Bi-functional Metasurface Polarizer for Reflected and Transmitted Waves

Manipulating the polarizations of electromagnetic waves by flexible and diverse means is desirable for a myriad of microwave systems. More recently, metasurfaces offer the promising alternatives to conventional polarization manipulating components because of the flexibility of their geometry could be arbitrarily customized. In this context, a bi-layered metasurface was presented to simultaneously manipulate the polarized states of reflected and transmitted microwaves. No matter whether the incident electromagnetic wave is x-polarized or y-polarized, the reflected and transmitted waves will be converted into orthogonal y- polarized waves at the operating frequency. The designed metasurface has a high polarization conversion rate(PCR) above 90% for both normal and oblique incidence. The experimental results verify the correctness of the simulated results. Finally, axial ratio and surface current distributions were employed to reveal the physics of polarization manipulation. The proposed metasurface will be beneficial to the design of flexible and versatile polarization converters and has great potential for applications in polarization controlled devices and also is believed extendable to higher frequency regimes.

Multidirectional Anisotropic Total Variation and Its Application in the Tomography of the Surrounding Rock of Coal Mining Faces

The computed tomography (CT) reconstruction algorithm is one of the crucial components of the CT system. To date, total variation (TV) has been widely used in CT reconstruction algorithms. Although TV utilizes the a priori information of the longitudinal and lateral gradient sparsity of an image, it introduces some staircase artifacts. To overcome the current limitations of TV and improve imaging quality, we propose a multidirectional anisotropic total variation (MATV) that uses multidirectional gradient information. The surrounding rock of coal mining faces uses principles of tomography similar to those of medical X-rays. The velocity distribution for the surrounding rock can be obtained by the first-arrival traveltime tomography of the transmitted waves in the coal mining face. Combined with the geological data, we can interpret the geological hazards in the coal mining face. To perform traveltime tomography, we first established the objective function of the first-arrival traveltime tomography of the transmitted waves based on the MATV regularization and then used the split Bregman method to solve the objective function. The simulated data and real data show that the MATV regularization method proposed in this paper can better maintain the boundaries of geological anomalies and reduce the artifacts compared with the isotropic total variation regularization method and the anisotropic total variation regularization method. Furthermore, this approach describes the distribution of geological anomalies more accurately and effectively and improves imaging accuracy.

Ultra-wideband Transmissive Linear Polarization Device Based on Graphene

In order to achieve both adjustable wideband and high Polarization Conversion Rate (PCR) of the transmitted waves, a novelty tri-layered structure is proposed for terahertz applications. The Rhombus Hollow Square (RHS) is built up by top and bottom gold gratings on Silicon Dioxide and Polyamide substrate with graphene strips. The proposed polarizer broadens the bandwidth and has well performance. As chemical potential increases, the bandwidth is also broadened by adjusting the graphene. From 0.5 THz to 3 THz, the PCR is greater than 90%, and the relative bandwidth up to 142.9%. The transmission and absorption of polarizer are analyzed at the oblique incidence with chemical potential 0.1eV. By simulating and analyzing the performance, a new result of maintaining broadband and high transmittance in oblique incidence is obtained.

Experimental and Modeling Studies of Stress Wave Propagation and Energy Dissipation Mechanism in Layered Composite Structures

Four composite structures (SiC/UHMWPE/TC4, SiC/TC4/UHMWPE, SiC/UHMWPE/MR/TC4, and SiC/TC4/MR/UHMWPE) were prepared using silicon carbide (SiC) ceramics, ultrahigh molecular weight polyethylene (UHMWPE), titanium alloy (TC4), and metal rubber (MR). The transmitted waves, failure forms, stress wave propagations, and energy dissipations of the composite structures were studied through Split Hopkinson Pressure Bar (SHPB) tests and numerical simulations. The results show that MR in composite structures can delay, attenuate, and smooth the stress wave, thereby reducing SiC damage. UHMWPE on the back of SiC provides cushioning for SiC, while TC4 on the back of SiC aggravates the damage of SiC. The composite structures with MR mainly dissipate the impact energy by reflecting energy, and the energy dissipation performance is better than that of composite structures without MR. A comprehensive comparison of transmitted waves, damage forms, stress wave propagations, and energy dissipations of the four composite structures shows that SiC/UHMWPE/MR/TC4 structure has the best impact resistance. Increasing the thickness of MR in the composite structures can improve the impact resistance, but there are also stress concentration and interface tensile stress.

Preliminary Determination of the Optimal Parameters When Using an Ultrasonic Probe to Measure Cavern Geometry Where a Metal Borehole Pipe Is Present

In order to determine the optimal parameters when using an ultrasonic probe to measure cavern geometry when a metal borehole pipe is present, an investigation was firstly carried out on influence of a vertical metal plates with a thickness from 1 mm to 15 mm immersed in water on transmitted and reflected ultrasonic waves. The results obtained will be used as an indicator for the measurement of underground geometry in which the ultrasonic probe is placed inside a metal pipe lining a borehole. These studies were performed both by experiment and computer simulation. The results show that the wavelength of the incident ultrasonic signals should be equal to half the thickness of the metal plate or an integer times smaller than this thickness. When the thickness of the barrier is unknown, an ultrasonic signal with linear frequency modulation (LFM) should be used. Due to the reverberation of the ultrasonic waves inside the pipe for caverns filled with water, the distance from the transducer to the cavern wall can be measured if it is longer than three times of the pipe diameter. Frequency analysis of both the reflected and the transmitted waves enables an optimal frequency of the incident ultrasonic wave to be selected, which can be used in the measurement of cavern geometry in conditions in which the ultrasonic probe is inside a metal pipe.

Electromagnetic Wave Scattering from a Moving Medium with Stationary Interface across the Interluminal Regime

This paper extends current knowledge on electromagnetic wave scattering from bounded moving media in several regards. First, it complements the usual dispersion relation of moving media, ω(θk) (θk: phase velocity direction, associated with the wave vector, k), with the equally important impedance relation, η(θS) (θS: group velocity direction, associated with the Poynting vector, S). Second, it explains the interluminal-regime phenomenon of double-downstream wave transmission across a stationary interface between a regular medium and the moving medium, assuming motion perpendicular to the interface, and shows that the related waves are symmetric in terms of the energy refraction angle, while being asymmetric in terms of the phase refraction angle, with one of the waves subject to negative refraction, and shows that the wave impedances of the two transmitted waves are equal. Third, it generalizes the problem to the case where the medium moves obliquely with respect to the interface. Finally, it highlights the connection between this problem and a spacetime modulated medium.

Experimental Characterization of 2 × 2 Electronically Reconfigurable 1 Bit Unit Cells for a Beamforming Transmitarray at X Band

This paper proposes a reconfigurable unit cell for a transmitarray operating at the X band. The unit cell consists of an active patch, a passive patch, and a phase shifter. The active patch has two PIN diodes that change the phase of 180° of the transmitted waves. The passive and active patches both have circular slots to enhance the bandwidth of the transmitted wave. We also propose a new type of experimental characterization technique to measure the performance of the unit cells at the X band without fabricating the entire transmitarray. Instead of a 1 unit cell as described in the literature, we propose 2 × 2 unit cells to measure the performance of unit cells using the X band waveguide. The waveguide consists of a WR-90 section and a rectangular to square waveguide transition section that can be fit to our proposed structure. A good agreement between simulated and measured results was found.

Diffraction of a Gaussian Beam with Limited cross Section by a Volume Phase Grating under Waveguide Mode Resonance

In this work, the diffraction of a Gaussian beam on a volume phase grating was researched theoretically and numerically. The proposed method is based on rigorous coupled-wave analysis (RCWA) and Fourier transform. The Gaussian beam is decomposed into plane waves using the Fourier transform. The number of plane waves is determined using the sampling theorem. The complex reflected and transmitted amplitudes are calculated for each RCWA plane wave. The distribution of the fields along the grating for the reflected and transmitted waves is determined using inverse Fourier transform. The powers of the reflected and transmitted waves are determined based on these distributions. Our method shows that the energy conservation law is satisfied for the phase grating. That is, the power of the incident Gaussian beam is equal to the sum of the powers of the reflected and transmitted beams. It is demonstration of our approach correctness. The numerous studies have shown that the spatial shapes of the reflected and transmitted beams differ from the Gaussian beam under resonance. In additional, the waveguide mode appears also in the grating. The spatial forms of the reflected and transmitted beams are Gaussian in the absence of resonance. It was found that the width of the resonance curves is wider for the Gaussian beam than for the plane wave. However, the spectral and angular sensitivities are the same as for the plane wave. The resonant wavelengths are slightly different for the plane wave and the Gaussian beam. Numerical calculations for four refractive index modulation coefficients of the grating medium were carried out by the proposed method. The widths of the resonance curves decrease with the increasing in the refractive index modulation. Moreover, the reflection coefficient also increases.

An Analysis Method for Time-, Frequency-, and Energy-Domain Characteristics of Downhole Microseismic Signals and Its Application

Based on SOS microseismic (MS) monitoring system of a certain mine, time-, frequency-, and energy-domain characteristics of MS signals are analyzed by comprehensively applying one-dimensional discrete Fourier transform (DFT) and one-dimensional discrete wavelet transform (DWT). The technology is employed to explore MS signals from incident and transmitted waves on the fault plane. The research results show that, within the time domain, the transmitted waves are found to have the rapidly reducing velocity and a few wave crests; within the frequency domain, the fault plane presents a low-pass filtering effect on incident waves; within the energy domain, the fault generally shows the vibration-attenuating effect on MS signals. The analysis method is used for the No. 3307 working face of a certain mine to realize the prediction on a buried fault in the middle part of the working face, and it is validated to have a favorable effect.