There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

Metasurfaces: From microwaves to visible

A decade after the first report, the family of two-dimensional (2D) carbides and nitrides (MXenes) includes structures with three, five, seven, or nine layers of atoms in an ordered or solid solution form. Dozens of MXene compositions have been produced, resulting in MXenes with mixed surface terminations. MXenes have shown useful and tunable electronic, optical, mechanical, and electrochemical properties, leading to applications ranging from optoelectronics, electromagnetic interference shielding, and wireless antennas to energy storage, catalysis, sensing, and medicine. Here we present a forward-looking review of the field of MXenes. We discuss the challenges to be addressed and outline research directions that will deepen the fundamental understanding of the properties of MXenes and enable their hybridization with other 2D materials in various emerging technologies.

The world of two-dimensional carbides and nitrides (MXenes)

A compact representation is given of the electric- and magnetic-type dyadic Green's functions for plane-stratified, multilayered, uniaxial media based on the transmission-line network analog along the aids normal to the stratification. Furthermore, mixed-potential integral equations are derived within the framework of this transmission-line formalism for arbitrarily shaped, conducting or penetrable objects embedded in the multilayered medium. The development emphasizes laterally unbounded environments, but an extension to the case of a medium enclosed by a rectangular shield is also included.

/pdf/multilayered-media-green-s-functions-in-integral-equation-icv3t901yt.pdf

Multilayered media Green's functions in integral equation formulations

The particle swarm optimization (PSO) is a recently developed evolutionary algorithm (EA) based on the swarm behavior in the nature. This paper presents recent advances in applying a versatile PSO engine to real-number, binary, single-objective and multiobjective optimizations for antenna designs, with a randomized Newtonian mechanics model developed to describe the swarm behavior. The design of aperiodic (nonuniform and thinned) antenna arrays is presented as an example for the application of the PSO engine. In particular, in order to achieve an improved peak sidelobe level (SLL), element positions in a nonuniform array are optimized by real-number PSO (RPSO). On the other hand, in a thinned array, the on/off state of each element is determined by binary PSO (BPSO). Optimizations for both nonuniform arrays and thinned arrays are also expanded to multiobjective cases. As a result, nondominated designs on the Pareto front enable one to achieve other design factors than the peak SLL. Optimized antenna arrays are compared with periodic arrays and previously presented aperiodic arrays. Selected designs fabricated and measured to validate the effectiveness of PSO in practical electromagnetic problems

Advances in Particle Swarm Optimization for Antenna Designs: Real-Number, Binary, Single-Objective and Multiobjective Implementations

A metamaterial surface, composed of a periodic arrangement of Hilbert Curve inclusions above a conducting ground plane, is analyzed numerically and is shown to possess the properties of a high impedance surface by investigating the phase and magnitude of the reflection coefficient, /spl Gamma/, for a plane wave of normal incidence. A parametric study is presented with respect to the iteration order of the Hilbert curve, the surface height above the ground plane, and the separation distance between the neighboring Hilbert elements within the surface array.

/pdf/high-impedance-metamaterial-surfaces-using-hilbert-curve-39btj8d05t.pdf

High impedance metamaterial surfaces using Hilbert-curve inclusions

A completely passive radio frequency tag is proposed, utilizing the scattering from electrically small but resonant inclusions. When placing these space-filling curve inclusions in an array and scaling each element within the array such that each element has its own separate resonant frequency, a radio frequency barcode can be developed from the radar cross section of the array. The narrow bandwidth inherent to such inclusions can be helpful in packing the overall signature into a relatively small frequency spectrum. Such radio frequency tags may have potential use in some applications of radio frequency identification systems.

/pdf/space-filling-curve-rfid-tags-1pqkifij73.pdf

Space-filling curve RFID tags

Highly integrated, flexible, and ultrathin wireless communication components are in significant demand due to the explosive growth of portable and wearable electronic devices in the fifth-generation (5G) network era, but only conventional metals meet the requirements for emerging radio-frequency (RF) devices so far. Here, it is reported on Ti3 C2 Tx MXene microstrip transmission lines with low-energy attenuation and patch antennas with high-power radiation at frequencies from 5.6 to 16.4 GHz. The radiation efficiency of a 5.5 µm thick MXene patch antenna manufactured by spray-coating from aqueous solution reaches 99% at 16.4 GHz, which is about the same as that of a standard 35 µm thick copper patch antenna at about 15% of its thickness and 7% of the copper weight. MXene outperforms all other materials evaluated for patch antennas to date. Moreover, it is demonstrated that an MXene patch antenna array with integrated feeding circuits on a conformal surface has comparable performance with that of a copper antenna array at 28 GHz, which is a target frequency in practical 5G applications. The versatility of MXene antennas in wide frequency ranges coupled with the flexibility, scalability, and ease of solution processing makes MXene promising for integrated RF components in various flexible electronic devices.

Solution-Processed Ti 3 C 2 T x MXene Antennas for Radio-Frequency Communication

In this paper, a 3-D diffraction tomographic algorithm is proposed for real-time through-the-wall radar imaging (TWRI). The spectral expansion of the three-layered medium dyadic Green's function is employed to derive a linear relationship between the spatial Fourier transforms of the image and the scattered field under Born approximation. Then, the image can be efficiently reconstructed with inverse fast Fourier transform (IFFT). The linearization of the inversion scheme and the easy implementation of the algorithm with FFT/IFFT make the diffraction tomographic TWRI algorithm suitable for on-site applications. The 3-D polarimetric TWRI is investigated using the proposed algorithm for the enhanced target detection and feature extraction as well as mitigation of the wall effect in the cross-polarization. Numerical and experimental results are presented to show the effectiveness and high efficiency of the proposed algorithm for 3-D real-time TWRI.

Three-Dimensional Real-Time Through-the-Wall Radar Imaging With Diffraction Tomographic Algorithm

We have applied a moment-method-based simulation code to perform a detailed parametric study of the bandwidth, cross-polarization level, and the feed location characteristics of the matched Hilbert antenna. It is shown that a properly chosen off-center near-the-end feed point may provide an approximate 50 /spl Omega/ real input impedance at the fundamental resonant frequency. The role of different iteration orders for Hilbert antennas, approximately matched to the 50 /spl Omega/ line, on the bandwidth and cross-polarization level of such antennas is investigated numerically. As was shown by K.J. Vinoy et al., the radiation patterns resemble those of a linear dipole and, for a fixed-area-matched Hilbert antenna, the resonant frequency can be lowered by increasing the iteration order n. However, for such matched antennas, the bandwidth for VSWR<2 is decreased and the cross-polarization level may be different for the higher iteration-order antennas.

/pdf/bandwidth-cross-polarization-and-feed-point-characteristics-33jx8ojx24.pdf

Ahmad Hoorfar

Papers

High impedance metamaterial surfaces using Hilbert-curve inclusions

Space-filling curve RFID tags

Solution-Processed Ti 3 C 2 T x MXene Antennas for Radio-Frequency Communication

Three-Dimensional Real-Time Through-the-Wall Radar Imaging With Diffraction Tomographic Algorithm

Bandwidth, cross-polarization, and feed-point characteristics of matched Hilbert antennas