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The electromagnetic wave with spiral wavefront is a vortex beam carrying orbital angular momentum. The phase singularity of the vortex beam has special applications in the fields of particle manipulation and communication. In this paper, a terahertz (THz) wide-band vortex beam generator based on reflective metasurface is proposed and simulated. The device consists of a metasurface-dielectric-metal three-layer structure, and the top layer is a metasurface composed of two orthogonal I-shaped metal structural units. The intermediate layer of polyimide medium, and the bottom layer is of metal as a reflecting plate. The CST microwave studio is used to simulate the reflection performance of unit cell. The structure parameters are optimized to obtain the better performance. A set of optimed structure parameters is determined. According to the phase principle of Pancharatnam-Berry (P-B), by rotating the angle of the top-layer I-type metal structure, the reflection amplitudes of the unit cell structure at different rotation angles are required to approximately equal while the phase changes linearly with rotation angle and reaches a range of 2lπ for the topological charge number l. These cell structures are arranged according to the phase principle mentioned above. The metasurfaces of different topological charge numbers are designed to generate the corresponding vortex beams. In this paper, the metasurfaces with topological charge numbers 1 and 2 are designed. The reflection amplitude and phase of the circularly polarized THz beam incident vertically on the metasurface are simulated by using CST microwave studio. The simulation results show that in a frequency range of 0.8−1.4 THz, the metasurface can convert the circularly polarized terahertz beam into a vortex beam with a different topological charge number. In addition, in order to illustrate that the metasurface designed can produce a higher topological charge number of vortex beam, a metasurface with a topological charge number of 3 is designed as an example. The reflection amplitude and phase of the circularly polarized THz beam at a frequency of 1.1 THz is simulated. The results show that the designed metasurface can produce a vortex beam with a topological charge number of 3. The higher topological charges of vortex beam can also be generated according to the corresponding phase arrangement. The device has a relatively wide operating bandwidth, simple structure, high conversion efficiency, and has the potential application in terahertz vortex beam generation.

In this paper, the Burgers’ equation is transformed into the linear diffusion equation by using the Hopf–Cole transformation. The obtained linear diffusion equation is discretized in space by the local discontinuous Galerkin method. The temporal discretization is accomplished by the total variation diminishing Runge–Kutta method. Numerical solutions are compared with the exact solution and the numerical solutions obtained by Adomian’s decomposition method, finite difference method, B-spline finite element method and boundary element method. The results show that the local discontinuous Galerkin method is one of the most efficient methods for solving the Burgers’ equation. Even with small viscosity coefficient, it can get the satisfied solution.

Cylinder container is a thin cylindrical shell sealed by a top and bottom plate. The internal noise reduction of the cylinder container is very important for vehicle, ship and other projects. A finite element model of the cylinder container vibration coupled with internal acoustic field is set up and the acoustic response feature is determined. Then, two optimization models are adopted: (1) the acoustic response is taken as the object function and the structure weight is taken as constraints; (2) the structure weight is taken as the object function and the acoustic response is taken as constraints. The design variables are thickness distribution for shell, bottom plate and top plate. The optimization problem is solved by the feasible direction method and a 30 dB reduction of the pressure level is achieved for optimum designs. The results show the effectiveness of the method.

We investigate the transmission properties of planar H fractal structures in THz range. 3D EM simulations using HFSS are used to design the parameters of the fractals and to evaluate the optical properties. We observe the transmission spectra with pass bands and stop bands, which show the subwavelength transmission through the non-metallic gaps. This unique transmission property through subwavelength apertures makes it potentially useful frequency selective components in THz region. We experimentally demonstrate its behavior by designing and fabricating four prototype planar fractals in the range of 0-1.5THz and characterize using a polarizing Fourier transform spectrometer. We find good agreement between the models and measurements.

In this manuscript, we present a new antenna design applied to the THz band, based on using a Vivaldi construction and theory. Design of pass band is 155–180 GHz, Using three-dimensional (3D) finite-integral time-domain simulations and Optimization. The antenna adopt the medium of polyethylene for reducing Power Loss and use choke groove for restrain back flow and reflection, the use of array to further improve its directionality. The interesting THz antenna properties suggest a promising application in the modern THz communication systems.

Optical transmission properties of subwavelength planar fractals in terahertz (THz) frequency regime are studied by means of time-domain spectroscopy. The transmission spectra with multiple pass bands and stop bands are observed. The tunable photonic band gaps are realized by changing the angle between the principle axis of planar fractal and the polarization of THz wave. The possible application of the subwavelength optical component is discussed. We attribute the detected transmittance from subwavelength fractals to localized resonances.

We present a time-resolved ultrafast measurement in terahertz (THz) frequency region by means of the free-space electro-optic sampling. The fast delay scan technique is used to suppress the noise with low frequency and to improve the signal-to-noise ratio of the system. The transmission spectra of different materials are obtained. The optical properties of these materials in a THz region are shown. The broadening of spectrum and chirping phenomena are illustrated. We find that polystyrene is an excellent material for the THz application.

The electronic band structures, densities of states (DOSs), and projected densities of states (PDOSs) of the wurtzite In1−xGaxN with x = 0, 0.0625, 0.125 are studied using the generalized-gradient approximation (GGA) and GGA + U in density functional theory. Our calculations suggest that in the case of wurtzite InN it is important to apply an on-site Hubbard correction to both the d states of indium and the p states of nitrogen in order to recover the correct energy level symmetry and obtain a reliable description of the InN band structure. The method is used to study the electronic properties of the wurtzite In1−xGaxN. The conduction band minimum (CBM) energy increases, while the valence band maximum (VBM) energy decreases with the increase of the gallium concentration. The effect leads to broadening the band gap (BG) and the valence band width (VBW). Furthermore, the compressive strain in the crystal can cause the BG and the VBW to increase with the increase of gallium concentrations.