Borislav Vasić

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...

Self-focusing media using graded photonic crystals: Focusing, Fourier transforming and imaging, directive emission, and directional cloaking

Using numerical simulations, we investigate the realization of self-focusing media using two-dimensional graded photonic crystals and their applications for imaging and non-imaging purposes. The two-dimensional graded photonic crystals consist of spatially varying cylindrical holes drilled in a dielectric host. By controlling the gradient of the refractive index and the thickness of the self-focusing medium, it is possible to obtain either a focusing lens with Fourier transforming capabilities or an imaging lens, which produces inverted images. Non-imaging applications include a simple antenna for directive emission obtained from the focusing lens, whereas a directional cloak is obtained by modifying the imaging lens. Graded photonic crystal based devices work well up to the Bragg frequencies. They are compact, made from lossless dielectrics, and compatible with planar lithographic techniques, so they can find applications in a broad frequency range, even at the optical frequencies.

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.

Atomic force microscopy based manipulation of graphene using dynamic plowing lithography

Tapping mode atomic force microscopy (AFM) is employed for dynamic plowing lithography of exfoliated graphene on silicon dioxide substrates. The shape of the graphene sheet is determined by the movement of the vibrating AFM probe. There are two possibilities for lithography depending on the applied force. At moderate forces, the AFM tip only deforms the graphene and generates local strain of the order of 0.1%. For sufficiently large forces the AFM tip can hook graphene and then pull it, thus cutting the graphene along the direction of the tip motion. Electrical characterization by AFM based electric force microscopy, Kelvin probe force microscopy and conductive AFM allows us to distinguish between the truly separated islands and those still connected to the surrounding graphene.

Graphene induced spectral tuning of metamaterial absorbers at mid-infrared frequencies

In order to expand bandwidth of the resonant metamaterial absorbers, we investigate their spectral tuning at mid-infrared frequencies using graphene. We consider the absorbers with square metallic patches, cross-shaped resonators, and split ring resonators. Their resonances can be blue shifted by increasing graphene conductivity. Among these structures, split ring resonators produce the largest electric fields enabling huge spectral shifts, almost 30%. In addition, the tuning can be used for switching the mid-infrared waves at the absorber resonance. Here, the reflectance is zero, so even a small spectral shift of the resonance results in a huge increase of the reflectance.

Spectroscopic imaging ellipsometry and Fano resonance modeling of graphene

In this work, we have examined the optical properties of exfoliated graphene on an Si/SiO2 substrate using spectroscopic imaging ellipsometry in the visible range (360–800 nm). Measured spectra were analyzed by an optical model based on the Fresnel coefficient equations. The optical model was supported by correlated Raman and atomic force microscopy measurements. The complex refractive index of graphene was obtained by inversion of the measured ellipsometry data. The Fano line-shape was used to parameterize the optical properties. Measurements were highly reliable due to the numerous advantages of the spectroscopic imaging ellipsometric technique combined with the proper choice of substrate and experimental set-up. Thickness maps of the graphene sample were obtained from spatially resolved imaging ellipsometry spectra with a spot size of 1 μm. The data showed the presence of a water layer on the surface of the sample, and the thickness was mapped showing the distribution of water over graphene in ambient ...

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%.

Nano-Anchors with Single Protein Capacity Produced with STED Lithography

Acrylate nanoanchors of subdiffraction-limited diameter are written with optical stimulated emission depletion (STED) lithography. After incubation, 98% of all nanoanchors are loaded quickly with fluorescently labeled antibodies. Controlling the size of the nanoanchors allows for limiting the number of the antibodies. Direct stochastic optical reconstruction microscopy (dSTORM) imaging, statistical distribution of fluorescence, quantitative fluorescence readout, and single molecule blinking consistently prove that 80% of the nanoanchors with a 65 nm diameter are carrying only one antibody each, which are functional as confirmed with live erythrocytes.

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.