Response Of Metamaterials Research Articles

Terahertz optically tunable dielectric metamaterials without microfabrication

We theoretically investigate the terahertz (THz) dielectric response of a semiconductor slab hosting a tunable grating photogenerated by the interference of two tilted infrared (IR) plane waves. In the case where the grating period is much smaller than the THz wavelength, we numerically evaluate the ordinary and extraordinary component of the effective permittivity tensor by resorting to electromagnetic full-wave simulation coupled to the dynamics of charge carriers excited by IR radiation. We show that the photo-induced metamaterial optical response can be tailored by varying the grating and it ranges from birefringent to hyperbolic to anisotropic negative dielectric without resorting to microfabrication.

An electromechanically reconfigurable plasmonic metamaterial operating in the near-infrared

Current efforts in metamaterials research focus on attaining dynamic functionalities such as tunability, switching and modulation of electromagnetic waves. To this end, various approaches have emerged, including embedded varactors, phase-change media, the use of liquid crystals, electrical modulation with graphene and superconductors, and carrier injection or depletion in semiconductor substrates. However, tuning, switching and modulating metamaterial properties in the visible and near-infrared range remain major technological challenges: indeed, the existing microelectromechanical solutions used for the sub-terahertz and terahertz regimes cannot be shrunk by two to three orders of magnitude to enter the optical spectral range. Here, we develop a new type of metamaterial operating in the optical part of the spectrum that is three orders of magnitude faster than previously reported electrically reconfigurable metamaterials. The metamaterial is actuated by electrostatic forces arising from the application of only a few volts to its nanoscale building blocks-the plasmonic metamolecules-that are supported by pairs of parallel strings cut from a flexible silicon nitride membrane of nanoscale thickness. These strings, of picogram mass, can be driven synchronously to megahertz frequencies to electromechanically reconfigure the metamolecules and dramatically change the transmission and reflection spectra of the metamaterial. The metamaterial's colossal electro-optical response (on the order of 10(-5)-10(-6) m V(-1)) allows for either fast continuous tuning of its optical properties (up to 8% optical signal modulation at up to megahertz rates) or high-contrast irreversible switching in a device only 100 nm thick, without the need for external polarizers and analysers.

A Broadband Negative Index Metamaterial at Optical Frequencies

A broadband metamaterial presenting negative indices across hundreds of nanometers in the visible and near‐infrared spectral regimes is demonstrated theoretically, using transformation optics to design the metamaterial constituents. The approach begins with an infinite plasmonic waveguide that supports a broadband but dark (i.e, not easily optically accessed) negative index mode. Conformal mapping of this waveguide to a finite split‐ring‐resonator‐type structure transforms this mode into a bright (i.e, efficiently excited) resonance composed of degenerate electric and magnetic dipoles. A periodic array of such resonators exhibits negative refractive indices at optical frequencies in multiple regions exceeding 200 nm in bandwidth. The metamaterial response is confirmed through simulations of plane‐wave refraction through a metamaterial prism. These results illustrate the power of transformation optics for new metamaterial designs and provide a foundation for future broadband metamaterial devices.

Enhancement of Second-Harmonic Generation from Metal Nanoparticles by Passive Elements

We prepare arrays of gold nanoparticles that include both noncentrosymmetric particles with a second-order nonlinear optical response (active particles) and centrosymmetric particles with no second-order response (passive particles). The plasmon resonances of the active and passive particles are at distinct wavelengths, yet the passive particles modify the electromagnetic modes of the structure in such a way that second-harmonic generation from the active particles is enhanced. Our results provide a completely new concept for optimizing the nonlinear responses of metamaterials.

ENHANCED NONLINEARITIES IN DOUBLE-FISHNET NEGATIVE-INDEX PHOTONIC METAMATERIALS

We numerically analyze the optical response and nonlinear susceptibilities of flshnet metamaterials with the holes inflltrated by a third-order nonlinear dielectric. Through full-wave simulations and employing a nonlinear parameter retrieval method, we conflrm and quantify the enhanced nonlinearities, showing bulk third-order nonlinear susceptibilities that are up to two orders of magnitude larger than the nonlinear dielectric. We also use the retrieved parameters to calculate the material flgure of merits and the conversion e-ciencies, showing material flgure of merits up to two orders of magnitude larger and conversion e-ciencies up to four orders of magnitude larger than the nonlinear dielectric alone. Though these results are calculated using one-unit-cell thick structures, the large magnitude of the enhancement still makes these structures attractive, allowing reasonable conversion e-ciencies supported by even subwavelength slabs.

Resonance properties of optical all-dielectric metamaterials using two-dimensional multipole expansion

We examine the electromagnetic response of metamaterial unit elements consisting of dielectric rods embedded in a nonmagnetic background medium. We establish a theoretical framework in which the response is described through the electric and magnetic multipole moments that are simultaneously generated via the polarization currents that are excited upon the incidence of plane waves. The corresponding dipole and quadrupole polarizabilities are then calculated as a function of the Mie scattering coefficients, and their resonances are mapped for the case of dielectric cylindrical rods as a function of the geometry and the material parameters used. The results provide critical insight into the anisotropic response of two-dimensional rod-type metamaterials and can be used as a unified methodology in the calculation of exotic effective electromagnetic parameters involved in phenomena such as optical magnetism.

Competing nonlinearities with metamaterials

We suggest an approach for creating metamaterials with sign-varying nonlinear response. We demonstrate that microwave metamaterials with such competing nonlinearities can be created by loading split-ring resonators (“meta-atoms” of the structure) with pairs of varactor diodes and photodiodes exhibiting nonmonotonic resonance frequency shift with changing incident microwave power. Additionally, the nonlinear response of such metamaterials can be controlled by illuminating the meta-atoms by light.

Near-field interactions in electric inductive–capacitive resonators for metamaterials

Near-field interactions in an array of electric inductive–capacitive (ELC) resonators are investigated analytically, numerically and experimentally. The measurement and simulation results show that inter-cell coupling plays an important role in determining the response of metamaterials. A quasistatic dipole–dipole interaction model, together with a Lagrangian formalism, quantitatively explains the interplay between the electric and magnetic couplings in the resonator array. Depending on the alignment of the resonators, the couplings can cause resonance shifting and/or splitting. The knowledge obtained from this study is crucial in designing metamaterials with ELC resonators.

Carrier-concentration-dependent resonance frequency shift in a metamaterial loaded semiconductor

We examined the electromagnetic responses of near-infrared metamaterials consisting of split-ring resonators fabricated on GaInAs semiconductor layers with different doping levels on an InP substrate. The inductance-capacitance (LC) resonances of the split-ring resonators could be controlled entirely from 52 to 63 THz by changing the carrier concentrations from 2.6×1018 to 2.7×1019 cm−3. Our results show the feasibility of semiconductor-based tunable metamaterials.

Terahertz-field-induced insulator-to-metal transition in vanadium dioxide metamaterial

Electron-electron interactions can render an otherwise conducting material insulating, with the insulator-metal phase transition in correlated-electron materials being the canonical macroscopic manifestation of the competition between charge-carrier itinerancy and localization. The transition can arise from underlying microscopic interactions among the charge, lattice, orbital and spin degrees of freedom, the complexity of which leads to multiple phase-transition pathways. For example, in many transition metal oxides, the insulator-metal transition has been achieved with external stimuli, including temperature, light, electric field, mechanical strain or magnetic field. Vanadium dioxide is particularly intriguing because both the lattice and on-site Coulomb repulsion contribute to the insulator-to-metal transition at 340 K (ref. 8). Thus, although the precise microscopic origin of the phase transition remains elusive, vanadium dioxide serves as a testbed for correlated-electron phase-transition dynamics. Here we report the observation of an insulator-metal transition in vanadium dioxide induced by a terahertz electric field. This is achieved using metamaterial-enhanced picosecond, high-field terahertz pulses to reduce the Coulomb-induced potential barrier for carrier transport. A nonlinear metamaterial response is observed through the phase transition, demonstrating that high-field terahertz pulses provide alternative pathways to induce collective electronic and structural rearrangements. The metamaterial resonators play a dual role, providing sub-wavelength field enhancement that locally drives the nonlinear response, and global sensitivity to the local changes, thereby enabling macroscopic observation of the dynamics. This methodology provides a powerful platform to investigate low-energy dynamics in condensed matter and, further, demonstrates that integration of metamaterials with complex matter is a viable pathway to realize functional nonlinear electromagnetic composites.

Visible transmission response of nanoscale complementary metamaterials for sensing applications

Metamaterials (MMs) have shown huge potential in sensing applications by detecting their optical properties, which can be designed to operate at frequencies from visible to mid-IR. Here we constructed complementary split ring resonator (CSRR) based metamaterials in nanoscale with unit length of 100 nm and slit width of 30 nm, and observed obvious responses in the visible waveband from 600 to 900 nm. These visible responses show a good tunability with the structure’s geometry, and are well suited for dielectric detection. We demonstrated good refractive index sensing of CSRR based metamaterials in the visible region under both 0° and 90° polarized incidence. Our results extend the study of CSRR based metamaterials to the visible region, which is expected to deepen the understanding of the response mechanism of CSRRs and benefit their sensing applications in the visible region.

Electric toroidal dipole response in split-ring resonator metamaterials

Toroidal dipoles are usually masked by other stronger effects of classical electric and magnetic multipoles, such as dipoles, quadrupoles, octupoles and so forth. So observation of the toroidal response is extremely rare. For the first time we present an electric toroidal dipole model based on metamaterials, which is composed of split-ring resonators. The transmission and reflection spectra show that there are two resonances at the frequency of 5.0 GHz and 11.2 GHz. Calculated radiated power dispersion for various multipole moments derived from the current density proved that these two resonances are electric dipole response and elusive electric toroidal dipole response respectively. Our experimental evidence of electric toroidal dipole response brings attentions to the often ignored toroidal multipoles.

Toroidal dipole response in a multifold double-ring metamaterial

The toroidal response is numerically investigated in a multifold double-ring metamaterials at the antibonding magnetic-dipole mode (i.e., antiparallel magnetic dipoles in one double-ring fold). This intriguing toroidal resonance in metamaterials is considered as a result of the magnetoelectric effect due to the broken balance of the electric near-field environment. We demonstrate that the toroidal dipole response in metamaterials can improve the quality factor of the resonance spectrum. In viewing of the design flexibility on the double-ring geometry, such toroidal metamaterials will offer advantages in application potentials of toroidal dipolar moment.

Wide-band negative permeability of nonlinear metamaterials

We propose a novel way to achieve an exceptionally wide frequency range where metamaterial possesses negative effective permeability. This can be achieved by employing a nonlinear response of metamaterials. We demonstrate that, with an appropriate design, a frequency band exceeding 100% is available for a range of signal amplitudes. Our proposal provides a significant improvement over the linear approach, opening a road towards broadband negative refraction and its applications.

Optical tuning and ultrafast dynamics of high-temperature superconducting terahertz metamaterials

Abstract Through the integration of semiconductors or complex oxides into metal resonators, tunable metamaterials have been achieved by a change of environment using an external stimulus. Metals provide high conductivity to realize a strong resonant response in metamaterials; however, they contribute very little to the tunability. The complex conductivity in high-temperature superconducting films is highly sensitive to external perturbations, which provides new opportunities in achieving tunable metamaterials resulting directly from the resonant elements. Additionally, superconducting metamaterials are expected to enable strong nonlinear response and quantum effects, particularly when Josephson junctions are integrated into the metamaterial resonant elements. Here we demonstrate ultrafast dynamical tuning of resonance in the terahertz (THz) frequency range in YBa2Cu3O7-δ (YBCO) split-ring resonator (SRR) arrays excited by near infrared femtosecond laser pulses. The photoexcitation breaks the superconducting Cooper pairs to create quasiparticles. This dramatically modifies the imaginary part of the complex conductivity and consequently the metamaterial resonance on an ultrafast timescale, although the real conductivity does not change significantly. We observed resonance switching accompanied by substantial frequency tuning as a function of photoexcitation fluence, which also strongly depends on the nanoscale thickness of the superconducting films. All of our experimental results agree with calculations using an analytical model, which takes into account the contributions of the complex conductivity of the YBCO films to SRR resistance and kinetic inductance. The theoretical calculations reveal that the increasing SRR resistance upon increasing photoexcitation fluence is responsible for the reduction of resonance strength, and changes in both the resistance and kinetic inductance cause the resonance frequency shifts.

Rational design and optimization of plasmonic nanoarrays for surface enhanced infrared spectroscopy.

We present an approach for rational design and optimization of plasmonic arrays for ultrasensitive surface enhanced infrared absorption (SEIRA) spectroscopy of specific protein analytes. Motivated by our previous work that demonstrated sub-attomole detection of surface-bound silk fibroin [Proc. Natl. Acad. Sci. U.S.A. 106, 19227 (2009)], we introduce here a general framework that allows for the numerical optimization of metamaterial sensor designs in order to maximize the absorbance signal. A critical feature of our method is the explicit compensation for the perturbative effects of the analyte's refractive index which alters the resonance frequency and line-shape of the metamaterial response, thereby leading to spectral distortion in SEIRA signatures. As an example, we leverage our method to optimize the geometry of periodic arrays of plasmonic nanoparticles on both Si and CaF2 substrates. The optimal geometries result in a three-order of magnitude absorbance enhancement compared to an unstructured Au layer, with the CaF2 substrate offering an additional factor of three enhancement in absorbance over a traditional Si substrate. The latter improvement arises from increase of near-field intensity over the Au nanobar surface for the lower index substrate. Finally, we perform sensitivity analysis for our optimized arrays to predict the effects of fabrication imperfections. We find that <20% deviation from the optimized absorbance response is readily achievable over large areas with modern nanofabrication techniques.

Electromagnetic field energy density in homogeneous negative index materials

An exact separation of both electric and magnetic energies into stored and lost energies is shown to be possible in the special case when the wave impedance is independent of frequency. A general expression for the electromagnetic energy density in such a dispersive medium having a negative refractive index is shown to be accurate in comparison with numerical results. Using an example metamaterial response that provides a negative refractive index, it is shown that negative time-averaged stored energy can occur. The physical meaning of this negative energy is explained as the energy temporarily borrowed by the field from the material. This observation for negative index materials is of interest when approaching properties for a perfect lens. In the broader context, the observation of negative stored energy is of consequence in the study of dispersive materials.

Birefringent nanostructured composite materials

We use a very efficient recursive method to calculate the effective optical response of materials made up of arbitrarily shaped dielectric inclusions arranged in periodic 2D arrays within a metal matrix with a lattice constant much smaller than the wavelength of the incident light, so that we may neglect retardation. The starting point of the calculation is a digitized image of the system. The geometrical shape of the inclusions and their orientation in the 2D array induce a birefringent optical response of the whole metamaterial that can be tailored to specific needs.

Modal analysis method to describe weak nonlinear effects in metamaterials

We apply a rigorous eigenmode analysis to study the electromagnetic properties of linear and weakly nonlinear metamaterials. The nonlinear response can be totally described by the linear eigenmodes when weak nonlinearities are attributed to metamaterials. We use this theory to interpret intrinsic second-harmonic spectroscopy on metallic metamaterials. Our study indicates that metamaterial eigenmodes play a critical role in optimizing a nonlinear metamaterial response to the extent that a poorly optimized modal pattern overwhelms the widely recognized benefits of plasmonic resonant field enhancements.

Using a discrete dipole approximation to predict complete scattering of complicated metamaterials

We develop a numerical technique for simulating metamaterial electromagnetic response based on an adaptation of the discrete dipole approximation (DDA). Our approach reduces each constituent metamaterial element within the composite to a point dipole with electric and magnetic polarizabilities, rather than assuming a homogenized effective material. We first validate the approach by computing the scattering cross-section for a collection of densely spaced isotropic dipole moments arranged within a cylindrical area, and compare with the known result from Mie theory. The discrete dipole approach has considerable advantages for the design of gradient and transformation optical media based on metamaterials, since the absence of local periodicity in other common design approaches leaves them with questionable validity. Several variants of iconic cloaking structures are investigated to illustrate the method, in which we study the impact that different configurations of dipolar elements can have on cloak performance. The modeling of a complex medium as polarizable dipoles provides a much closer connection to actual metamaterial implementations, and can address key nonlocal phenomena, such as magnetoelectric coupling, not accessible to most current numerical metamaterial approaches.