Isotropic Negative Index Research Articles

Structures for Metamaterial Superlenses Used in Medical Imaging

The metamaterial superlenses for imaging proteins, viruses, and DNA have to present a high resolution, which cannot be ensured by classical lenses. Such a material for lenses exceeds the Abbe-Rayleigh diffraction limit, leading to a nanoscale level of resolution, a several times better than the classical diffraction limits. We have illustrated here a hotspot size of ca. 0.25 – 0.28 λ, corresponding to a numerical aperture of about 1.4. The used metamaterial structures are isotropic negative index metamaterials (NIMs), with negligible losses. Material combinations of metallic nanoparticles (with dimensions of tens of nm) inserted in a dielectric slab have been considered for study. Microcomponents periodicity in the layer is of a few hundred of naometers. Material properties evolve in function of the constituent’s nature and dimensions. Refraction index in function of wavelength was determined and represented on graphs in order to illustrate the domain of negative values and the manner in which it can be controlled at structure level. Analysis was performed in visible and IR range by simulation methods, using the HFSS program and a proper algorithm based on physical considerations.

Transformation optics design of a planar near field magnifier for sub-diffraction imaging.

It is well established that, under certain conditions, imaging systems with either isotropic negative index, or hyperbolic (indefinite) media can achieve super-resolution. However, achieving sub-diffraction limited imaging along with uniform aberration-free magnification can be challenging. In this article, we design, simulate, and evaluate the performance of planar 2D near-field magnifying lenses, based on the transformation-optic design principle. Specifically, we investigate a grid-relaxed transformation, that results in material properties that are more amenable to implementation. We discuss possible design choices in terms of: material properties, achievable resolution enhancement, adverse effect of loss tangent, magnification factor, and other design constraints affecting the imaging performance. We also present imaging performance results for a planar, near-field, 3× magnifier operating on a standard resolution target, based on a rigorous, 3D, electromagnetic simulation. This computational intensive result was achieved using cylindrical harmonic decomposition and the 2.5D electromagnetic simulation technique. Further, we investigate and propose a path to achieve higher magnification factors using cascaded elements.

Thermotunable Terahertz Negative-Index Metamaterials with Dielectric Spheres Embedded in Semiconductor Host

A possibility to realize thermotunable isotropic negative-index metamaterials up to terahertz regime is theoretically investigated. The proposed composite metamaterials consist of dielectric spheres embedded randomly in a semiconductor host medium. As the variation in the intrinsic carrier density in InSb due to a variation in temperature thus changes the plasma frequency, we show theoretically the provided composite metamaterials whose index of refraction is thermally tunable and potentially less propagation loss. Furthermore, the effects of the radius of the dielectric sphere on the modulation frequency and bandwidth are discussed. Finally, we find that the design parameters for the composites can be scaled for application in the higher frequency regions.

Spatial frequency maps of power flow in metamaterials and photonic crystals: Investigating backward-wave modes across the electromagnetic spectrum

We present spatial frequency maps of power flow in metamaterials and photonic crystals in order to provide insights into their electromagnetic responses and further our understanding of backward power in periodic structures. Since 2001, many different structures across the electromagnetic spectrum have been presented in the literature as exhibiting an isotropic negative effective index. Although these structures all exhibit circular or spherical equifrequency contours that resemble those of left-handed media, here we show through $k$-space diagrams that the distribution of power in the spatial frequency domain can vary considerably across these structures. In particular, we show that backward power arises from high-order right-handed harmonics in photonic crystals, magnetodielectric crystals, and across the layers of coupled-plasmonic-waveguide metamaterials, while arising from left-handed harmonic pairs in split-ring resonator and wire composites, plasmonic crystals, and along the layers of coupled-plasmonic-waveguide metamaterials. We also show that the fishnet structure exhibits the same left-handed harmonic pairs as the latter group. These observations allow us to categorize different metamaterials according to their spatial spectral source of backward power and identify the mechanism behind negative refraction at a given interface. Finally, we discuss how $k$-space maps of power flow can be used to explain the high or low transmittance of power into different metamaterial or photonic crystal structures.

Modulation instability of structured-light beams in negative-index metamaterials

One of the most fundamental properties of isotropic negative-index metamaterials (NIMs), namely opposite directionality of the Poynting vector and the wavevector, enable many novel linear and nonlinear regimes of light–matter interactions. Here, we predict distinct characteristics of azimuthal modulation instability (MI) of optical vortices with different topological charges in NIMs with Kerr-type and saturable nonlinearity. We derive an analytical expression for the spatial modulation-instability gain for the Kerr-nonlinearity case and show that a specific condition relating the diffraction and the nonlinear lengths must be fulfilled for the azimuthal MI to occur. Finally, we investigate the rotation of the necklace beams due to the transfer of orbital angular momentum of the generating vortex on the movement of solitary necklace beams. We show that the direction of rotation is opposite in positive- and negative-index materials.

Broadband 3D isotropic negative-index metamaterial based on fishnet structure

In this paper, we numerically demonstrate a broadband 3D isotropic negative index metamaterial (NIM) at microwave frequency ranges, which is composed of double periodic array metallic fishnet structure (FS) etched on the six sides of a cubic dielectric substrate. The electric and magnetic L-C resonance circuit models are constructed to demonstrate the broadband resonance properties of the proposed 3D metamaterial. The finite integration technology (FIT) simulation and standard S parameters retrieval methods are used to calculate and analyze the negative characteristics, isotropy and polarization of the 3D model. The numerical results show that the negative index bandwidth is about 7 GHz and relative bandwidth can be up nearly to 63%, the negative-index pass band is independent of the polarization of incident waves and is almost the same for different oblique incident angles. Thus, the proposed metamaterial is good candidate as a broad-band 3D isotropic NIMs.

Proposed isotropic negative index in three-dimensional optical metamaterials

A simple route toward achieving an isotropic optical negative index in three dimensions is theoretically proposed. We show that, in contrast with previous studies, the plasmonic ring resonators, symmetrically split with an odd number of gaps, have both degenerate electric and magnetic resonances and thus provide an isotropic negative index if randomly distributed in a host medium. For an even number of gaps, the electric and magnetic dipoles sufficiently overlap only by additional symmetry breaking, which is demonstrated to allow unique control of the relative contribution from higher-order multipolar moments.

Metallo-dielectric core–shell nanospheres as building blocks for optical three-dimensional isotropic negative-index metamaterials

Materials showing electromagnetic properties that are not attainable in naturally occurring media, so-called metamaterials, have been lately, and still are, among the most active topics in optical and materials physics and engineering. Among these properties, one of the most attractive ones is the sub-diffraction resolving capability predicted for media having an index of refraction of −1. Here, we propose a fully three-dimensional, isotropic metamaterial with strong electric and magnetic responses in the optical regime, based on spherical metallo-dielectric core–shell nanospheres. The magnetic response stems from the lowest, magnetic-dipole resonance of the dielectric shell with a high refractive index, and can be tuned to coincide with the plasmon resonance of the metal core, responsible for the electric response. Since the response does not originate from coupling between structures, no particular periodic arrangement needs to be imposed. Moreover, due to the geometry of the constituents, the metamaterial is intrinsically isotropic and polarization independent. It could be realized with current fabrication techniques with materials such as silver (core) and silicon or germanium (shell). For these particular realistic designs, the metamaterials present a negative index in the range of 1.2–1.55 μm.

Intra-connected three-dimensionally isotropic bulk negative index photonic metamaterial

Isotropic negative index metamaterials (NIMs) are highly desired, particularly for the realization of ultra-high resolution lenses. However, existing isotropic NIMs function only two-dimensionally and cannot be miniaturized beyond microwaves. Direct laser writing processes can be a paradigm shift toward the fabrication of three-dimensionally (3D) isotropic bulk optical metamaterials, but only at the expense of an additional design constraint, namely connectivity. Here, we demonstrate with a proof-of-principle design that the requirement connectivity does not preclude fully isotropic left-handed behavior. This is an important step towards the realization of bulk 3D isotropic NIMs at optical wavelengths.

Compact planar far-field superlens based on anisotropic left-handed metamaterials

Pendry's perfect lens has spurred intense interest for its practical realization at visible frequencies. However, fabrication of low-loss isotropic left-handed metamaterials is a current challenge. In this work, we theoretically show that under specific conditions anisotropic metamaterial slabs can emulate Pendry's perfect-lens phenomenon on a plane. Geometric optics leads to a new lens formula for this special anisotropic metamaterial superlens, which allows significant shrinkage of the metamaterial slab thickness for a certain range of far-field operation. Conversely, such anisotropic metamaterial superlens with the same thickness as its isotropic analog can operate for much larger distances between object and lens. We present numerical simulations which confirm our theoretical calculations. In particular, we find subdiffraction focusing that rivals the perfect isotropic negative-index metamaterial lens performance and obeys the new lens formula as predicted. In addition, we demonstrate that it is possible to attain far-field superfocusing with a metamaterial slab as thin as half the free-space wavelength. We believe this work will inspire new anisotropic metamaterial designs and opens a promising route for the realization of compact far-field superlenses in the visible regime.

A phase conjugate mirror inspired approach for building cloaking structures with left-handed materials

In this paper, we propose and examine a new cloaking method, which was inspired by the close correspondence between a phase conjugate mirror and the interface between a pair of matched right-handed material (RHM) and left-handed material (LHM) media. Using this method, we show that a symmetric conducting shell embedded in the interface junction of an isotropic RHM layer and an isotropic negative index or LHM layer can serve as a limited cloaking structure. The proposed structure presents an anomalously small scattering cross-section to an incident propagating electromagnetic (EM) field. The interior of the shell can be used to shield small objects from interrogation. We report the results of 2D finite-element-method (FEM) simulations that were performed to verify the principle, and discuss the limitations of the proposed structure.

Nested structures approach in designing an isotropic negative-index material for infrared

We propose a new generic approach for designing isotropic metamaterial with nested cubic structures. As an example, a three-dimensional isotropic unit cell design "Split Cube in Cage" (SCiC) is shown to exhibit an effective negative refractive index on infrared wavelengths. We report on the refractive index reaching -2.3 and the figure of merit as high as 2.7. The structure exhibits potential for application as a building block of isotropic negative-index materials.

Optically isotropic negative index of refraction metamaterial

AbstractWe report a metamaterial with optically isotropic negative refraction index, n, and small loss factor, k = |Im (n)/Re (n)| ≤ 0.4, that occurs near the visible/UV line. The metamaterial presented consists of two media. One medium (randomly distributed gold spherical nanoparticles with Drude‐like permittivity, in free space) ensures negative permittivity, and another medium (spherical SiC nanoparticles) ensures negative permeability via the Mie resonance within the same frequency range. We have found, self‐consistently, in the framework of both the effective medium approach and rigorous first‐principles based Finite Difference Time Domain (FDTD) simulations, that the metamaterial exhibits negative refraction index behavior. This result stands for both the random distribution of the SiC particles, and the regular simple‐cubic lattice of the same particles. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Effects of material composition on the superlens frequency of photonic crystals

We use the plane-wave (PW) method to calculate the superlens frequency of photonic crystals with various material compositions. At this frequency, photonic crystal behaves like a medium with isotropic negative index equal to −1. The relationship between the frequency and material compositions is derived from the calculated data. For the TE and TM modes, the relationship has the same format. From the relationship, a case has been chosen and, under these conditions, the wave-propagating field through the photonic crystal has been calculated by the finite-difference time-domain method. A good agreement is obtained between the results from the PW method and the finite-difference time-domain calculation. This is very useful for fabricating photonic crystal superlens material at an appropriate frequency.

Omnidirectional gaps of one-dimensional photonic crystals containing indefinite metamaterials

The explicit dispersion relation for one-dimensional photonic crystals consisting of alternative layers of indefinite metamaterial and ordinary positive-index material is obtained and analyzed in detail. It is shown that such a photonic crystal possesses an omnidirectional zero-averaged refractive-index gap when the indefinite metamaterial is a cutoff or anticutoff medium, yet it possesses an omnidirectional zero-effective phase gap when the photonic crystal contains two different always-cutoff media. The appearance of these two omnidirectional gaps does not require that all tensor elements of permittivity and permeability have negative values, quite different from that for the photonic crystal containing isotropic negative-index or single-negative material. Moreover, the new zero-averaged refractive-index gap and zero-effective phase gap are also insensitive to the incident angle and polarization of light and are invariant upon the change of scale length. These properties provide more degrees of freedom to design new types of optical devices.

Construction of a polarization insensitive lens from a quasi-isotropic metamaterial slab

We propose to employ the quasi-isotropic metamaterial (QIMM) slab to construct a polarization insensitive lens, in which both E- and H-polarized waves exhibit the same refocusing effect. For shallow incident angles, the QIMM slab will provide some degree of refocusing in the same manner as an isotropic negative index material. The refocusing effect allows us to introduce the ideas of paraxial beam focusing and phase compensation by the QIMM slab. On the basis of angular spectrum representation, a formalism describing paraxial beams propagating through a QIMM slab is presented. Because of the negative phase velocity in the QIMM slab, the inverse Gouy phase shift and the negative Rayleigh length of paraxial Gaussian beam are proposed. We find that the phase difference caused by the Gouy phase shift in vacuum can be compensated by that caused by the inverse Gouy phase shift in the QIMM slab. If certain matching conditions are satisfied, the intensity and phase distributions at object plane can be completely reconstructed at image plane. Our simulation results show that the superlensing effect with subwavelength image resolution could be achieved in the form of a QIMM slab.

Negative index and indefinite media waveguide couplers

We study the coupling interaction between dielectric waveguides and coupling elements made from negative-refracting media. The coupling configuration consists of a length of dielectric waveguide, which terminates either directly into or near a planar layer composed of the negative-refracting medium, and is followed by a second waveguide. Radiation output from the first waveguide is refocused at the position of the second waveguide, so that the negative-refracting layer serves as a coupler between the waveguides. Because both isotropic negative-index layers and bilayers of indefinite media can recover the near-field, evanescent components of a source field distribution, the coupling between the input and output waveguides can be highly efficient – in principle providing perfect, lossless coupling. We present simulations and some initial experimental results illustrating the coupling effect, and speculate on the potential for optical fiber couplers and integrated modulators.

Coated nonmagnetic spheres with a negative index of refraction at infrared frequencies

A three-dimensional lattice of micron-scale coated spheres is shown to have an isotropic negative index of refraction at infrared frequencies. The materials used are entirely non-magnetic. The Mie scattering theory of the constituent spheres is used in the effective medium theory. The physical mechanisms and procedures are presented in the design of a negative effective permeability with solid polaritonic spheres, as well as a negative effective permittivity with solid Drude spheres. It is then shown that a collection of polaritonic spheres coated with a thin layer of Drude material can exhibit a negative index of refraction at infrared frequencies. Comparison with numerical photonic band structure calculations verifies the theory.

Realization of 3D Isotropic Negative Index Materials using Massively Parallel and Manufacturable Microfabrication and Micromachining Technology

AbstractIn this paper, we present a method to realize a three dimensional (3D) homogeneous and isotropic negative index materials (3D-NIMs) fabricated using a low cost and massively parallel manufacturable microfabrication and microassembly technique. The construction of self-assembled 3D-NIM array was realized through two dimensional (2-D) planar microfabrication techniques exploiting the as-deposited residual stress imbalance between a bi-layer consisting of e-beam evaporated metal (650nm of chromium) and a structural layer of 500nm of low stress silicon nitride deposited by LPCVD on a silicon substrate.A periodic continuation of a single rectangular unit cell consisting of split-ring resonators (SRR) and wires were fabricated to generate a 3D assembly by orienting them along all three Cartesian axes. The thin chromium and silicon nitride bi-layer is formed as hinges. The strain mismatch between the two layers curls the structural layer (flap) containing the SRR upwards. The self-assembled out-of-plane angular position depends on the thickness and material composing the bi-layer. This built-in stress-actuated assembly method is suitable for applications requiring a thin dielectric layer for the SRR. The split-ring resonators and other structures are created on the membrane which is then assembled into the 3-D configuration.

Experimental demonstration of labyrinth-based left-handed metamaterials

In this present work, we propose and demonstrate a resonant structure that solves two major problems related to the split-ring resonator structure. One of the problems related to the split-ring resonator structure is the bianisotropy, and the other problem is the electric coupling to the magnetic resonance of the split-ring resonator structure. These two problems introduce difficulties in obtaining isotropic left-handed metamaterial mediums. The resonant structure that we propose here solves both of these problems. We further show that in addition to the magnetic resonance, when combined with a suitable wire medium, the structure that we propose exhibits left-handed transmission band. We believe that the structure we proposed may have important consequences in the design of isotropic negative index metamaterial mediums.