The design of a broadband tunable absorber is proposed based on a thin vanadium dioxide metasurface, which is composed of a simple array of vanadium dioxide and a bottom gold film. When the ...conductivity of vanadium dioxide is equal to 2000 Ω -1 cm -1 , simulated absorptance exceeds 90% with 71% bandwidth from 0.47 to 0.99 THz and full width at half maximum is 98% from 0.354 to 1.036 THz with center frequency of 0.695 THz. Simulated results show that absorptance peak can be tuned from 5% to 100% when the conductivity changes continually from 10 Ω -1 cm -1 to 2000 Ω -1 cm -1 . The designed absorber may have useful applications in terahertz spectrum such as energy harvesting, thermal emitter, and sensing.
When underwater pressure wave is generated, moving water particles cut off the geomagnetic field and produce induced currents, which will simultaneously induce electromagnetic field in the whole ...space. Due to the large distribution range and slow attenuation of this pseudo‐radiation, it is possible to observe above the sea surface. In this work, we introduce a novel long‐ and short‐term memory neural network and the corresponding training algorithm, to model the multi‐physics process instead of solving magneto‐hydrodynamics equations via numerical methods. Compared with commercial software, the proposed approach is much faster and easier to apply, which puts forward a feasible alternative for predicting the electromagnetic field distribution excited by underwater pressure wave.
Identifying unbalanced phase currents is crucial for control and fault alarm rates in power grids, especially in urban distribution networks. The zero-sequence current transformer, specifically ...designed for measuring unbalanced phase currents, offers advantages in measurement range, identity, and size, compared to using three separate current transformers. However, it cannot provide detailed information on the unbalance status beyond the total zero-sequence current. We present a novel method for identifying unbalanced phase currents based on phase difference detection using magnetic sensors. Our approach relies on analyzing phase difference data from two orthogonal magnetic field components generated by three-phase currents, as opposed to the amplitude data used in previous methods. This enables the differentiation of unbalance types (amplitude unbalance and phase unbalance) through specific criteria and allows for the simultaneous selection of an unbalanced phase current in the three-phase currents. In this method, the amplitude measurement range of magnetic sensors is no longer a critical factor, allowing for an easily attainable wide identification range for current line loads. This approach offers a new avenue for unbalanced phase current identification in power systems.
Here, a thermally switching absorber based on a one-dimensional photonic crystal containing a phase change material is proposed, which operates in the terahertz range. Vanadium dioxide (VO
2
) is ...utilized as the phase-change material in the structure, which shows semiconductor-to-metal transition with varying temperatures. The frequency of switching is regulated in such a way that according to the VO
2
thickness, the absorption band displays switching properties from low to high frequencies and vice versa, and also from narrow to broadband absorption at the same frequency when the temperature increases from 300 to 350 K. The absorptivity in both bands is obtained at over 90%. Field distribution profile and the impedance matching technique elucidate the physical mechanism of absorption peaks. At 300 K, maximum absorption is realized by localizing the intensity at the defect layer, and at 350 K, the Tamm state excitation makes it possible to achieve perfect absorption. Also, relative impedance matching of the structure at the peak frequencies with vacuum impedance explains high absorption. Finally, the effects of incidence angle and polarization of light that influence the absorption peaks are analyzed. According to the results, the proposed absorber, despite showing switching features between two bands, also can be adjusted by incident angle for both TE and TM polarizations. This work may have potential applications in designing terahertz switches, filters, and sensors.
One fundamental difficulty in low-frequency subsurface electromagnetic exploration is the low-frequency breakdown phenomenon in numerical computation. It makes the discretized linear system very ...poorly conditioned and thus difficult to solve. This issue is present in both integral equation and partial differential equation solution methods, and thus has attracted many researchers who have proposed various methods to overcome this difficulty. In this paper, we propose a new mixed spectral element method (mixed SEM) to eliminate this low-frequency breakdown problem and apply this method to solve the subsurface electromagnetic exploration problem. Since Gauss' law is now explicitly enforced in the mixed SEM to make the system matrix well-conditioned even at extremely low frequency, we can solve the linear system from dc to high frequencies. With the proposed method, we study the surface-to-borehole electromagnetic system for hydrocarbon exploration. Numerical examples show that the mixed SEM is accurate and efficient, and has significant advantages over conventional methods.
Traditional identification tags in the radio or optical domain are unsatisfactory in terms of size and security. In this paper, we theoretically demonstrate an ultracompact and chipless ...identification tag in the terahertz band using multi-resonant metasurfaces based on graphene. Benefiting from the exceptional electrical and optical properties of graphene, the proposed tags encoding different bit sequences have a uniform geometric shape and size, which is significant for large-scale fabrication processes. Also, their operating frequency could be independently tuned by altering the doping level of each graphene loop without changing their geometric parameters. Tag principles are revealed by absorption spectra and the physical mechanism is investigated with electric field distributions. In addition, we further evaluate the multilayer graphene-based terahertz identification tag for multi-bit coding application, which breaks the spatial constraints in a single layer. Therefore, the proposed terahertz identification tags based on graphene may have remarkable potential values in future chipless identification applications.
An active toroidal switch is numerically presented in a terahertz metamaterial. Transmission of toroidal dipole can be tuned by the conductivity of vanadium dioxide, and its modulation depth can be ...up to 32%. The resonance of toroidal dipole is verified by the distribution of electric current. Due to the unique field distribution of the toroidal mode, the proposed design may find potential applications in enhanced absorption and sensing.
We propose a domain decomposition method based on the spectral element method (DDM-SEM) for elastic wave computation in frequency domain. It combines the high accuracy of the spectral element method ...and the high degree of parallelism of a domain decomposition technique, which makes this method suitable for accurate and efficient simulations of large scale problems in elastodynamics. In the DDM-SEM, the original large-scale problem is divided into a number of well designed subdomains. We use the spectral element method independently for each subdomain, and the neighboring subdomains are connected by a frequency-domain version of Riemann transmission condition (RTC) for elastic waves. For the proposed method, we can employ the non-conforming meshes and different interpolation orders in different subdomains to maximize the efficiency. By separating the internal and boundary unknowns of each subdomain, an efficient and naturally parallelizable block LDU direct solver is developed to solve the final system matrix. Numerical experiments verify its accuracy and efficiency, and show that the proposed DDM-SEM can be a promising numerical tool for accurately and effectively solving large and multi-scale problems of elastic waves. It is potentially valuable for the frequency domain seismic inversion where multiple source illuminations are required.
Orbital angular momentum (OAM) has made it possible to regulate classical waves in novel ways, which is more energy- or information-efficient than conventional plane wave technology. This work aims ...to realize the transition of antenna radiation mode through the rapid design of an anisotropic dielectric lens. The deep learning neural network (DNN) is used to train the electromagnetic properties of dielectric cell structures. Nine variable parameters for changing the dielectric unit structure are present in the input layer of the DNN network. The trained network can predict the transmission phase of the unit cell structure with greater than 98% accuracy within a specific range. Then, to build the corresponding relationship between the phase and the parameters, the gray wolf optimization algorithm is applied. In less than 0.3 s, the trained network can predict the transmission coefficients of the 31 × 31 unit structure in the arrays with great accuracy. Finally, we provide two examples of neural network-based rapid anisotropic dielectric lens design. Dielectric lenses produce the OAM modes +1, -1, and -1, +2 under TE and TM wave irradiation, respectively. This approach resolves the difficult phase matching and time-consuming design issues associated with producing a dielectric lens.
As underwater disturbances (natural or artificial) occur in the ocean, moving seawater crossing the geomagnetic fields will produce weak circular currents. These currents can induce measurable ...magnetic fields, which might be useful for monitoring ocean internal waves using aeromagnetic survey. In this research, a spectral-element method (SEM) based on Gauss–Lobatto–Legendre (GLL) polynomials is presented to characterize the magnetic field induced by the underwater pressure waves. A concise mathematical model is established through combining the acoustic wave equations and Maxwell’s equations. Specifically, the acoustic–magnetic coupling simulation adopts the nodal-based SEM for acoustic analysis and edge-based SEM for electromagnetic analysis. The proposed SEM has spectral accuracy, as the error exponentially decreases with the order of the basis functions. Additionally, by adopting an independent modeling and mesh scheme in two solvers, respectively, the waste of computing resources is avoided. The experimental analysis demonstrates that the induced magnetic fields mechanically propagate with the acoustic wave, producing the pseudo-radiation phenomenon. The signals of these magnetic fields may extend for tens of kilometers and exist for hours under certain circumstances, which provide a theoretical basis for underwater target identification via high-sensitivity atomic magnetometer.
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