arXiv subjects

Thomas Koschny

Publications and source records attributed to Thomas Koschny.

At least 55 records · Page 3Linked to original sources

arXiv2010

Self-consistent calculations of loss compensated fishnet metamaterials

We present a computational approach, allowing for a self-consistent treatment of three-dimensional (3D) fishnet metamaterial coupled to a gain material incorporated into the nanostructure. We show numerically that one can compensate the losses by incorporating gain material inside the fishnet structure. The pump rate needed to compensate the loss is much smaller than the bulk gain and the figure of merit (FOM = |Re(n)/Im(n)|) increases dramatically with the pump rate. Transmission, reflection, and absorption data, as well as the retrieved effective parameters, are presented for the fishnet structure with and without gain material. Kramers-Kronig relations of the effective parameters are in excellent agreement with the retrieved results with gain.

physics.optics↗

arXiv2010

Chiral metamaterials: retrieval of the effective parameters with and without substrate

After the prediction that strong enough optical activity may result in negative refraction and negative reflection, more and more artificial chiral metamaterials were designed and fabricated at difference frequency ranges from microwaves to optical waves. Therefore, a simple and robust method to retrieve the effective constitutive parameters for chiral metamaterials is urgently needed. Here, we analyze the wave propagation in chiral metamaterials and follow the regular retrieval procedure for ordinary metamaterials and apply it in chiral metamaterial slabs. Then based on the transfer matrix technique, the parameter retrieval is extended to treat samples with not only the substrate but also the top layers. After the parameter retrieval procedure, we take two examples to check our method and study how the substrate influences on the thin chiral metamaterials slabs. We find that the substrate may cause the homogeneous slab to be inhomogeneous, i.e. the reflections in forward and backward directions are different. However, the chiral metamaterial where the resonance element is embedded far away from the substrate is insensitive to the substrate.

physics.optics↗

arXiv2010

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.

physics.optics↗

arXiv2010

Reducing Ohmic Losses in Metamaterials by Geometric Tailoring

Losses in metamaterials render the applications of such exotic materials less practical unless an efficient way of reducing them is found. We present two different techniques to reduce ohmic losses at both lower and higher frequencies, based on geometric tailoring of the individual magnetic constituents. We show that an increased radius of curvature, in general, leads to the least losses in metamaterials. Particularly at higher THz frequencies, bulky structures outperform the planar structures.

physics.optics↗

arXiv2010

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.

physics.optics↗

arXiv2009

Negative refractive index response of weakly and strongly coupled optical metamaterials

e present a detailed study of the retrieved optical parameters, electrical permittivity, magnetic permeability and refractive index, of the coupled fishnet metamaterial structures as a function of the separation between layers. For the weak coupling case, the retrieved parameters are very close to the one-functional-layer results and converge relatively fast. For the strong coupling case, the retrieved parameters are completely different than the one unit fishnet results. We also demonstrate that the high value of the figure of merit (FOM=|Re(n)/Im(n)|) for the strongly coupled structures is due to the fact that the real part of the negative $n$ moves away from the maximum of the imaginary part of n (close to the resonance), where the losses are high.

physics.optics↗

arXiv2009

Lasing in metamaterial nanostructures

A self-consistent computational scheme is presented for one dimensional (1D) and two dimensional (2D) metamaterial systems with gain incorporated into the nanostructures. The gain is described by a generic four-level system. The loss compensation and the lasing behavior of the metamaterial system with gain are studied. A critical pumping rate exists for compensating the losses of the metamaterial. There exists a wide range of input signals where the composite system behaves linearly. Nonlinearities arise for stronger signals due to gain depletion. The retrieved effective parameters are presented for one layer of gain embedded in two layers of Lorentz dielectric rods and split ring resonators with two different gain inclusions: (1) gain is embedded in the gaps only and (2) gain is surrounding the SRR. When the pumping rate increases, there is a critical pumping rate that the metamaterial system starts lasing.

physics.optics↗

arXiv2009

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.

physics.optics↗

arXiv2009

Optical anisotropic metamaterials: Negative refraction and focusing

We design three-dimensional (3D) metallic nanowire media with different structures and numerically demonstrate that they can be homogeneous effective indefinite anisotropic media by showing that their dispersion relations are hyperbolic. For a finite slab, a nice fitting procedure is exploited to obtain the dispersion relations from which we retrieve the effective permittivities. The pseudo focusing for the real 3D wire medium agrees very well with the homogeneous medium having the effective permittivity tensor of the wire medium. Studies also show that in the long-wavelength limit, the hyperbolic dispersion relation of the 3D wire medium can be valid even for evanescent modes.

physics.optics↗

arXiv2009

Nonplanar Chiral Metamaterials with Negative Index

We demonstrate experimentally and numerically that nonplanar chiral metamaterials give giant optical activity, circular dichroism, and negative refractive index. The transmission, reflection, and the retrieval results of the experiments agree pretty well with the simulations. This is an important step toward the design and fabrication of three-dimensional isotropic chiral metamaterials.

physics.optics↗

arXiv2008

Nonlinear properties of split-ring resonators

In this letter, the properties of split-ring resonators (SRRs) loaded with high-Q capacitors and nonlinear varactors are theoretically analyzed and experimentally measured. We demonstrate that the resonance frequency $f_m$ of the nonlinear SRRs can be tuned by increasing the incident power. $f_m$ moves to lower and higher frequencies for the SRR loaded with one varactor and two back-to-back varactors, respectively. For high incident powers, we observe bistable tunable metamaterials and hysteresis effects. Moreover, the coupling between two nonlinear SRRs is also discussed.

cond-mat.mtrl-sci↗

arXiv2008

Bulk Negative Index Photonic Metamaterials for Direct Laser Writing

We show the designs of one- and two-dimensional photonic negative index metamaterials around telecom wavelengths. Designed bulk structures are inherently connected, which render their fabrication feasible by direct laser writing and chemical vapor deposition.

physics.optics↗

arXiv2008

Transmission in the vicinity of the Dirac point in hexagonal Photonic Crystals

We use a scattering matrix approach to simulate the transmission through a hexagonal Photonic Crystal in the vicinity of the Dirac point. If the crystal is oriented so that the propagation direction perpendicular to the surface corresponds to the Gamma-K direction, no oblique transmission is possible for a very long (infinite) structure. For a finite structure with width, W, and length, L, the length dependence of the transmission is given by T_total = Gamma_0 W/L. For T_total all waves with a wavevector parallel to the surface, k_||=n 2pi/W, described by a channel number, n, must be considered. We show the transmission at the Dirac point follows the given scaling law and this scaling law is related to the behavior of the individual channels. This leads to the establishment of a criterion for the maximum length for this scaling behavior when the total transmission reaches a constant value. We also compare this scaling behavior to the results in other frequency regions.

physics.optics↗

arXiv2008

Wide-angle perfect absorber/thermal emitter in the THz 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.

physics.optics↗

arXiv2008

An efficient way to reduce losses of left-handed metamaterials

We propose a simple and effective way to reduce the losses in left-handed metamaterials by manipulating the values of the effective parameters R, L, and C. We investigate the role of losses of the short-wire pairs and the fishnet structures. Increasing the effective inductance to capacitance ratio, L/C, reduces the losses and the figure of merit can increase substantially, especially at THz frequencies and in the optical regime.

cond-mat.mtrl-sci↗

arXiv2008

Experimental Verification of the Quantized Conductance of Photonic Crystal Waveguides

We report experiments that demonstrate the quantization of the conductance of photonic crystal waveguides. To obtain a diffusive wave, we have added all the transmitted channels for all the incident angles. The conductance steps have equal height and a width of one half the wavelength used. Detailed numerical results agree very well with the novel experimental results.

cond-mat.mtrl-sci↗

arXiv2008

Surface waves in photonic crystal slabs

Photonic crystals with a finite size can support surface modes when appropriately terminated. We calculate the dispersion curves of surface modes for different terminations using the plane wave expansion method. These non-radiative surface modes can be excited with the help of attenuated total reflection technique. We did experiments and simulations to trace the surface band curve, both in good agreement with the numerical calculations.

physics.optics↗

arXiv2008

Size Dependence and Convergence of the Retrieval Parameters of Metamaterials

We study the dependence of the retrieval parameters, such as the electric permittivity, the magnetic permeability and the index of refraction, $n$, on the size of the unit cell of a periodic metamaterial. The convergence of the retrieved parameters on the number of the unit cells is also examined. We have concentrated our studies on the so-called fishnet structure, which is the most promising design to obtain negative $n$ at optical wavelengths. We find that as the size of the unit cell decreases, the magnitude of the retrieved effective parameters increases. The convergence of the effective parameters of the fishnet as the number of the unit cells increases is demonstrated but found to be slower than for regular split ring resonators and wires structures. This is due to a much stronger coupling between the different unit cells in the fishnet structure.

cond-mat.mtrl-sci↗