Goran Isić

Controlling electromagnetic fields with graded photonic crystals in metamaterial regime.

Engineering of a refractive index profile is a powerful method for controlling electromagnetic fields. In this paper, we investigate possible realization of isotropic gradient refractive index media at optical frequencies using two-dimensional graded photonic crystals. They consist of dielectric rods with spatially varying radii and can be homogenized in broad frequency range within the lowest band. Here they operate in metamaterial regime, that is, the graded photonic crystals are described with spatially varying effective refractive index so they can be regarded as low-loss and broadband graded dielectric metamaterials. Homogenization of graded photonic crystals is done with Maxwell-Garnett effective medium theory. Based on this theory, the analytical formulas are given for calculations of the rods radii which makes the implementation straightforward. The frequency range where homogenization is valid and where graded photonic crystal based devices work properly is discussed in detail. Numerical simulations of the graded photonic crystal based Luneburg lens and electromagnetic beam bend show that the homogenization based on Maxwell-Garnett theory gives very good results for implementation of devices intended to steer and focus electromagnetic fields.

Localized surface plasmon resonances in graphene ribbon arrays for sensing of dielectric environment at infrared frequencies

High confinement of surface plasmon polaritons in graphene at infrared frequencies enhances the light-matter interaction and can be used for the sensing of the environment. The considered sensing platform consists of parallel graphene ribbons which enables efficient coupling of an electromagnetic field into localized surface plasmons. Changes in the environment are then detected by measuring the resulting frequency shifts of the plasmonic resonances. It is shown that the graphene ribbons have the sensitivity comparable to the sensitivity of noble metal nanoparticles at visible frequencies, which enable sensing of only several nanometers thick films at wavelengths around ten microns. At the same time, the tunability of graphene plasmons enables a design of broadband substrates for surface enhanced infrared absorption of thin films. By changing the Fermi level in graphene, the plasmonic resonance of graphene ribbons can be adjusted to desired vibrational mode which facilitates detection of multiple absorpti...

Spectral and directional reshaping of fluorescence in large area self-assembled plasmonic-photonic crystals.

Spectral and directional reshaping of fluorescence from dye molecules embedded in self-assembled hybrid plasmonic-photonic crystals has been examined. The hybrid crystals comprise two-dimensional hexagonal arrays of dye-doped dielectric nanospheres, capped with silver semishells. Comparing the reshaped fluorescence spectra with measured transmission/reflection spectra and numerical calculations reveals that the spectral and directional reshaping of fluorescence is the result of its coupling to photonic crystal Bloch modes and to void plasmons localized inside the silver caps.

Radiation and scattering from imperfect cylindrical electromagnetic cloaks

The design of electromagnetic invisibility cloaks is based on singular mappings prescribing zero or infinite values for material parameters on the inner surface of the cloak. Since this is only approximately feasible, an asymptotic analysis is necessary for a sound description of cloaks. We adopt a simple and effective approach for analyzing electromagnetic cloaks - instead of the originally proposed singular mapping, nonsingular mappings asymptotically approaching the ideal one are considered. Scattering and radiation from this type of imperfect cylindrical cloaks is solved analytically and the results are confirmed by full-wave finite element simulations. Our analysis sheds more light on the influence of this kind of imperfection on the cloaking performance and further explores the physics of cloaking devices.

Coordinate transformation based design of confined metamaterial structures

The coordinate transformation method is applied to bounded domains to design metamaterial devices for steering spatially confined electromagnetic fields. Both waveguide and free-space beam applications are considered as these are analogous within the present approach. In particular, we describe devices that bend the propagation direction and squeeze confined electromagnetic fields. Two approaches in non-magnetic realization of these structures are examined. The first is based on using a reduced set of material parameters, and the second on finding non-magnetic transformation media. It is shown that transverse-magnetic fields can be bent or squeezed to an arbitrary extent and without reflection using only dielectric structures.

Spectroscopic ellipsometry of few-layer graphene

The optical properties of few-layer graphene (FLG) films were measured in the ultraviolet and visible spectrum using a spectroscopic ellipsometer equipped with a 50-mu m nominal microspot size. The FLG thickness was found by atomic force microscopy. Measurements revealed that the microspot is larger than the FLG flake. The ellipsometric data was interpreted using the island-film model. Comparison with graphite and recently published graphene data showed reasonable agreement, but with some features that could not be explained. The error margin for the optical constants was estimated to be +/- 10%.

Spectroscopic ellipsometry and the Fano resonance modeling of graphene optical parameters

We investigate the optical response of graphene via spectroscopic ellipsometry in the ultraviolet and visible ranges. The optical conductance of graphene is described by a Fano model. The parameters of this model are extracted from our spectroscopic ellipsometry measurements, and the complex refractive index of graphene is obtained. Graphenes dispersion relation shows that the density of states function has a logarithmic van Hove singularity corresponding to the M point of the Brillouin zone. The exciton binding energy is calculated as the difference between the resonant and the saddle point energies.

Electrically tunable terahertz polarization converter based on overcoupled metal-isolator-metal metamaterials infiltrated with liquid crystals

Large birefringence and its electrical modulation by means of Fréedericksz transition makes nematic liquid crystals (LCs) a promising platform for tunable terahertz (THz) devices. The thickness of standard LC cells is in the order of the wavelength, requiring high driving voltages and allowing only a very slow modulation at THz frequencies. Here, we first present the concept of overcoupled metal-isolator-metal (MIM) cavities that allow for achieving simultaneously both very high phase difference between orthogonal electric field components and large reflectance. We then apply this concept to LC-infiltrated MIM-based metamaterials aiming at the design of electrically tunable THz polarization converters. The optimal operation in the overcoupled regime is provided by properly selecting the thickness of the LC cell. Instead of the LC natural birefringence, the polarization-dependent functionality stems from the optical anisotropy of ultrathin and deeply subwavelength MIM structures. The dynamic electro-optic control of the LC refractive index enables the spectral shift of the resonant mode and, consequently, the tuning of the phase difference between the two orthogonal field components. This tunability is further enhanced by the large confinement of the resonant electromagnetic fields within the MIM cavity. We show that for an appropriately chosen linearly polarized incident field, the polarization state of the reflected field at the target operation frequency can be continuously swept between the north and south pole of the Poincaré sphere. Using a rigorous Q-tensor model to simulate the LC electro-optic switching, we demonstrate that the enhanced light-matter interaction in the MIM resonant cavity allows the polarization converter to operate at driving voltages below 10 Volt and with millisecond switching times.

Phase-breaking effects in double-barrier resonant tunneling diodes with spin-orbit interaction

Several recent theoretical studies showed that the spin-orbit interaction in narrow gap InGaAs/InAlAs double-barrier resonant tunneling structures might yield a highly spin-polarized current in the ballistic limit. In this paper, a nonequilibrium Green’s function model is used to examine the effect of phase-breaking on the spin-dependent transport of carriers. The scattering is described as a local interaction with a bath of scatterers and treated in the self-consistent first Born approximation. Elastic and inelastic scatterers, with scattering strengths that cause a few millielectron volt broadening of quasibound states, have been found to significantly reduce the spin polarization. The magnitude of spin polarization has been found to be dominantly determined by the quasibound state broadening, while the interaction details are not significant.

Oblique incidence ellipsometric characterization and the substrate dependence of visible frequency fishnet metamaterials

We use spectroscopic ellipsometry to investigate the angular-dependent optical modes of fishnet metamaterials fabricated by nanoimprint lithography. Spectroscopic ellipsometry is demonstrated as a fast and efficient method for metamaterial characterization and the measured polarization ratios significantly simplify the calibration procedures compared to reflectance and transmittance measurements. We show that the modes can be well identified by a combination of comparing different substrates and considering the angular dependence of the Woods anomalies. The lack of angular dispersion of the anti-symmetric gap-modes does not agree with the model and requires further theoretical investigation.