Thomas Koschny

Metamaterial with negative index due to chirality

Artificial magnetism, negative permeability and negative refractive index are demonstrated in 3D-chiral metamaterial that shows giant polarization rotation and circular dichroism. ∗ As presented at the Quantum Electronics and Laser Sciences Conference (CLEO/QELS 2008), San Jose, CA, USA, 5 May 2008, paper QMA4.

Low-loss metamaterials based on classical electromagnetically induced transparency

We demonstrate theoretically that electromagnetically induced transparency can be achieved in metamaterials, in which electromagnetic radiation is interacting resonantly with mesoscopic oscillators rather than with atoms. We describe novel metamaterial designs that can support a full dark resonant state upon interaction with an electromagnetic beam and we present results of its frequency-dependent effective permeability and permittivity. These results, showing a transparency window with extremely low absorption and strong dispersion, are confirmed by accurate simulations of the electromagnetic field propagation in the metamaterial.

Resonant and antiresonant frequency dependence of the effective parameters of metamaterials.

We present a numerical study of the electromagnetic response of the metamaterial elements that are used to construct materials with negative refractive index. For an array of split ring resonators (SRR) we find that the resonant behavior of the effective magnetic permeability is accompanied by an antiresonant behavior of the effective permittivity. In addition, the imaginary parts of the effective permittivity and permeability are opposite in sign. We also observe an identical resonant versus antiresonant frequency dependence of the effective materials parameters for a periodic array of thin metallic wires with cuts placed periodically along the length of the wire, with roles of the permittivity and permeability reversed from the SRR case. We show in a simple manner that the finite unit cell size is responsible for the antiresonant behavior.

Wide-angle perfect absorber/thermal emitter in the terahertz regime

We show that a perfect absorber/thermal emitter exhibiting an absorption peak of 99.9% can be achieved in metallic nanostructures that can be easily fabricated. The very high absorption is maintained for large angles with a minimal shift in the center frequency and can be tuned throughout the visible and near-infrared regime by scaling the nanostructure dimensions. The stability of the spectral features at high temperatures is tested by simulations using a range of material parameters. Since the beginning of the last century it is known that a perfect thermal emitter follows Planck’s law of blackbody radiation. 1 Realistic structures, however, generally do not follow Planck’s law but exhibit a smaller emission. The properties of these emitters strongly depend on the materials and their shapes. From the absorption spectra of a structure the emission properties can be deduced since Kirchhoff’s law directly relates the absorption with the emissivity. The emission is then determined by multiplying the emissivity with the blackbody radiation spectrum. Using photonic crystals, 2,3 it has been shown that this approach is also valid for periodically structured materials. For a number of applications such as thermophotovoltaic converters, it is necessary to control the spectral properties to achieve, e.g., selective emitters in a narrow frequency band corresponding to the band gap of solar cells. 4 In the case of structured metallic surfaces, the changes in the emission spectra are based on surface waves coupled to the external radiation through the periodic surface. 5,6 Alternatively, microcavity resonances can also be used to create narrow-band thermal radiation. 7 Unfortunately, most of the recent designs 6,8 for perfect absorbers/ emitters only work for one incident angle and one polarization. So, there is a need for wide-angle perfect absorber/ emitter nanostructures. In this Brief Report, we suggest a structure which exhibits a large absorption in the terahertz regime for a wide range of angles with respect to the surface. We show that the absorption characteristics are maintained even if the uncertainties in the estimated changes in the material parameters, due to high temperatures, are considered. The proposed structure can be easily manufactured with today’s planar microfabrication techniques. We also comment on the impact of deviations in the geometrical parameters caused by fabricational tolerances. The small size of the structure, in comparison to the wavelength together with the relatively straightforward fabrication, allows for easy integration into various devices, such as perfect thermal emitters, perfect absorbers, bolometers, and very effective light extraction light-emitting diodes LEDs. The suggested structure is shown in Fig. 1. It consists of a metal back plate black with a thickness larger than 200 nm. This is much larger than the typical skin depth in the terahertz regime and avoids transmission through the structure. In this case the reflection is the only factor limiting the absorption. The thickness of the back plate can be adjusted to the specific needs of the final application, e.g., to obtain good heat transport to sensors or to obtain a better stability. On top of the metal plate a spacer layer of silicon nitride SiN is deposited with a thickness Dt. The structure is terminated by an array of metallic stripes with a rectangular cross section. Their arrangement is described by a lattice constant a and their shape is given by a width Ww and a thickness Wt. In this setup a strong resonance with a large field enhancement in the dielectric spacer layer and in between the stripes can be obtained, as will be shown later. Adjusting the size of the metal stripes on the top, the coupling to this resonance can be tuned and the reflection can be minimized. Due to the scalability of Maxwell’s equations, in principle, the structure can be simulated using dimensionless units by dividing all sizes by the lattice constant and using =a / as frequency. However, the Drude model used to describe the metal requires frequencies in terahertz and therefore the lattice constant must be assigned in the simulation. If a shift in the frequencies of the spectral features by adjusting the lattice constant is intended, a different simulation must be done since changes in the dielectric constant would not be considered. In the simulation frequency-dependent material parameters are required. We calculate those using standard methods and adjust their values to take into consideration the high temperatures. The tungsten parts plate and stripes are described by a Drude model

Negative refractive index due to chirality

We demonstrate experimentally and numerically that metamaterials based on bilayer cross wires give giant optical activity, circular dichroism, and negative refractive index. The presented chiral design offers a much simpler geometry and more efficient way to realize negative refractive index at any frequency. We also developed a retrieval procedure for chiral materials which works successfully for circularly polarized waves.

A comparison of graphene, superconductors and metals as conductors for metamaterials and plasmonics

Recent advancements in metamaterials and plasmonics have promised a number of exciting applications, in particular at terahertz and optical frequencies. Unfortunately, the noble metals used in these photonic structures are not particularly good conductors at high frequencies, resulting in significant dissipative loss. Here, we address the question of what is a good conductor for metamaterials and plasmonics. For resonant metamaterials, we develop a figure-of-merit for conductors that allows for a straightforward classification of conducting materials according to the resulting dissipative loss in the metamaterial. Application of our method predicts that graphene and high-Tc superconductors are not viable alternatives for metals in metamaterials. We also provide an overview of a number of transition metals, alkali metals and transparent conducting oxides. For plasmonic systems, we predict that graphene and high-Tc superconductors cannot outperform gold as a platform for surface plasmon polaritons, because graphene has a smaller propagation length-towavelength ratio.

Unifying approach to left-handed material design

We show that equivalent circuits offer a qualitative and even quantitative simple explanation for the behavior of various types of left-handed (or negative-index) metamaterials. This allows us to optimize design features and parameters while avoiding trial and error simulations or fabrications. In particular, we apply this unifying circuit approach in accounting for the features and in optimizing the structure employing parallel metallic bars on the two sides of a dielectric film.

Wide-angle and polarization-independent chiral metamaterial absorber

We propose a resonant microwave absorber based on a chiral metamaterial. We show, with both numerical simulations and experimental measurements, that the absorber works well for a very wide range of incident angles for different polarizations. The proposed absorber has a compact size and the absorption is close to one for a wide range of incident angles and it is a good candidate for potential applications.

Classical analogue of electromagnetically induced transparency with a metal-superconductor hybrid metamaterial.

Metamaterials are engineered materials composed of small electrical circuits producing novel interactions with electromagnetic waves. Recently, a new class of metamaterials has been created to mimic the behavior of media displaying electromagnetically induced transparency (EIT). Here we introduce a planar EIT metamaterial that creates a very large loss contrast between the dark and radiative resonators by employing a superconducting Nb film in the dark element and a normal-metal Au film in the radiative element. Below the critical temperature of Nb, the resistance contrast opens up a transparency window along with a large enhancement in group delay, enabling a significant slowdown of waves. We further demonstrate precise control of the EIT response through changes in the superfluid density. Such tunable metamaterials may be useful for telecommunication because of their large delay-bandwidth products.

Chiral metamaterials with negative refractive index based on four “U” split ring resonators

A uniaxial chiral metamaterial is constructed by double-layered four “U” split ring resonators mutually twisted by 90°. It shows a giant optical activity and circular dichroism. The retrieval results reveal that a negative refractive index is realized for circularly polarized waves due to the large chirality. The experimental results are in good agreement with the numerical results.