Showing papers on "Transformation optics published in 2014"
TL;DR: A bilayer thermal cloak made of bulk isotropic materials is demonstrated, and it has been validated as an exact cloak and the robustness of this scheme is validated in both 2D (including oblique heat front incidence) and 3D configurations.
Abstract: Invisibility has attracted intensive research in various communities, e.g., optics, electromagnetics, acoustics, thermodynamics, dc, etc. However, many experimental demonstrations have only been achieved by virtue of simplified approaches due to the inhomogeneous and extreme parameters imposed by the transformation-optic method, and usually require a challenging realization with metamaterials. In this Letter, we demonstrate a bilayer thermal cloak made of bulk isotropic materials, and it has been validated as an exact cloak. We experimentally verified its ability to maintain the heat front and its heat protection capabilities in a 2D proof-of-concept experiment. The robustness of this scheme is validated in both 2D (including oblique heat front incidence) and 3D configurations. The proposed scheme may open a new avenue to control the diffusive heat flow in ways inconceivable with phonons, and also inspire new alternatives to the functionalities promised by transformation optics.
431 citations
TL;DR: A viable recipe for controlling and manipulating heat signatures using thermal metamaterials to empower cloaking and camouflage in heat conduction is demonstrated.
Abstract: Thermal camouflage and cloaking can transform an actual heat signature into a pre-controlled one. A viable recipe for controlling and manipulating heat signatures using thermal metamaterials to empower cloaking and camouflage in heat conduction is demonstrated. The thermal signature of the object is thus metamorphosed and perceived as multiple targets with different geometries and compositions, with the original object cloaked.
336 citations
TL;DR: A transformational metasurface Luneburg lens based on the quasi-conformal mapping method, which has weakly anisotropic constitutive parameters, is presented in this article.
Abstract: We present a transformational metasurface Luneburg lens based on the quasi-conformal mapping method, which has weakly anisotropic constitutive parameters. We design the metasurface lens using inhomogeneous artificial structures to realize the required surface refractive indexes. The transformational metasurface Luneburg lens is fabricated and the measurement results demonstrate very good performance in controlling the radiated surface waves.
138 citations
TL;DR: In this paper, the optical properties of parity-time symmetric metamaterials composed of planar plasmonic waveguides were theoretically investigated, and it was shown how the initially isotropic material becomes both asymmetric and unidirectional.
Abstract: We theoretically investigate the optical properties of parity-time ($\mathcal{PT}$)-symmetric three-dimensional metamaterials composed of strongly coupled planar plasmonic waveguides. By tuning the loss-gain balance, we show how the initially isotropic material becomes both asymmetric and unidirectional. Investigation of the band structure near the material's exceptional point reveals several interesting optical properties, including double negative refraction, Bloch power oscillations, unidirectional invisibility, and reflection and transmission coefficients that are simultaneously equal to or greater than unity. The highly tunable optical dispersion of $\mathcal{PT}$-symmetric metamaterials provides a foundation for designing an unconventional class of three-dimensional bulk synthetic media, with applications ranging from lossless subdiffraction-limited optical lenses to nonreciprocal nanophotonic devices.
124 citations
TL;DR: In this paper, a prestressed plate theory is proposed to explain the shifting of the resonant frequency induced by the magnetic field and coincides very well with the experimental results, and the tunability of magneto-acoustic metamaterials is attributed to the competition between the magnetic-field-induced prestress and the structural flexural rigidity.
Abstract: Magnetically controlled acoustic metamaterials are designed and experimentally studied. Magneto-acoustic metamaterials are fabricated by covering an aluminum circular ring with magnetorheological elastomer. The resonant frequency of the structured elastomer is actively tunable by external gradient magnetic field, allowing for values of effective mass density of metamaterials to be adjusted in the low-frequency region. A prestressed plate theory is proposed to explain the shifting of the resonant frequency induced by the magnetic field and coincides very well with the experimental results. It is found that the tunability of magneto-acoustic metamaterials is attributed to the competition between the magnetic-field-induced prestress and the structural flexural rigidity. The proposed magneto-acoustic metamaterials realize the dynamic tuning of effective mass density with non-contact and fast-response gradient magnetic fields, providing a degree of freedom for full control of sound.
94 citations
94 citations
20 Aug 2014
TL;DR: A new class of one-dimensional optical transformations that exploits the mathematical framework of supersymmetry (SUSY) is introduced that can be utilized to synthesize photonic configurations with identical reflection and transmission characteristics, down to the phase, for all incident angles, thus rendering them perfectly indistinguishable to an external observer.
Abstract: Transformation optics aims to identify artificial materials and structures with desired electromagnetic properties by means of pertinent coordinate transformations. In general, such schemes are meant to appropriately tailor the constitutive parameters of metamaterials in order to control the trajectory of light in two and three dimensions. Here, we introduce a new class of one-dimensional optical transformations that exploits the mathematical framework of supersymmetry (SUSY). This systematic approach can be utilized to synthesize photonic configurations with identical reflection and transmission characteristics, down to the phase, for all incident angles, thus rendering them perfectly indistinguishable to an external observer. Along these lines, low-contrast dielectric arrangements can be designed to fully mimic the behavior of a given high-contrast structure that would have been otherwise beyond the reach of available materials and existing fabrication techniques. Similar strategies can also be adopted to replace negative-permittivity domains, thus averting unwanted optical losses.
93 citations
TL;DR: A type of all-dielectric metamaterials based on split bar resonators based on nano gap at the centre of the resonant elements results in large local field enhancement and light localization in the surrounding medium, which can be employed for strong light-matter interactions.
Abstract: Strong subwavelength field enhancement has often been assumed to be unique to plasmonic nanostructures. Here we propose a type of all-dielectric metamaterials based on split bar resonators. The nano gap at the centre of the resonant elements results in large local field enhancement and light localization in the surrounding medium, which can be employed for strong light-matter interactions. In a Fano-resonant dielectric metamaterial comprising pairs of asymmetric split silicon bars, the enhancement of electric field amplitude in the gap exceeds 120 while the averaged electromagnetic energy density is enhanced by more than 7000 times. An optical refractive index sensor with a potential sensitivity of 525 nm/RIU is designed based on the proposed metamaterials. The proposed concept can be applied to other types of dielectric nanostructures and may stimulate further research of dielectric metamaterials for applications ranging from nonlinear optics and sensing to the realization of new types of active lasing devices.
86 citations
TL;DR: To extract the plasmonic modes of the structure into the far field, two types of 1D grating with triangular and rectangular profile are implemented, obtaining a 10-fold radiative enhancement at visible frequencies.
Abstract: Hyperbolic metamaterials can enhance spontaneous emission, but the radiation-matter coupling is not optimized if the light source is placed outside such media. We demonstrate a 3-fold improvement of the Purcell factor over its outer value and a significant enlargement in bandwidth by including the emitter within a Si/Ag periodic multilayer metamaterial. To extract the plasmonic modes of the structure into the far field we implement two types of 1D grating with triangular and rectangular profile, obtaining a 10-fold radiative enhancement at visible frequencies.
78 citations
TL;DR: This work shows how the geometric formalism of transformation optics can be applied to describe the Cherenkov cone in an arbitrary anisotropic medium, leading to enhanced sensitivity for particle identification at higher momentum.
Abstract: In high energy physics, unknown particles are identified by determining their mass from the Cherenkov radiation cone that is emitted as they pass through the detector apparatus. However, at higher particle momentum, the angle of the Cherenkov cone saturates to a value independent of the mass of the generating particle, making it difficult to effectively distinguish between different particles. Here, we show how the geometric formalism of transformation optics can be applied to describe the Cherenkov cone in an arbitrary anisotropic medium. On the basis of these results, we propose a specific anisotropic metamaterial to control Cherenkov radiation, leading to enhanced sensitivity for particle identification at higher momentum.
75 citations
TL;DR: This work experimentally demonstrates a three-dimensional, transmitting, continuously multidirectional cloak in the visible regime using ray optics, and provides a concise formalism that quantifies and produces perfect optical cloaks in the small-angle ('paraxial') limit.
Abstract: Despite much interest and progress in optical spatial cloaking, a three-dimensional (3D), transmitting, continuously multidirectional cloak in the visible regime has not yet been demonstrated. Here we experimentally demonstrate such a cloak using ray optics, albeit with some edge effects. Our device requires no new materials, uses isotropic off-the-shelf optics, scales easily to cloak arbitrarily large objects, and is as broadband as the choice of optical material, all of which have been challenges for current cloaking schemes. In addition, we provide a concise formalism that quantifies and produces perfect optical cloaks in the small-angle (‘paraxial’) limit.
TL;DR: In this paper, the dispersion of optical waves propagating in nanowire media results from coupling of transverse and longitudinal electromagnetic modes supported by the nanowires and derive the nonlocal effective medium approximation for this dispersion.
Abstract: We present an analytical description of the nonlocal optical response of plasmonic nanowire metamaterials that enable negative refraction, subwavelength light manipulation, and emission lifetime engineering. We show that dispersion of optical waves propagating in nanowire media results from coupling of transverse and longitudinal electromagnetic modes supported by the nanowires and derive the nonlocal effective medium approximation for this dispersion. We derive the profiles of electric field across the unit cell, and use these expressions to solve the long-standing problem of additional boundary conditions in calculations of transmission and reflection of waves by nonlocal nanowire media. We verify our analytical results with numerical solutions of Maxwell's equations and discuss generalization of the developed formalism to other uniaxial metamaterials.
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TL;DR: A novel and general method for spatially manipulating DC currents has been proposed and experimentally verified by only using bilayer bulk natural conductive materials and shows distinctive advantages with respect to homogeneity, isotropy, and independence of complicated microfabrication techniques.
Abstract: The principle of transformation optics has been applied to various wave phenomena (e.g., optics, electromagnetics, acoustics and thermodynamics). Recently, metamaterial devices manipulating dc currents have received increasing attention which usually adopted the analogue of transformation optics using complicated resistor networks to mimic the inhomogeneous and anisotropic conductivities. We propose a distinct and general principle of manipulating dc currents by directly solving electric conduction equations, which only needs to utilize two layers of bulk natural materials. We experimentally demonstrate dc bilayer cloak and fan-shaped concentrator, derived from the generalized account for cloaking sensor. The proposed schemes have been validated as exact devices and this opens a facile way towards complete spatial control of dc currents. The proposed schemes may have vast potentials in various applications not only in dc, but also in other fields of manipulating magnetic field, thermal heat, elastic mechanics, and matter waves.
TL;DR: In this article, a new class of one-dimensional optical transformations that exploits the mathematical framework of supersymmetry (SUSY) is introduced, which can be utilized to synthesize photonic configurations with identical reflection and transmission characteristics, down to the phase, for all incident angles, thus rendering them perfectly indistinguishable to an external observer.
Abstract: Transformation optics aims to identify artificial materials and structures with desired electromagnetic properties by means of pertinent coordinate transformations. In general, such schemes are meant to appropriately tailor the constitutive parameters of metamaterials in order to control the trajectory of light in two and three dimensions. Here we introduce a new class of one-dimensional optical transformations that exploits the mathematical framework of supersymmetry (SUSY). This systematic approach can be utilized to synthesize photonic configurations with identical reflection and transmission characteristics, down to the phase, for all incident angles, thus rendering them perfectly indistinguishable to an external observer. Along these lines, low-contrast dielectric arrangements can be designed to fully mimic the behavior of a given high-contrast structure that would have been otherwise beyond the reach of available materials and existing fabrication techniques. Similar strategies can also be adopted to replace negative-permittivity domains, thus averting unwanted optical losses.
TL;DR: This work proposes an extension to arbitrary orientations of the principal axes of anisotropy and oblique incidence of the anisotropic effective slabs to determine the effective permeability and permittivity of metamaterials.
Abstract: Electromagnetic or acoustic metamaterials can be described in terms of equivalent effective, in general anisotropic, media and several techniques exist to determine the effective permeability and permittivity (or effective mass density and bulk modulus in the context of acoustics). Among these techniques, retrieval methods use the measured reflection and transmission coefficients (or scattering coefficients) for waves incident on a metamaterial slab containing few unit cells. Until now, anisotropic effective slabs have been considered in the literature but they are limited to the case where one of the axes of anisotropy is aligned with the slab interface. We propose an extension to arbitrary orientations of the principal axes of anisotropy and oblique incidence. The retrieval method is illustrated in the electromagnetic case for layered media, and in the acoustic case for array of tilted elliptical particles.
TL;DR: In this article, a momentum conservation approach is introduced to manipulate light at distance using metasurfaces, where a specified field existing on one side of the surface and a desired field transmitted from the opposite side, a general momentum boundary condition is established, which determines the amplitude, phase and polarization transformation to be induced by the surface.
Abstract: A momentum conservation approach is introduced to manipulate light at distance using metasurfaces. Given a specified field existing on one side of the metasurface and specified desired field transmitted from the opposite side, a general momentum boundary condition is established, which determines the amplitude, phase and polarization transformation to be induced by the metasurface. This approach, named momentum transformation, enables a systematic way to synthesize metasurfaces with complete control over the reflected and transmitted fields. Several synthesis illustrative examples are provided: a vortex hypergeometric-Gaussian beam and a “delayed-start” accelerated beam for Fresnel region manipulation, and a pencil beam radiator and a holographic repeater for Frauenhofer region manipulation.
01 Jan 2014
TL;DR: In this article, the authors used transformation optics (TO) to create anisotropic modulated-impedance metasurfaces able to transform planar surface waves (SW) into a predefined curved wavefront surface wave.
Abstract: Metasurfaces constitute a class of thin metamaterials, which are used from microwave to optical frequencies to create new antennas and microwave devices. This chapter describes how to use transformation optics (TO) to create anisotropic modulated-impedance metasurfaces able to transform planar surface waves (SW) into a predefined curved-wavefront surface wave. In fact, the modulated anisotropic impedance imposes a local modification of the dispersion equation and, at constant operating frequency, of the local wavevector. The general effects of metasurface modulation are similar to those obtained by TO in volumetric inhomogeneous metamaterials, namely readdressing the propagation path of an incident wave; however, significant technological simplicity is gained.
TL;DR: In this article, the authors proposed a bilayered asymmetrically split ring metamaterials for multi-band coherent perfect absorption (CPA) in a standing wave formed by two coherent counterpropagating beams.
Abstract: We show multi-band coherent perfect absorption (CPA) in simple bilayered asymmetrically split ring metamaterials. The selectivity of absorption can be accomplished by separately excited electric and magnetic modes in a standing wave formed by two coherent counterpropagating beams. In particular, each CPA can be completely switched on/off by the phase of a second coherent wave. We propose a practical scheme for realizing multi-band coherent perfect absorption of 100% that is allowed to work from microwave to optical frequency.
TL;DR: It is demonstrated that resonant optical forces generated within all-dielectric planar photonic metamaterials at near-infrared illumination wavelengths can be an order of magnitude larger than in corresponding plasmonic metammaterials.
Abstract: We demonstrate that resonant optical forces generated within all-dielectric planar photonic metamaterials at near-infrared illumination wavelengths can be an order of magnitude larger than in corresponding plasmonic metamaterials, reaching levels many tens of times greater than the force resulting from radiation pressure. This is made possible by the dielectric structures’ freedom from Joule losses and the consequent ability to sustain Fano-resonances with high quality factors that are unachievable in plasmonic nanostructures. Dielectric nano-optomechanical metamaterials can thus provide a functional platform for a range of novel dynamically controlled and self-adaptive nonlinear, tunable/switchable photonic metamaterials.
TL;DR: In this paper, the use of inhomogeneous metamaterial lenses is proposed to enable suitable radiation properties for arbitrary-shape antenna arrays, which is generalized to allow an arbitrary physical arrangement coated with a suitable lens to exhibit the same radiating features of an arbitrary reference virtual array in free space.
Abstract: The use of inhomogeneous metamaterial lenses is proposed to enable suitable radiation properties for arbitrary-shape antenna arrays. Towards this end, the Quasi-Conformal Transformation Optics (QCTO) methodology is generalized to allow an arbitrary physical arrangement coated with a suitable lens to exhibit the same radiating features of an arbitrary reference virtual array in free space. A representative numerical example, concerned with a two-dimensional layout, is presented to assess the effectiveness of the proposed method as well as the enhanced features of the resulting metamaterial-coated arrays with respect to standard conformal arrangements.
TL;DR: Three amplitude cloaks that can hide very large spatial objects over the entire visible spectrum using only passive, off-the-shelf optics are demonstrated.
Abstract: We demonstrate three amplitude cloaks that can hide very large spatial objects over the entire visible spectrum using only passive, off-the-shelf optics. The cloaked region for all of the devices exceeds 106 mm3, with the largest exceeding 108 mm3. Although unidirectional, these cloaks can hide the cloaked object, even if the object is transversely illuminated or self-illuminated. Due to the small usable solid angle, but simple scaling, these cloaks may be of value in hiding small field-of-view objects such as mid- to high-earth orbit satellites from earth-based observation. Active phase front manipulation can also make these cloaks invisible to some forms of image homodyning.
TL;DR: This work derives general coupled-mode equations describing the nonlinear interaction of electromagnetic modes in periodic media with loss and gain based on the Lorentz reciprocity theorem and predicts novel effects on self- and cross-phase modulation in multilayer nonlinear fishnet metamaterials.
Abstract: We derive general coupled-mode equations describing the nonlinear interaction of electromagnetic modes in periodic media with loss and gain. Our approach is rigorously based on the Lorentz reciprocity theorem, and it can be applied to a broad range of metal–dielectric photonic structures, including plasmonic waveguides and metamaterials. We verify that our general results agree with the previous analysis of particular cases, and predict novel effects on self- and cross-phase modulation in multilayer nonlinear fishnet metamaterials.
TL;DR: In this article, a superconductor-ferromagnetic metamaterial was used to concentrate the magnetostatic energy in its interior and in other situations to amplify the energy on its exterior.
Abstract: Concentrating magnetic energy in a desired volume is an important requirement for many technologies. Here, we experimentally realize a superconductor-ferromagnetic metamaterial that allows to concentrate the magnetostatic energy in its interior and in other situations to amplify the energy in its exterior. We show that surrounding two distant current loops with two such metamaterials enhance the magnetostatic coupling between them. We also demonstrate that a ferromagnetic-only metamaterial, without superconducting parts, achieves these properties with only a slight decrease in performance. Results may be applied to increase the sensitivity of magnetic sensors or for enhancing wireless power transmission, where efficiency depends critically on the magnetic coupling strength between source and receiver.
TL;DR: In this article, a generalized Floquet analysis is presented to discuss wave propagation in twisted metamaterials, and the eigenmodes support specific circular polarization properties based on a lattice effect even when achiral inclusions are considered.
Abstract: Twisted metamaterials, i.e. arrays of identical planar metasurfaces stacked with a sequential rotation, offer new possibilities for manipulating and engineering wave propagation. This paper presents a generalized Floquet analysis to discuss wave propagation in these kind of structures. The authors show that the eigenmodes support specific circular polarization properties based on a lattice effect even when achiral inclusions are considered.
TL;DR: In this article, a generalized Bloch method is adopted to find the dominant modes of the volumetric homogenized metamaterial, and different levels of equivalence are defined, denoted as external equivalence, dispersion equivalence and double-mode equivalence.
Abstract: Different procedures are presented for the retrieval of effective constitutive parameters of metamaterial structures realized by multilayer lossless periodic structures. These structures are modeled as Floquet-wave based multiport networks connected by pieces of transmission lines. A generalized Bloch method is then adopted to find the dominant modes of the volumetric homogenized metamaterial. Starting from the results of this analysis, the constitutive tensors of an equivalent homogeneous material are defined. Different levels of equivalence are defined, denoted as “external equivalence,” “dispersion equivalence,” “single-mode equivalence,” and “double-mode equivalence.” The corresponding homogenization processes are characterized by an increasing level of completeness (in the description of the metamaterial behavior) and of sophistication (concerning the processing of the full-wave analysis outcomes). The defined homogenization procedure accounts for dispersion in time (frequency) and space (wavenumber), as well as for anisotropy.
TL;DR: In this paper, a simplified quadratic cloak has been proposed which provides finite material properties at the inner radius of the cloak and impedance match with the free space at the outer radius of a cloak simultaneously.
Abstract: The presence of extreme material properties and undesired scattering make practical implementation of transformation optics based cloaks an extremely challenging issue. To overcome these problems, a simplified quadratic cloak has been proposed which provides finite material properties at the inner radius of the cloak and impedance match with the free space at the outer radius of the cloak simultaneously. The simultaneous occurrence of finite material properties and impedance match with the free space at the respective boundaries reduces the scattering cross section significantly. The material properties of the proposed simplified quadratic cloak can be realized with two dimensional (2D) metamaterials. The performance of the proposed cloak is examined by plotting the normalized total scattering cross section (SCSt,norm), and the scattering patterns in azimuthal plane. The proposed simplified quadratic cloak shows 10 dB reduction of the scattered field in both the forward and backward directions with respec...
TL;DR: It is demonstrated that for wavelengths leading to ℜ(εz) ≈ 0, near-perfect absorption arises and persists over a range of frequencies and subwavelength structure thicknesses.
Abstract: We investigate the interaction of polarized electromagnetic waves with hyperbolic metamaterial structures, whereby the in-plane permittivity component $\epsilon_x$ is opposite in sign to the normal component $\epsilon_z$. We find that when the thickness of the metamaterial is smaller than the wavelength of the incident wave, hyperbolic metamaterials can absorb significantly higher amounts of electromagnetic energy compared to their conventional counterparts. We also demonstrate that for wavelengths leading to $\Re(\epsilon_z) \approx 0$, near-perfect absorption arises and persists over a range of frequencies and subwavelength structure thicknesses.
TL;DR: In this paper, a new class of dielectric sheet antennas is presented based on transformation optics, and the authors successfully implement the transformation with a consideration of surface wave eigenmode properties, and carry out a practical perforated design by drilling holes to synthesize the required Dielectric constants for their potential realization.
Abstract: A new class of dielectric sheet antennas is presented based on transformation optics. Specifically, we successfully implement the transformation with a consideration of surface wave eigenmode properties, and carry out a practical perforated design by drilling holes to synthesize the required dielectric constants for their potential realization. The full wave simulation shows that our surface wave transformation lens antenna is capable of obtaining a greatly improved directivity, while maintaining the original low profile.
TL;DR: By incorporating the conducting layer developed in the present paper right between the cloaked region and the cloaking region, arbitrary electromagnetic contents can be nearly cloaked.
Abstract: The approximate cloaking is investigated for time-harmonic Maxwell's equations via the approach of transformation optics. The problem is reduced to certain boundary effect estimates due to an inhomogeneous electromagnetic inclusion with an asymptotically small support but an arbitrary content enclosed by a thin high-conducting layer. Sharp estimates are established in terms of the asymptotic parameter, which are independent of the material tensors of the small electromagnetic inclusion. The result implies that the “blow-up-a-small-region” construction via the transformation optics approach yields a near-cloak for the electromagnetic waves. A novelty lies in the fact that the geometry of the cloaking construction of this work can be very general. Moreover, by incorporating the conducting layer developed in the present paper right between the cloaked region and the cloaking region, arbitrary electromagnetic contents can be nearly cloaked. Our mathematical technique extends the general one developed in [H. Y...
TL;DR: In this paper, a chiral metamaterial with a broken four-fold symmetry of chiral structure was proposed to make the polarization plane of a linearly polarized electromagnetic wave reflected from its surface almost perpendicular to the polarization planes of the incident wave.
Abstract: Despite chiral metamaterials being widely appreciated for their giant optical activity and negative refractive index with respect to the transmitted electromagnetic waves, little research efforts are devoted to effects occurring upon optical reflection from such metamaterials. Here, we theoretically demonstrate highly energy-efficient cross-polarization conversion for a normally incident wave reflecting from metamaterial with a broken four-fold symmetry of chiral structure. We do this by designing metamaterial that makes the polarization plane of a linearly polarized electromagnetic wave reflected from its surface almost perpendicular to the polarization plane of the incident wave. Using reflection from this metamaterial, one can also freely convert between left-handed and right-handed circular polarizations without significant energy loss. The proposed chiral metamaterial may prove useful in electromagnetic communication systems, polarization controllable antennas, and on-chip biomedical sensors.