Proposal and analysis of artificial dielectric lens with metallic corrugated structures for terahertz wave band

Optical devices for the terahertz wave band are being developed now and require better designs. This paper proposes an artificial dielectric lens with metallic corrugated structures for the terahertz wave band. A periodic analysis model extracted from the full model by assuming periodicity confirms the phase delay, which produces the focusing effect. Full model analysis also confirms the focusing effect. The full model analysis also confirms that the focusing length is longer as the spacing of corrugated baffles is wider. The focusing length is longer the metallic groove width is wider. The focusing length is longer as the groove depth is shallower. The lens shape without grooves does not produce the focusing effect. The results of the full model analysis are qualitatively consistent with those of the periodic model ones. This implies that the design for an exact size lens is possible by using the periodic model.     

Parallel plate lens with metal hole array for terahertz wave band

Optical devices for terahertz wave band from 0.1 to 10 THz are rapidly expanding and require better designs. This paper proposes and designs a parallel plate lens with metal hole array for the terahertz wave band. The fast wave effect is due to the parallel plate. For this lens, the parallel plate spacing and hole array dimensions control the phase velocity and the focusing effect. It is not necessary to control the phase through the lens shape, which is flat, itself. The periodic analysis model extracted from the full model confirms the phase control by the metal hole array dimensions. The periodic model can be used for efficient iterative design. The full wave analysis results are also obtained by ANSYS HFSS and the focusing effect is confirmed. Phase control using both the parallel plate and the hole array enhances the focusing effect over the focusing effect controlled only by the metal hole array dimensions.     

Focusing effect measurements of artificial dielectric multilayer lens with metal rectangular chips for terahertz wave band

This paper presents the design and fabrication of an artificial multilayer lens comprising ten layers of metal rectangular chips on a cyclo olefin polymer, which has low loss in the terahertz band. Measurements of the focusing effect are also presented. The focusing effect is produced by the rectangular metallic chips which act as an electrical dipole. Simulations are performed using ANSYS HFSS. The lenses are fabricated by laser processing or semiconductor etching. The focusing effect is confirmed by measurement using terahertz near-field microscopy, although some discrepancies are observed between the simulated and measured results.     

Influence of cutting off position of plasma filament formed by two-color femtosecond laser on terahertz generation

Femtosecond laser filamentation is a method of generating terahertz, which has wide application in terahertz subwavelength resolution imaging. In this paper, the plasma filament formed by femtosecond laser focusing was terminated with an alumina ceramics at different positions and the influence of the cutting off position of the plasma filament on the terahertz wave was studied. The results showed that the terahertz amplitude increases as the position approaches the end of the filament gradually. The stability of amplitude and peak frequency of the terahertz generated by the filament formed by two-color femtosecond laser via a lens with a longer focal length is lower than that through a lens with a shorter focal length, especially the terahertz amplitude at the end of the filament. The study will be helpful for future researchers in the field of THz sub-wavelength imaging utilizing femtosecond laser filament.     

Free wave propagation in two-dimensional periodic plates

The propagation of flexural waves in a two-dimensional periodic plate which rests on an orthogonal array of equi-spaced simple line supports has been investigated. A type of plane wave motion has been considered. An energy method has been developed to predict the frequency of wave propagation in terms of the propagation constants. A Galerkin type of analysis has been used, incorporating assumed complex modes of wave motion for the identical rectangular elements of the periodic plate. Expressions for the frequency have been obtained firstly by using simple polynomial modes for the plate displacements, and then (alternatively) by using characteristics beam function modes. The use of these different modes has first been demonstrated by applying them to the analysis of wave propagation in periodic beams. A single polynomial mode which satisfies the geometric and wave-boundary conditions of the periodic plate element leads to an elegant expression relating the frequency and the wave propagation constants in the first propagation band. The frequencies so obtained compare well with those found from a multi-mode, characteristic beam function analysis. The latter involves much more algebra, is solved as an eigenvalue problem, and yields the frequencies in as many propagation bands as are desired. The bounding frequencies and corresponding wave motions in the first and higher propagation bands have been identified, and it has been shown that the propagation bands can overlap. Consideration has been given to one-dimensional “strip” structures which are equivalent to the two-dimensional plate when a plane wave in a general direction is propagating. Furthermore, it is shown that the natural frequencies of finite rectangular periodic plates can be obtained very simply from the results of the wave propagation analysis.     

Analysis and design of concave lens with metallic slit array for terahertz wave band

This paper presents a three-dimensional concave lens constructed from a metallic slit array. The effective refractive index is estimated to be $0<n<1$ . The actual lens requires spacers in the slits. A lens using a low loss dielectric material for the terahertz frequency band is designed. The fast wave effect is enhanced in order to mitigate the slow wave effect of the dielectric material. Full wave analysis results are obtained by ANSYS HFSS and the focusing effect of the three-dimensional concave lens is confirmed in the terahertz frequency band.     

Optically induced lensing effect on a Bose-Einstein condensate expanding in a moving lattice

We report the experimental observation of a lensing effect on a Bose-Einstein condensate expanding in a moving 1D optical lattice. The effect of the periodic potential can be described by an effective mass dependent on the condensate quasimomentum. By changing the velocity of the atoms in the frame of the optical lattice, we induce a focusing of the condensate along the lattice direction. The experimental results are compared with the numerical predictions of an effective 1D theoretical model. In addition, a precise band spectroscopy of the system is carried out by looking at the real-space propagation of the atomic wave packet in the optical lattice.     

Focusing properties of radially polarized Bessel-like beam with radial cosine phase wavefront by a high numerical aperture objective

By the use of the vector diffraction theory, the focusing properties of radially polarized Bessel-like beam with radial cosine wave front phase through a high numerical-aperture (NA) lens are investigated theoretically and numerically in this work. The wave front phase distribution is a cosine function of radial coordinate. Calculation results show that focus shift is considerably influenced by changing frequency parameter C and topological charge. An increase on the focus shift C, the total intensity pattern changes remarkably and it focuses along the optical propagation axis. Thus, the focal shift direction can be adjusted by the change of the frequency parameter in cosine function. In this paper, the generation of the focal spot allows an increase in focal length in the axial direction of the incident beam propagation. Under higher numerical aperture (NA = 0.95), the effect of the frequency parameter and topological charge on the focal pattern gets stronger.     

Effects of wave propagation anisotropy on the wave focusing by negative refractive sonic crystal flat lenses

<正>In this study,wave propagation anisotropy in a triangular lattice crystal structure and its associated waveform shaping in a crystal structure are investigated theoretically.A directional variation in wave velocity inside a crystal structure is shown to cause bending wave envelcpes.The authors report that a triangular lattice sonic crystal possesses six numbers of a high symmetry direction,which leads to a wave convergence caused by wave velocity anisotropy inside the crystal.However,two of them are utilized mostly in wave focusing by an acoustic flat lens.Based on wave velocity anisotropy,the pseudo ideal imaging effect obtained in the second band of the flat lens is discussed.     

基于蜷曲空间结构的近零折射率声聚焦透镜

研究基于蜷曲空间结构的近零折射率声聚焦透镜.根据近零折射率材料的声波方向选择机理,采用蜷曲空间结构为基本单元进行排列,设计具有特定入射与出射界面的几何结构,对透射声波的出射方向进行调控,实现了平面声波与柱面声波的聚焦效应,并深入讨论了透镜内部刚性散射体对声聚焦性能的影响.在此基础上,改变近零折射率透镜的出射界面,可以精确调控声波阵面的形状与方向.该类型透镜具有单一的单元结构、高聚焦性能及高鲁棒性等优点.研究结果为设计新型近零折射率声聚焦透镜提供了理论指导与实验参考,同时也为研究声波阵面的调控提供了新思路.     

Scattering of Circularly Polarized Terahertz Waves on a Graphene Nanoantenna

We present a surface current method to model the graphene rectangular nanoantenna scattering in the terahertz band with Comsol.Compared with the equivalent thin slab method,the results obtained by the surface current method are more accurate and efficient.Then the electromagnetic scattering of circularly polarized terahertz waves on graphene nanoantennas is numerically analyzed by utilizing the surface current method.The dependences of the antenna resonant frequency with the circularly polarized wave on width and length are consistent with those for the linear polarized waves.These results are proved to be useful to design efficient nanoantennas in terahertz wireless communications.     

有限宽平板波导中的克尔空间孤子及全光器件的设计

利用数值模拟的方法研究了倾斜入射的傍轴光束在有限宽非线性平板波导内的传输规律.研究发现,当不同波长相同束宽的光束各以临界功率倾斜入射波导时,如果倾斜角、入射点都一样,它们将沿大致相同的周期性的Z字形路径传输;如果光束功率进一步增大,相邻反射点之间的间距随传输距离有逐渐变大的趋势,Z字形路径不再有严格的周期性;当功率相当大时,光束将沿波导z方向传输,不再在波导的两个边界之间来回反射.利用倾斜入射光束在波导内的传输路径随功率而变的特点,设计了一个功率开关和一个光时分解复用器. 关键词： 空间光孤子 有限宽非线性平板波导 全光器件     

Frequency Selective Properties of Two-Dimensional Dielectric Gratings: TE- and TM-Polarizations

This work analyzes the propagation characteristics of electromagnetic waves and the frequency selectivity of dielectric gratings with periodicity in two dimensions, having in mind their application as dichroic surfaces in the millimeter wave band. The moment method and the Green's functions, along with the volume equivalence theorem are used. Particular cases were analyzed to compare with results available in the specialized literature and agreement was observed. This work gives evidence that the inclusion of the periodicity along a second dimension allows an additional adjusting parameter of the frequency selective characteristics for the design requirements of dielectric gratings.     

Electromechanical tailoring of structure with periodic piezoelectric circuitry

In this paper, we study the integration of piezoelectric transducers and electrical circuitry onto a mechanical host structure to form an electromechanical periodic system. This scheme enables us to take advantage of the unique wave propagation characteristics of spatially periodic systems and thus manipulate the energy flow/distribution in the host mechanical structure. Different from most previous investigations where spatial periodicity is typically due to the periodic discontinuities in the mechanical structure, in the proposed scheme the mechanical structure can remain to be continuous without being altered. Instead, the electrical circuitry consists of periodic branches which, when coupled to the mechanical structure through the piezoelectric transducers, yields repetitive subsystems in the electromechanical integrated system. The circuitry elements can be online tunable, effectively making the energy manipulation adaptive to excitation variations. A transfer matrix-based approach is adopted in the dynamic analysis of the integrated system, where each subsystem is represented by two state vectors with a transfer matrix relating them. Given the repetitive nature of the subsystems due to periodicity, analysis on the entire system is conducted by focusing on one single subsystem. Systematic analysis is carried out to demonstrate the characteristics of wave propagation and attenuation in terms of propagation constants as well as the mechanical and electrical responses. Effects of circuitry tuning are also discussed through detailed parametric studies. Implications and potential applications of the proposed scheme are illustrated.     

基于波传播法的周期复合板振动带隙衰减特性研究

运用波传播法对有限和无限周期对边简支复合板的振动带隙衰减特性进行了研究.在建立相邻板结构边界连续方程的基础上, 分别运用传递矩阵和Bloch定理建立了有限和无限周期复合板的耦合运动方程, 并详细对比分析了有限和无限周期复合板带隙衰减特性的关联关系.研究表明: 周期板结构的振动带隙频率范围与激励方式和激励位置是相关的, 若周期复合板在宽度方向按某阶模态进行线激励, 则该激励下的振动带隙与无限周期复合板在该阶模态下的振动带隙是一致的; 若周期板在点激励作用, 则该点激励下的振动带隙是参与振动的各阶模态振动带隙的交集. 此外, 还进一步研究了结构阻尼对振动衰减带隙的影响. 关键词： 周期复合板 带隙衰减特性 波传播法 结构阻尼     

Diffraction of terahertz waves after passing through a Fresnel lens

The spatiotemporal and spectral characteristics of ultrawide-band terahertz pulses after passing through a Fresnel lens are studied by using the scalar diffraction theory. The simulation shows that the transmitted terahertz waveforms compress with increasing propagation distance, and the multi-frequency focusing phenomenon at different focal points is observed. Additionally, the distribution of terahertz fields in a plane perpendicular to the axis is also discussed, and it is found that the diffraction not only induces focusing on-axis but also inhibits focusing at off-axis positions. Therefore, the Fresnel lens may be a useful alternative approach to being a terahertz filter. Moreover, the terahertz pulses travelling as a basic mode of a Gaussian beam are discussed in detail.     

Flat lenses constructed by graded negative index-based photonic crystals with tuned configurations

Flat lenses are designed by means of graded negative refractive index-based photonic crystals （PCs） constructed using air-holes tuned with different shapes. By gradually modifying the filling factor along the transverse direction, we obtain the graded negative index-based lenses for the purpose of focusing an incident plane wave. The finite-difference and timedomain （FDTD） algorithm is adopted for numerical calculation. Our calculation results indicate that these lenses can finely focus incident plane waves. Moreover, for the same size of air-holes, the focusing properties of the lens with rectangular air-holes are better than those with the other shaped air-holes. The graded negative index PCs lenses could possibly enable new applications in optoelectronic systems.     

Wave propagation along transversely periodic structures

The dispersion curves for guided waves have been of constant interest in the last decades, because they constitute the starting point for NDE ultrasonic applications. This paper presents an evolution of the semianalytical finite element method, and gives examples that illustrate new improvements and their importance for studying the propagation of waves along periodic structures of infinite width. Periodic boundary conditions are in fact used to model the infinite periodicity of the geometry in the direction normal to the direction of propagation. This method allows a complete investigation of the dispersion curves and of displacement/stress fields for guided modes in anisotropic and absorbing periodic structures. Among other examples, that of a grooved aluminum plate is theoretically and experimentally investigated, indicating the presence of specific and original guided modes.     

全向入射条件下一维固流周期结构中低频声裂隙变化特性研究

利用传递矩阵法, 从理论上建立了全向入射条件下一维固-流周期结构中的声传播模型, 在此基础上计算、分析并比较了无限周期结构的声能带结构和有限周期结构中的声传输特性. 研究结果表明, 当声波以一定的入射角入射时, 固-流周期结构的低频通带区域存在一个声裂隙, 该声裂隙所对应的入射角大小与构成周期结构的固体层和流体层的密度或结构尺寸无关, 而仅取决于构成该周期性结构材料的波速. 关键词： 传递矩阵 全向入射 固-流周期结构 声裂隙     

Dispersion properties of a periodic semiconductor structure in a magnetic field directed along the periodicity axis

The band spectrum of natural waves in a periodic structure formed by alternating dielectric and semiconductor layers is investigated for the propagation of waves at some angle with respect to a magnetic field applied along the periodicity axis. A method is presented for deriving a dispersion equation, and its properties are analyzed numerically. It is shown that, in the absence of dissipation, there are two independent spectra of natural waves in the structure being considered, and the regions of existence of various types of such waves are classified. It is established that the transmission bands of the two spectra may supplement one another or overlap. It is found that, for the chosen magnetic-field direction, there exist numerous bands of transmission of cyclotron waves.