling between two spoof-localized-surface-plasmon (SLSP) resonators. Instead of observing in far field, the near-field energy transport is probed through the two SLSP resonators. By virtue of enhanced coupling between the two resonators stacked vertically, a spectral transmission dip with asymmetric Fano-like line-shape, similar to the far-field invisibility dips predicted by transformation optics, is observed. The underlying mode interference mechanism is further demonstrated by directly imaging the field maps of interfered waves that are tightly localized around the resonators. These near-field invisibility dips may find use in near-field sensing, on-chip switching, filters and logical operation elements.

Elastic Metamaterials Making Use of Chirality: A Review

Abstract: Metamaterials are artificially designed composite materials exhibiting unusual physical properties not easily found in nature. In most cases, these properties appear in a wave environment where local resonance comes into play. Therefore the design principle of these metamaterials relies intimately on creation of appropriate local resonances and their interplay with background waves. In this review, we show that the coupling between rotation and bulk deformations of two-dimensional chiral solids is able to provide a unique resonant mechanism in designing elastic metamaterials, and the recent advances in this area will be reviewed. We begin with a metamaterial with a single-negative parameter by integrating a chiral lattice with resonating inclusions, and demonstrate that this metamaterial not only is suitable for broadband vibration isolation but also provides a mixed-type resonance due to microstructure chirality. This mixed-type resonance is further explored to realize elastic metamaterial with double-negative parameters, which can refract elastic wave with a negative refraction angle. Finally, we present also recent development on micropolar constitutive models, which are potentially suitable for modeling chiral elastic metamaterials.

Optical effective media with independent control of permittivity and permeability based on conductive particles

Abstract: We propose and prove that it is possible to decouple the electric and magnetic response of an array of conductive nanoparticles and realize a very wide range of combinations of effective permittivities and permeabilities. The principle exploits the large differences in the Thomas-Fermi screening length for longitudinal electric fields and the classical penetration depth for time-varying transverse magnetic fields. This non-resonant principle allows frequency invariance of the effective material properties with a bandwidth spanning many octaves, orders of magnitude larger than previous resonant metamaterials. An effective medium with a record-high refractive index over broadband is demonstrated as an example.

3D printed broadband transformation optics based all-dielectric microwave lenses

Abstract: Quasi-conformal transformation optics is applied to design electromagnetic devices for focusing and collimating applications at microwave frequencies. Two devices are studied and conceived by solving Laplace's equation that describes the deformation of a medium in a space transformation. As validation examples, material parameters of two different lenses are derived from the analytical solutions of Laplace's equation. The first lens is applied to produce an overall directive in-phase emission from an array of sources conformed on a cylindrical structure. The second lens allows deflecting a directive beam to an off-normal direction. Full-wave simulations are performed to verify the functionality of the calculated lenses. Prototypes presenting a graded refractive index are fabricated through three-dimensional polyjet printing using solely dielectric materials. Experimental measurements carried out show very good agreement with numerical simulations, thereby validating the proposed lenses. Such easily realizable designs open the way to low-cost all-dielectric microwave lenses for beam forming and collimation.

PT-symmetric planar devices for field transformation and imaging

Abstract: The powerful tools of transformation optics (TO) allow an effective distortion of a region of space by carefully engineering the material inhomogeneity and anisotropy, and have been successfully applied in recent years to control electromagnetic fields in many different scenarios, e.g., to realize invisibility cloaks and planar lenses. For various field transformations, it is not necessary to use volumetric inhomogeneous materials, and suitably designed ultrathin metasurfaces with tailored spatial or spectral responses may be able to realize similar functionalities within smaller footprints and more robust mechanisms. Here, inspired by the concept of metamaterial TO lenses, we discuss field transformations enabled by parity-time (PT) symmetric metasurfaces, which can emulate negative refraction. We first analyze a simple realization based on homogeneous and local metasurfaces to achieve negative refraction and imaging, and we then extend our results to arbitrary PT-symmetric two-port networks to realize aberration-free planar imaging.

Superluminal media formed by photonic crystals for transformation optics-based invisibility cloaks

Abstract: We have developed an approach to building superluminal medium for transformation optics‐based devices, including invisibility cloaks, from photonic crystals. Analysis of dispersion diagrams of 2D arrays composed from dielectric rods has shown that at frequencies corresponding to the second bands formed due to bandgap opening at increase of rod permittivity, the medium formed by arrays exhibits refractive indices providing for superluminal phase velocities of propagating waves. It is further demonstrated that rod arrays with various lattice constants could be used for realizing a range of superluminal index values prescribed by transformation optics for cylindrical cloaks at arbitrary chosen operating frequency. The performed studies allowed for solving a row of problems with employment rod arrays in the cloak medium: in particular, formulating transformation optics‐based prescriptions for refractive index dispersion in the cloaking shell, defining the dimensions of array fragments capable of responding similar to infinite arrays, finding optimal distribution of linear arrays sets at their coiling to form concentric material layers in the cloaking shell, and employing interaction between neighboring array sets with various lattice constants to assist the realization of prescribed index dispersion. The performance of the superluminal medium formed by rod array sets was demonstrated on an example of a cloaking shell developed for microwave frequency range. In contrast to metamaterial-based cloak media, the developed media requires neither material homogenization, nor obtaining the effective parameters with peculiar values and Lorentz's type resonances in rods. Combination of these advantages and low losses makes photonic crystals perspective materials for invisibility cloaks operating in THz and optical ranges.

Complex-coordinate non-Hermitian transformation optics

Abstract: Transformation optics (TO) is conventionally based on real-valued coordinate transformations and, therefore, cannot naturally handle metamaterials featuring gain and/or losses. Motivated by the growing interest in non-Hermitian optical scenarios featuring spatial modulation of gain and loss, and building upon our previous studies, we explore here possible extensions of the TO framework relying on complex-valued coordinate transformations. We show that such extensions can be naturally combined with well-established powerful tools and formalisms in electromagnetics and optics, based on the 'complexification' of spatial and spectral quantities. This enables us to deal with rather general non-Hermitian optical scenarios, while retaining the attractive characteristics of conventional (real-valued) TO in terms of physically incisive modeling and geometry-driven intuitive design. As representative examples, we illustrate the manipulation of beam-like wave-objects (modeled in terms of 'complex source points') as well as radiating states ('leaky waves', modeled in terms of complex-valued propagation constants). Our analytical results, validated against full-wave numerical simulations, provide useful insight into the wave propagation in non-Hermitian scenarios, and may indicate new directions in the synthesis of active optical devices and antennas.

Cartographic distortions make dielectric spacetime analog models imperfect mimickers

Abstract: It is commonly assumed that if the optical metric of a dielectric medium is identical to the metric of a vacuum space-time then light propagation through the dielectric mimics light propagation in the vacuum. However, just as the curved surface of the Earth cannot be mapped into a flat plane without distortion of some surface features, so too is it impossible to project the behavior of light from the vacuum into a dielectric analog residing in Minkowski space-time without introducing distortions. We study the covariance properties of dielectric analog space-times and the kinematics of a congruence of light in the analog, and show how certain features can be faithfully emulated in the analog depending on the choice of projection, but that not all features can be simultaneously emulated without distortion. These findings indicate conceptual weaknesses in the idea of using analog space-times as a basis for transformation optics, and we show that a certain formulation of transformation optics closely related to analog space-times resolves these issues.

Symmetry-broken metamaterial for blocking, cloaking, and supertunneling of sound in a subwavelength scale

Abstract: Metamaterials offer extraordinary possibilities for manipulating the propagation of the sound wave in a subwavelength scale. However, the design of acoustic metamaterials remains challenging with traditional strategies, employing two different types of acoustic resonators simultaneously or using specific substructures with multiple resonances. Here we design a symmetry-broken metamaterial comprising of only Helmholtz resonators whose periodical spatial arrangements are broken. The symmetry-broken metamaterials form a hollow hexagonal resonant absorber with two significant resonances. One is the monopolar resonance presenting a collective in-phase pattern motion independent of angle. The other is the dipolar resonance originating from the multiple scattering of symmetry-broken metamaterials. By concentrating the sound energy in peaks of their modes, the hollow hexagonal resonant absorber with extremely small filling ratio can be effectively used to block the propagation of the sound wave in a low-frequency range between monopolar and dipolar resonances. Numerical results also show that the symmetry-broken metamaterials with careful arrangement can be applied to the sound cloaking and the sound supertunneling.

Invisibility Dips of Near-Field Energy Transport in a Spoof Plasmonic Metadimer

Abstract: Invisibility dips, i.e., minima in scattering spectrum associated with asymmetric Fano-like line-shapes, have been predicted with transformation optics in studying strong coupling between two plasmonic nanoparticles. This feature of strongly coupled plasmonic nanoparticles holds promise for sensor cloaking. It requires an extremely narrow gap between the two nanoparticles, preventing its experimental observation at optical frequencies. Here, the concept of spoof surface plasmons is used to facilitate the strong coupling between two spoof-localized-surface-plasmon (SLSP) resonators. Instead of observing in far field, the near-field energy transport is probed through the two SLSP resonators. By virtue of enhanced coupling between the two resonators stacked vertically, a spectral transmission dip with asymmetric Fano-like line-shape, similar to the far-field “invisibility dips” predicted by transformation optics, is observed. The underlying mode interference mechanism is further demonstrated by directly imaging the field maps of interfered waves that are tightly localized around the resonators. These near-field “invisibility dips” may find use in near-field sensing, on-chip switching, filters, and logical operation elements.