Issue No. 1, April

Abstract

We provide a compact review of some recent results on non-Hermitian metamaterials characterized by spatial modulation of loss and gain. First, we present a systematic synthesis procedure based on a complex-coordinate extension of the transformation-optics paradigm, which admits an insightful interpretation in conjunction with the “complex-source-point” formalism. Subsequently, we study some waveguiding phenomena that can occur in bilayers satisfying the parity-time symmetry condition in the “epsilon-near-zero” regime.

Abstract

Polarimetry enables to measure the state of polarization (SoP) of a light beam, which is essential in many disciplines. Typical polarimeters use bulky and expensive optical elements such as half-wave plates and grid polarizers. Plasmonic nanostructures may help to transform such bulky components into subwavelength metallic elements showing similar performance. Based on the concept of spin-orbit coupling, here we demonstrate a nanophotonic polarimeter that measures the Stokes parameters of a light beam over an ultrabroad bandwidth in a less than a square wavelength active region. Furthermore, the presented approach is applicable to any wavelength regime and technological platform, paving the way for the miniaturization of polarimeters.

Abstract

In photonic crystals composed of ferroelectrics, hybrid bands corresponding to new additional band gaps are expected to appear around the centre and boundaries of the Brillouin zone. We expect to discover the group-velocity anomaly modes related to the phonon-polariton branches in these hybrid bands. Numerical analyses conducted using the finite element method and experimental results obtained by performing terahertz time-domain spectroscopy underlie a discussion of the propagation characteristics of the group-velocity anomaly modes in the hybrid bands of one-dimensional photonic crystals fabricated from ferroelectric Li₂Ge₇O₁₅ single crystals. The electric-field intensity of the standing-wave mode at the end point of the dielectric band branch was found to be localized around all of the ferroelectric plates in the photonic crystal. In contrast, the group-velocity anomaly mode near the standing-wave mode is strongly localized around the first ferroelectric plate on the incident side and decays as it propagates through successive ferroelectric plates.

Abstract

We characterise from first principles the structure and bonding in 2D heterosystems made of bilayers or trilayers of graphene and graphene-like-materials (GLMs), stacked on top of each other, and functionalized using hydrogen. The effects of electron band gap opening and tuning, as well as formation of strongly bonded multilayers have been predicted. The linear and nonlinear optical and vibrational spectra were modelled for hydrogenated alternating graphene monolayers with insulating hexagonal boron nitride (h-BN) films. Here we focus mostly on the structural aspect of the 2D heterosystems. The simulated atomic and related electron structures indicate that submonolayer hydrogenation of the outer surfaces of multilayer systems induces covalent interlayer bonds and enables electron gap engineering in otherwise gapless graphene or wide-band gap h-BN. Calculated structural, vibrational, electronic and optical properties of the systems of interest aim to enabling in-situ noninvasive characterization of graphene based multilayers.

Abstract

Within the framework of the Quantum Field Theory, we discuss how to study electromagnetic properties of a multilayer graphene sample in the presence of electric and magnetic fields, both perpendicular to the graphene planes. We deal with the multilayer system by taking into account the quantum mechanical supersymmetric property of the monolayer Hamiltonian. We solve the Dirac equation for the graphene charge carriers by using the Ritus formalism. This formalism consists in the diagonalization of the operator (y_μΠ^μ )² with Π^μ  = p^μ − eA^μ and γ^μ the Dirac gamma matrices which contain information about the pseudo-spin. We calculate the charge carrier propagator for the monolayer case, and we obtain the photon polarization operator, the leading quantum correction to the classical Lagrangian density that encodes the electromagnetic properties of the system through the constitutive equations. With the quantum supersymmetrical properties of both, the monolayer and the multilayer graphene Hamiltonians, it is possible to extend our results to obtain the charge carrier propagator for the multilayer case.

Abstract

In this research aerogels were synthesized by homogenization of carbon nanotubes and chitosan under ultrasonic treatment and active magnetic stirring, followed by freeze-drying in order to remove the liquid from its structure. Freeze-drying is characterized by a certain ratio of pressure and temperature at which the solid phase, in our case - the ice, turns into a gas without passing through a liquid phase. Freeze-drying was carried out at a temperature of -5 ° C and a pressure of 30-80 Pa. After freeze-drying which lasted for 20 hours, the as-obtained aerogels were carbonized at temperature of 800 °C in an inert atmosphere. Surface morphology of resulting aerogels was studied using scanning electron microscopy. The hydrophobicity and sorption capacity of these aerogels to organic liquids characterized by different densities were investigated. In addition, composite aerogels with the presence of graphene layers in the structure were obtained and the influence of introduction of graphene on aerogel’s properties was analyzed. It was found that composite aerogels based on graphene and carbon nanotubes with chitosan as a glue matrix are characterized by a better-developed porosity of surface with a smaller pore sizes, and their sorption capacity for organic liquids is also higher compared with the aerogels based on carbon nanotubes.