Twisted Waves near a Plasma Cutoff

This work considers twisted wave propagation in inhomogeneous and unmagnetised plasma, and discusses the wave properties in the cutoff region. The qualitative differences between twisted waves described by a single Laguerre–Gauss (LG) mode, and light springs resulting from the superposition of two or more LG modes with different frequency and helicity are studied. The peculiar properties displayed by these waves in the nonuniform plasma are discussed. The pulse envelope of a light-spring shows a contraction at reflection, which resembles that of a compressed mechanical spring. The case of normal incidence is examined, and nonlinear ponderomotive effects are discussed, using theory and simulations.

A ReaxFF potential for Al - ZnO systems

A reactive field force (ReaxFF) potential has been created in order to model the structural effects of low percentage dopant aluminium in a zinc oxide system. The potential’s parameters were fitted to configurations computed with Density Functional Theory (DFT): cohesive energies, binding energies and forces were all considered for bulk crystals, surface structures and ZnAl alloys. As a first application of the model, the energetic deposition (0.1 - 40 eV) of an aluminium atom onto the polar surface of a ZnO (000 ̄1) is considered. For low energies the Al atom attaches to two preferred sites on the surface but as the energy increases above ≈ 15 eV subplantation is preferred at near normal incidence, with high diffusion barriers between stable sites. At these energies, reflection of the Al atom occurs at incident angles above ≈ 55◦.

Numerical Study of a Wide-angle Near-Perfect Absorber for the Visible Regime Incorporating Metal-Dielectric-Metal Subwavelength Grating Structure

A near-perfect absorber for the visible regime based on metal-dielectric-metal subwavelength grating structure with the refractory metals is designed and demonstrated numerically. The absorber presents an average absorption over 98.4% in the visible regime at normal incidence. Angle-relative analysis shows that the proposed structure has good angle-tolerance. The high average absorption (86.6%) in the visible region can be maintained with the incident angles up to 60°. Through the analysis of the magnetic field, the physical origin is verified that this excellent absorption performance mainly stems from the cooperative effect of surface plasmonic resonances and the intrinsic broadband spectral responses by the refractory metals. In addition, the dependence of the absorption spectrum of the proposed absorber on the structural parameters is analyzed. This work provides an idea for the design of high-performance absorbers and has potential applications in advanced light energy capture and integration systems.

Doppler backscattering systems on the Globus-M2 tokamak

Doppler backscattering (DBS) was successfully previously used on the Globus-M tokamak. The diagnostic was utilised in the form of either a single-frequency or a four-frequency dual homodyne system. It was used primarily for the study of zonal flows, filaments and Alfvén eigenmodes. These phenomena are worth being studied both on the periphery and in the core region of the plasma in a tokamak. For this specific reason two multifrequency DBS systems were installed on the upgraded Globus-M2 tokamak. The first four-frequency system with dual homodyne detection had already been used on the Globus-M tokamak and has lower probing frequencies which provide measurements from the periphery plasma. The second and new six-frequency DBS system was installed with a non-linear transmission line that was adapted to generate probing signals at frequencies 50, 55, 60, 65, 70 and 75 GHz. In general, the range of probing frequencies corresponds to the region of critical plasma densities from 5 × 1018 to 7 × 1019 m−3 at normal incidence. The pyramidal horn antennas are located inside the vacuum vessel with a special cardan-like rotator outside the camera so as to tilt antennas in the toroidal and poloidal directions. A previously developed code was applied to simulate 3D raytracing for all frequency channels. Calculations were carried out for different angles of incidence and for different electron density distributions in order to investigate the possibilities of the implementation of radial and poloidal correlation Doppler reflectometry. Examples of the DBS system application for study of plasma properties in the Globus-M2 tokamak are presented.

Frequency-tunable wide-angle polarization selection with a graphene-based anisotropic epsilon-near-zero metamaterial

In this paper, we achieve frequency-tunable wide-angle polarization selection based on an anisotropic epsilon-near-zero (AENZ) metamaterial mimicked by a subwavelength graphene/SiO2 multilayer. The physical mechanism of wide-angle polarization selection can be explained by the analysis of the iso-frequency curve (IFC). Under transverse electric polarization, only the incident lights which are close to normal incidence can transmit through the designed multilayer since the IFC of the AENZ metamaterial is an extremely small circle. However, under transverse magnetic polarization, all the incident lights can transmit through the designed multilayer since the IFC of the AENZ metamaterial is a flat ellipse. Therefore, polarization selection can work in a broad angular width. By changing the gate voltage applying to the graphene, the operating frequency of polarization selection can be flexibly tuned. The optimal operating angular width of high-performance polarization selection where the polarization selection ratio is larger than 102 reaches 54.9 degrees. This frequency-tunable wide-angle polarization selector would possess potential applications in liquid crystal display, read-write magneto-optical data storage, Q-switched lasing, and chiral molecule detection.

Migration velocity analysis based on focusing diffractions generated using the CRS wavefront attributes: Application to real land data

In land seismic data processing, the prestack time migration (PSTM) image remains the standard imaging output, but a reliable migrated image of the subsurface depends on the accuracy of the migration velocity model. We have adopted two new algorithms for time-domain migration velocity analysis based on wavefield attributes of the common-reflection-surface (CRS) stack method. These attributes, extracted from multicoverage data, were successfully applied to build the velocity model in the depth domain through tomographic inversion of the normal-incidence-point (NIP) wave. However, there is no practical and reliable method for determining an accurate and geologically consistent time-migration velocity model from these CRS attributes. We introduce an interactive method to determine the migration velocity model in the time domain based on the application of NIP wave attributes and the CRS stacking operator for diffractions, to generate synthetic diffractions on the reflection events of the zero-offset (ZO) CRS stacked section. In the ZO data with diffractions, the poststack time migration (post-STM) is applied with a set of constant velocities, and the migration velocities are then selected through a focusing analysis of the simulated diffractions. We also introduce an algorithm to automatically calculate the migration velocity model from the CRS attributes picked for the main reflection events in the ZO data. We determine the precision of our diffraction focusing velocity analysis and the automatic velocity calculation algorithms using two synthetic models. We also applied them to real 2D land data with low quality and low fold to estimate the time-domain migration velocity model. The velocity models obtained through our methods were validated by applying them in the Kirchhoff PSTM of real data, in which the velocity model from the diffraction focusing analysis provided significant improvements in the quality of the migrated image compared to the legacy image and to the migrated image obtained using the automatically calculated velocity model.

The simulation of the microwave shielding properties of the dual band pass frequency selective surface

Without proper caution, the microwave leakage from a microwave oven door can be harmful to users’ health. In practice, the leaked radiation has to be blocked while the visible light is allowed to pass for a visual inspection of the cooking progress inside the oven. To fulfil the requirements, the door design based on the principle of the frequency selective surface (FSS) was proposed and the gridded square loop pattern was chosen. In the simulation conducted by COMSOL Multiphysics software, the size of the proposed FSS was given as 40.7×40.7 mm with a dielectric thickness of 2.8 mm. Two important characteristics in terms of shielding effectiveness (SE) and optical transparency (OT) of the proposed FSS configuration at normal incidence were simulated and found to be 62.7 dB and 57.5%, respectively. The simulation results indicate that the proposed FSS is applicable to a safety design of a microwave oven door in suppressing the microwave leakage. Parametric studies on the characteristics due to geometrical dimensions and glass substrate thickness were also investigated. These parameters were found to affect the shielding and transmitting performances of the proposed FSS.

Ultrawideband electromagnetic metamaterial absorber utilizing coherent absorptions and surface plasmon polaritons based on double layer carbon metapatterns

An ultrawideband electromagnetic metamaterial absorber is proposed that consists of double-layer metapatterns optimally designed by the genetic algorithm and printed using carbon paste. By setting the sheet resistance of the intermediate carbon metapattern to a half of that of the top one, it is possible to find an optimal intermediate metapattern that reflects and absorbs the EM wave simultaneously. By adding an absorption resonance via a constructive interference at the top metapattern induced by the reflection from the intermediate one, an ultrawideband absorption can be achieved without increasing the number of layers. Moreover, it is found that the metapatterns support the surface plasmon polaritons which can supply an additional absorption resonance as well as boost the absorption in a broad bandwidth. Based on the simulation, the $$90\%$$ 90 % absorption bandwidth is confirmed from 6.3 to 30.1 GHz of which the fractional bandwidth is 130.77$$\%$$ % for the normal incidence. The accuracy is verified via measurements well matched with the simulations. The proposed metamaterial absorber could not only break though the conventional concept that the number of layers should be increased to extend the bandwidth but also provide a powerful solution to realize a low-profile, lightweight, and low cost electromagnetic absorber.

Artificial event horizons in Weyl semimetal heterostructures and their non-equilibrium signatures

We investigate transport in type-I/type-II Weyl semimetal heterostructures that realize effective black- or white-hole event horizons. We provide an exact solution to the scattering problem at normal incidence and low energies, both for a sharp and a slowly-varying Weyl cone tilt profile. In the latter case, we find two channels with transmission amplitudes analog to those of Hawking radiation. Whereas the Hawking-like signatures of these two channels cancel in equilibrium, we demonstrate that one can favor the contribution of either channel using a non-equilibrium state, either by irradiating the type-II region or by coupling it to a magnetic lead. This in turn gives rise to a peak in the two-terminal differential conductance which can serve as an experimental indicator of the artificial event horizon.