Response Of Metamaterials Research Articles

Magnetic field concentrator for probing optical magnetic metamaterials

Development of all dielectric and plasmonic metamaterials with a tunable optical frequency magnetic response creates a need for new inspection techniques. We propose a method of measuring magnetic responses of such metamaterials within a wide range of optical frequencies with a single probe. A tapered fiber probe with a radially corrugated metal coating concentrates azimuthally polarized light in the near-field into a subwavelength spot the longitudinal magnetic field component which is much stronger than the perpendicular electric one. The active probe may be used in a future scanning near-field magnetic microscope for studies of magnetic responses of subwavelength elementary cells of metamaterials.

Physical restrictions on the Casimir interaction of metal-dielectric metamaterials: An effective-medium approach

Using an effective-medium approach, we demonstrate that the Casimir interaction of structured metal-dielectric metamaterial slabs which effectively behave as either uniform nongyrotropic materials or bi-isotropic materials is attractive at all distances, independent of the emergence of artificial magnetism or strong magnetoelectric coupling, when the slabs stand in a vacuum. In particular, it is shown that the magnetic response of a metal-dielectric metamaterial is always diamagnetic at imaginary frequencies, and this explains in simple physical terms the impossibility of Casimir repulsion.

Metamaterial tuning by manipulation of near-field interaction

We analyze the near-field interaction between the resonant sub-wavelength elements of a metamaterial, and present a method to calculate the electric and magnetic interaction coefficients. We show that by adjusting the relative configuration of the neighboring split ring resonators it becomes possible to manipulate this near-field interaction, and thus tune the response of metamaterials. We use the results of this analysis to explain the experimentally observed tuning of microwave metamaterials.

Thermally tunable optical constants of vanadium dioxide thin films measured by spectroscopic ellipsometry

Smart materials with reversible tunable optical constants from visible to near-infrared wavelengths could enable excellent control over the resonant response in metamaterials, tunable plasmonic nanostructures, optical memory based on phase transition and thermally tunable optical devices. Vanadium dioxide (VO 2) is a promising candidate that exhibits a dramatic change in its complex refraction index or complex dielectric function arising from a structural phase transition from semiconductor to metal at a critical temperature of 70 °C. We demonstrated the thermal controllable reversible tunability of optical constants of VO 2 thin films. The optical/dielectric constants showed an abrupt thermal hysteresis which confirms clearly the electronic structural changes. Temperature dependence of dielectric constants as well as optical conductivity of sputtered VO 2 thin films was also reported and compared to previous theoretical and experimental reports.

Metamaterials design using gradient-free numerical optimization

We apply numerical optimization methods in combination with full-field electromagnetic simulations to tailor the broadband spectral response of gold and silver split-ring resonator metamaterials. A derivative-free, nonlinear mesh adaptive search technique was used to drive finite-difference time-domain simulations. This algorithm allows the designer to independently vary the spectral position of the two resonant peaks and their relative reflection amplitudes throughout a wide range of the near infrared. An application of this method is then shown to design split-ring resonator “notch filters,” with narrow pass bands at 1310, 1550, and 1800 nm which have an ∼45% change in reflectivity at the pass band and corresponding linewidths of ∼90 meV.

Efficient homogenization procedure for the calculation of optical properties of 3D nanostructured composites

We present a very efficient recursive method to calculate the effective optical response of metamaterials made up of arbitrarily shaped inclusions arranged in periodic 3D arrays. We apply it to dielectric particles embedded in a metal matrix with a lattice constant much smaller than the wavelength of the incident field, so that we may neglect retardation and factor the geometrical properties from the properties of the materials. If the conducting phase is continuous the low frequency behavior is metallic, and if the conducting paths are thin, the high frequency behavior is dielectric. Thus, extraordinary-transparency bands may develop at intermediate frequencies, whose properties may be tuned by geometrical manipulation.

Influence of Filling Medium of Holes on the Negative-Index Response of Sandwiched Metamaterials

We numerically study the negative index properties of sandwiched metamaterials, perforated with a square array of circle holes filled with different media. Transmission spectra indicate that the filling medium can effectively change the position of the localized resonant peak, while keeping the position of the other transmission peaks hardly changed. Reflection spectra and retrieved effective impedance verify that an appropriate choice of the filling medium can provide a perfect impedance match. Due to the perfect impedance match, the electromagnetic responses of the negative index band based on the internal surface plasmon polaritons change in many aspects, such as a stronger magnetic resonance, a higher figure of merit and a narrower negative refractive index band.

Optical properties of nanostructured metamaterials

AbstractWe present a very efficient recursive method to calculate the effective optical response of nanostructured metamaterials made up of particles with arbitrarily shaped cross sections arranged in periodic two‐dimensional arrays. We consider dielectric particles embedded in a metal matrix with a lattice constant much smaller than the wavelength. Neglecting retardation our formalism allows factoring the geometrical properties from the properties of the materials. If the conducting phase is continuous the low frequency behavior is metallic. If the conducting paths are nearly bloqued by the dielectric particles, the high frequency behavior is dielectric. Thus, extraordinary‐reflectance bands may develop at intermediate frequencies, where the macroscopic response matches vacuum. The optical properties of these systems may be tuned by adjusting the geometry.magnified image Sketch of a nanostructured metamaterial slab with a dielectric‐like or metallic‐like behavior depending on the frequency of the incoming light.

Influence of the substrate on negative index fishnet metamaterials

We analyze numerically the influence of the dielectric substrate on the optical response and effective macroscopic parameters of fishnet metamaterials. We show that the substrate can suppress the region of the negative refractive index due to the weakening of the gap plasmon associated with the magnetic resonance of the structure. We also demonstrate that the effect of substrate vanishes when the number of the functional layers increases.

Magnetic response of nanoscale left-handed metamaterials

Using detailed simulations we investigate the magnetic response of metamaterials consisting of pairs of parallel slabs or combinations of slabs with wires (including the fishnet design) as the length-scale of the structures is reduced from mm to nm. We observe the expected saturation of the magnetic resonance frequency when the structure length-scale goes to the sub-micron regime, as well as weakening of the effective permeability resonance and reduction of the spectral width of the negative permeability region. All these results are explained by using an equivalent resistor-inductor-capacitor (RLC) circuit model, taking into account the current-connected kinetic energy of the electrons inside the metallic parts through an equivalent inductance, added to the magnetic field inductance in the unit-cell. Using this model we derive simple optimization rules for achieving optical negative permeability metamaterials of improved performance. Finally, we analyze the magnetic response of the fishnet design and we explain its superior performance regarding the high attainable magnetic resonance frequency, as well as its inferior performance regarding the width of the negative permeability region.

An infrared invisibility cloak composed of glass

We propose to implement a nonmetallic low-loss cloak for the infrared range from identical chalcogenide glass resonators. Based on transformation optics for cylindrical objects, our approach does not require metamaterial response to be homogeneous and accounts for the discrete nature of elementary responses governed by resonator shape, illumination angle, and inter-resonator coupling. Air fractions are employed to obtain the desired distribution of the cloak effective parameters. The effect of cloaking is verified by full-wave simulations of the true multiresonator structure. The feasibility of cloak fabrication is demonstrated by prototyping glass grating structures with the dimensions characteristic for the cloak resonators.

Strong lateral displacement in polarization anisotropic extraordinary transmission metamaterial

In this paper, a deep numerical as well as experimental study of the anisotropy response of extraordinary transmission metamaterials constructed by stacking subwavelength hole arrays is presented. Two-dimensional (2D) dispersion diagrams for S- and P-polarization were obtained from simulation. From them, it was found that negative refraction can be obtained for the latter case for small angles of incidence. Additionally, it was found that double periodic and dielectric loaded hole arrays are optimal to enlarge the numerical aperture that leads to negative refraction. Several experiments are then presented in the V-band of the millimetre-wave range that show excellent agreement with the numerical calculations. Moreover, the richness of the anisotropic characteristic exhibited by the stacked hole array structure allows for designing structures with complex electromagnetic response other than solely negative refraction. Thus, the results presented here could be taken as a novel route to achieve exotic behaviour, such as negative refraction at other frequency ranges, like terahertz or the visible.

Tunable fishnet metamaterials infiltrated by liquid crystals

We analyze numerically the optical response and effective macroscopic parameters of fishnet metamaterials infiltrated with a nematic liquid crystal. We show that even a small amount of liquid crystal can provide tuning of the structures due to reorientation of the liquid crystal director. This enables switchable optical metamaterials, where the refractive index can be switched from positive to negative by an external field. This tuning is primarily determined by the shift in the cut-off wavelength of the holes, with only a small influence due to the change in plasmon dispersion.

External modulators for TeraHertz Quantum Cascade Lasers based on electrically-driven active metamaterials

We have designed, fabricated and measured electrically-driven active metamaterials which operate as external modulators for TeraHertz Quantum Cascade Lasers. The modulation is achieved by applying a voltage to the metamaterial layer which actively displaces carriers from the n-doped layer causing changes in damping and frequency location of the lowest metamaterial response. We demonstrate their operation at 2.4 and 2.8 TeraHertz and obtain a maximum modulation depth of ∼60% with a large degree of modulation linearity.

Singular, hypersingular and singular free electromagnetic fields at wedge tips in metamaterials

The engineering response of metamaterials has a dramatic impact on the physics, optics and engineering communities, because they offer electromagnetic properties that are difficult or impossible to achieve with conventional materials. In this paper, an asymptotic analysis of the electromagnetic fields at multi-material wedges composed of metamaterials is proposed. This is made possible by removing the assumption of positive electric permittivities and magnetic permeabilities, an hypothesis which usually applies to conventional materials. Exploring the whole range of variability of these electromagnetic properties, it is shown that, in addition to the classical real eigenvalues 0 ⩽ λ < 1 leading to power-law singularities of the type O( r λ−1 ) as r → 0, it is also possible to find imaginary eigenvalues leading to hypersingular solutions, as well as nonsingular configurations for a suitable choice of the negative electric permittivities and magnetic permeabilities of the media. Moreover, to fully characterize the asymptotic fields, the analysis is not only restricted to the determination of the lowest real and complex eigenvalues, but is also extended to the evaluation of the higher-order nonsingular ones. The obtained analytical results collected in synthetic diagrams are expected to have impact on the design of micro- and nano-electro-mechanical systems.

Discrete dipole approximation for the study of radiation dynamics in a magnetodielectric environment

We develop a general computational approach, based on the discrete dipole approximation, for the study of radiation dynamics near or inside an object with arbitrary linear dielectric permittivity, and magnetic permeability tensors. Our method can account for dispersion and losses and provides insight on the role of local-field corrections in discrete magnetodielectric structures. We illustrate our method in the case of a source inside a magneto-dielectric, isotropic sphere for which the spontaneous emission rate of a source can be computed analytically. We show that our approach is in excellent agreement with the exact result, providing an approach capable of handling both the electric and magnetic response of advanced metamaterials.

Commentary: Controlling electromagnetic metamaterials

The last two decades have been witness to two exciting and independent developments that have forever changed our conventional view of how light interacts with matter. One relates to coherent control via quantum interference, wherein the possibility of making an otherwise opaque medium transparent [1], now known as electromagnetically induced transparency (EIT), set off intense research activity. EIT essentially requires careful creation of atomic coherence, that results in diverse effects varying from almost freezing light in its tracks (slow light) to freezing atoms to nanoKelvin temperatures via velocity-selective coherent population trapping. The second development relates to metamaterials whose origins are very classical in nature. In electromagnetics, these designer materials were originally proposed for realizing a super-lens wherein the evanescent field becomes the work horse that accords sub-wavelength resolution in imaging [2]. Since then a variety of metamaterials have been proposed, where even the propagating fields can be dramatically controlled, as in electromagnetic cloaks wherein the fields are maneuvered around an obstacle so as to make it invisible. The biggest technological contraints in realizing large-scale device applications of metamaterials have been two. The first is the large dissipation associated with an inherently resonant phenomenon. The second arises due to the very design of metamaterial; once the metamaterial structures (inclusions) are fabricated, they offer little maneuverability in terms of the operating frequency. However, both EIT and metamaterials have truly lifted the tedium associated with the usual classical linear phenomena involving light. It is commonly believed this is just the beginning of a long journey, where our inherent drive to control gainfully these and many other wondrous effects will be the prime mover of future developments. The marriage of these two diverse developments is highlighted here, with a word of caution: one can only naively guess the surprises this relationship will bring forth. In order to achieve a composite material combining the attributes of EIT and metamaterials, one simple design involves immersing the metamaterial in a dilute atomic gas whose frequency-selective absorption can be exploited to manipulate the metamaterial response [3]. Furthermore, a combination of light fields accords extra control over the metamaterial via the absorption and dispersion of the atomic gas through the atomic coherence (quantum) route, based on effects like EIT. The price of working at near-resonant conditions is the large dispersion with frequency accompanied by large loss. EIT-based control exploits the large frequency-dispersion and yet provides substantially decreased loss which is even lower than the metallic losses in a narrow-bandwidth regime. The large variation of the refractive index of the EIT medium results in the freezing of currents in the metallic inclusions of the metamaterial, thereby lowering loss. The most critical issues that govern this alliance arise from the very nature of the two partners. EIT is a quantum phenomenon, whereas metamaterials are described classically. Quantum phenomena are extremely susceptible to the surroundings, and using a solid or liquid medium severely restricts the performance of the quantum partner. For example, rareearth ions implanted in crystals have been shown to exhibit EIT only at extremely low temperatures where the phonon noise is sufficiently suppressed such that the ground-state

Tilted response of fishnet metamaterials at near-infrared optical wavelengths

We study experimentally the transmission properties of ${\text{Au-TiO}}_{2}\text{-Au}$ fishnet metamaterials in the near-infrared spectral range and analyze the change in the transmission resonances at varying angles of incidence and different input polarizations. The results show that the main transmission peak through the fishnet is due to the excitation of hole modes. This high-transmission region is significantly influenced by surface plasmon coupling when the incident electric field has a component normal to the metal plates, while little change with respect to tilt is observed when the electric field is parallel to the two metal films of the fishnet.

Terahertz response of microfluidic-jetted three-dimensional flexible metamaterials

We demonstrate the fabrication and characterization of three-dimensional (3D) metamaterials in the terahertz (THz) range using the microfluidic-jetted technique. This technique has proven a convenient technique to fabricate metamaterial structures at the micrometer scale. The metamaterials are fabricated using dodecanethiol functionalized gold nanoparticles on flexible polyimide substrates. The metamaterials consist of alternate layers of single split-ring resonator and microstrip arrays that are stacked to form a 3D metamaterial medium. The fabricated metamaterials, with lattice sizes of 180 microm, are characterized using THz time-domain spectroscopy within 0.1 to 2 THz in the transmission mode. Numerical simulation is performed to calculate the effective metamaterials parameter.

Dynamic response of metamaterials in the terahertz regime: Blueshift tunability and broadband phase modulation

Terahertz time-domain spectroscopy is used to probe the electromagnetic properties of metamaterials, which are dynamically photoexcited, using synchronized femtosecond near-infrared laser pulses. Blueshift tunability of the electric dipole metamaterial’s resonance, as well as a broadband phase tunability reaching ∼π/4, are demonstrated. Numerical simulations show the observations are due to changes in the complex index of the photoexcited semiconductor substrate.