Perfect invisibility concentrator with simplified material parameters

We present a series of invisibility concentrators with simplified material parameters beyond transformation optics. One of them can achieve the perfect invisible effect at frequencies of Fabry–Pérot resonances, while others have very small scattering. The required materials are feasible in practice. Analytical calculations and numerical simulations confirm the functionalities of these devices.     

Macroscopic invisibility cloak for visible light

Invisibility cloaks, a subject that usually occurs in science fiction and myths, have attracted wide interest recently because of their possible realization. The biggest challenge to true invisibility is known to be the cloaking of a macroscopic object in the broad range of wavelengths visible to the human eye. Here we experimentally solve this problem by incorporating the principle of transformation optics into a conventional optical lens fabrication with low-cost materials and simple manufacturing techniques. A transparent cloak made of two pieces of calcite is created. This cloak is able to conceal a macroscopic object with a maximum height of 2 mm, larger than 3500 free-space-wavelength, inside a transparent liquid environment. Its working bandwidth encompassing red, green, and blue light is also demonstrated.     

正N边形柱的隐身条件的严格推导及其隐身特性验证

基于坐标变换理论,提出并推导了正N边形柱的隐身条件,并得到了相应隐身罩材料本构参数张量的通解表达式. 根据导出的本构参数张量,利用电磁仿真软件分别对N取不同值时的三个典型算例进行仿真验证. 仿真结果证实了所得到的本构参数张量的正确性. 考虑到损耗对于隐身效果的影响分析,这些分析结果为隐身物理机理的进一步理解,以及降低对称度隐身罩的设计奠定了理论基础.     

Acoustic carpet invisibility cloak with two open windows using multilayered homogeneous isotropic material

We present a method for designing an open acoustic cloak that can conceal a perturbation on flat ground and simultaneously meet the requirement of communication and matter interchange between the inside and the outside of the cloak. This cloak can be constructed with a multilayered structure and each layer is an isotropic and homogeneous medium. The design scheme consists of two steps: firstly, we apply a conformal coordinate transformation to obtain a quasi-perfect cloak with heterogeneous isotropic material; then, according to the profile of the material distribution, we degenerate this cloak into a multilayered-homogeneous isotropic cloak, which has two open windows with negligible disturbance on its invisibility performance. This may greatly facilitate the fabrication and enhance the applicability of such a carpet-type cloak.     

正N边形柱的隐身条件的严格推导及其隐身特性验证

基于坐标变换理论,提出并推导了正N边形柱的隐身条件,并得到了相应隐身罩材料本构参数张量的通解表达式. 根据导出的本构参数张量,利用电磁仿真软件分别对N取不同值时的三个典型算例进行仿真验证. 仿真结果证实了所得到的本构参数张量的正确性. 考虑到损耗对于隐身效果的影响分析,这些分析结果为隐身物理机理的进一步理解,以及降低对称度隐身罩的设计奠定了理论基础. 关键词： 坐标变换 非均匀各向异性介质 本构参数张量 隐身     

Multifrequency optical invisibility cloak with layered plasmonic shells

Here, we theoretically suggest the possibility of employing a multilayered plasmonic shell as a cloak for reducing the total scattering cross section of a particle, simultaneously at different frequencies in the optical domain. By exploiting the frequency dispersion of plasmonic materials and their inherent negative polarizability, it is shown, theoretically and with numerical simulations, how covering a dielectric or conducting object of a certain size with this multilayered cloak may reduce its "visibility" by several orders of magnitude simultaneously at multiple frequencies.     

Scattering characteristics of nonmagnetic invisibility cloak with minimized scattering

A kind of transformation functions is proposed to realize the nonmagnetic invisibility cloak with minimized scattering on the basis of generalized transformation. By matching the impedance at the outer surface of the cloak, the transformations with two parameters are determined. The good performance of the cloak is indicated by the full wave simulation based on the finite element method. Furthermore, based on the calculation of total scattering cross section, it is shown that the scattering cross section is very sensitive to the different parameters even though the impedance at the exterior boundary matches perfectly with the free space. In addition, from the effective media theory, an alternating layered system composed of two isotropic materials is proposed to realize experimentally the cloak.     

Experimental investigation of omnidirectional multiphysics bilayer invisibility cloak with anisotropic geometry

Thermal-electric bilayer invisibility cloak can prevent the heat flux and electric current from touching the object without distorting the external temperature and electric potential fields simultaneously. In this paper, we design an omnidirectional thermal-electric invisibility cloak with anisotropic geometry. Based on the theory of neutral inclusion, the anisotropic effective thermal and electric conductivities of confocal elliptical bilayer core–shell structure are derived, thus obtaining the...     

Extraordinary surface voltage effect in the invisibility cloak with an active device inside

The electromagnetic field solution for a spherical invisibility cloak with an active device inside is established. Extraordinary electric and magnetic surface voltages are induced at the inner boundary of a spherical cloak, which prevent electromagnetic waves from going out. The phase and handness of polarized waves obliquely incident on such boundaries are kept in the reflected waves. The surface voltages due to an electric dipole inside the concealed region are found equal to the auxiliary scalar potentials at the inner boundary, which consequently gain physical counterparts in this case.     

Bandwidth enhancement of transformation optics-based cloak with reduced parameters

In this paper, dispersive cloak design with broad bandwidth and minimal scattering cross section is proposed by appropriately selecting a radial permeability for each shell in a discretized reduced cloak. The dispersive medium is constructed by artificially varying the inner radius of the cloak with frequency, and this variation results into unique material properties at every frequency. The variation of inner radius of the cloak with frequency is artificial since the actual physical dimension of inner radius remains invariant. The relation between bandwidth and geometrical parameters of cloak is obtained by ensuring that transformation media must satisfy the condition that group velocity must remain less than the speed of light along every direction for a finite frequency range. The proposed cloak provides \(8.9\,\%\) bandwidth with respect to the center frequency for \(50\,\%\) reduction in total scattering cross section, and at the design frequency, the minimum scattering cross section obtained is \(0.266\). The proposed dispersive cloak design is verified by numerical full-wave simulations results which also confirm good cloaking performance.     

Transparent device with homogeneous material parameters

A transparent device is an electromagnetic structure that is explicitly designed to be transparent to incoming electromagnetic waves. Based on coordinate transformation theory, a novel diamond-shaped transparent device is proposed and designed. It can protect electronic equipments inside without affecting their performance. Compared with a traditional transparent device, the material parameters of the novel transparent device feature homogeneity and non-singularity, which can easily be realized in actual applications. Full-wave simulations are made to validate the performance of the transparent device.     

Diamond-shaped acoustic concentrator with homogeneous material parameters

Based on coordinate transformation method, the two-dimensional and three-dimensional acoustic concentrators with diamond shape composed of nonsingular homogeneous materials are designed in this paper. The mass density tensor and bulk modulus of the acoustic concentrators are derived. Performance of the concentrators is confirmed by full-wave simulation. The work represents an important progress towards the practical realization of the metamaterial-assisted acoustic concentrator and expands the application of the coordinate transformation method.     

Characteristic material parameters of CIGS solar cell related with device performance

CIGS solar cells with power conversion efficiency (PCE) in the range of 1.82%–12.30% were obtained by using two-step process, and were further analyzed through various measurement techniques. Material parameters showed diverse values and some trends depending on the device performance. The lower performance device showed small integrated PL intensity, short minority life time, larger defect density and lower activation energy, whereas the higher performance device showed opposite values. We investigated relationship between material parameters and PCE of solar cells, and found that some physical parameters such as integrated PL intensity, minority life time, defect density, and difference between band gap and activation energy (E_g-E_a), which all reflect defect states in bulk and at pn interface, are strongly related with PCE and would be used as a good indicator to evaluate device performance quickly.     

Influence of laser parameters and material properties on micro drilling with femtosecond laser pulses

We report experimental results of drilling micro holes in Al, Mo, Ti, Cu, Ag, Au, and brass thin metal foils in air using 60-femtosecond Ti:sapphire laser pulses. The influence of laser parameters and material properties on drilling processes at the sub-10-micron scale has been examined. Comparison of hole drilling using 60-fs pulses with that using 50-ps and 10-ns pulses at an energy fluence of ~15 J/cm² indicates that a significant amount of melting is present in both ps and ns regimes, whereas with fs pulses, fewer and much smaller droplets from the molten phase are observed.     

高功率微波及材料特性参数对沿面击穿的影响

为研究高功率微波及材料特性参数对介质沿面闪络击穿过程的影响,采用自编的1D3V PIC-MCC程序,通过粒子模拟手段,得到了电子与离子数目、电子及离子密度分布、空间电荷场时空分布、电子平均能量、放电功率、表面沉积功率、激发电离损耗功率、电离频率等重要物理量。结果表明：电离频率随场强增加而增加,达到饱和后缓慢下降,强场诱发的二次电子数目更多导致本底沉积功率增高;电离频率随频率减小而增加,达到饱和后缓慢下降,频率太高会抑制次级电子倍增;因此,低频强场下击穿压力较大;反射引发表面电场下降及磁场增加效应,降低表面场强虽使表面击穿压力下降,但磁场的增加会导致二次电子倍增起振时间缩短,且会增加器件内部击穿风险;圆极化相对线极化诱导二次电子数目更多、本底沉积功率更高,击穿风险增加;短脉冲产生电子、离子总数少,平均能量低,沉积功率低,击穿风险低于长脉冲;脉冲上升时间的缩短和延长,只会提前或推后击穿时间,并不会改善击穿压力;材料二次电子发射率的增加会给击穿造成巨大压力,表面光滑度对击穿过程影响不大;电离频率和电子平均能量随释气压强增加均先增加后减小,低气压二次电子倍增占优,高气压碰撞电离占优。     

Structuring of material parameters in lithium niobate crystals with low-mass, high-energy ion radiation

Ferroelectric lithium niobate crystals offer a great potential for applications in modern optics. To provide powerful optical components, tailoring of key material parameters, especially of the refractive index n and the ferroelectric domain landscape, is required. Irradiation of lithium niobate crystals with accelerated ions causes strong structured modifications in the material. The effects induced by low-mass, high-energy ions (such as ³He with 41?MeV, which are not implanted, but transmit through the entire crystal volume) are reviewed. Irradiation yields large changes of the refractive index ??n, improved domain engineering capability within the material along the ion track, and waveguiding structures. The periodic modification of ??n as well as the formation of periodically poled lithium niobate (PPLN) (supported by radiation damage) is described. Two-step knock-on displacement processes, ³He??Nb and ³He??O causing thermal spikes, are identified as origin for the material modifications.     

高功率微波及材料特性参数对沿面击穿的影响

为研究高功率微波及材料特性参数对介质沿面闪络击穿过程的影响,采用自编的1D3V PIC-MCC程序,通过粒子模拟手段,得到了电子与离子数目、电子及离子密度分布、空间电荷场时空分布、电子平均能量、放电功率、表面沉积功率、激发电离损耗功率、电离频率等重要物理量。结果表明:电离频率随场强增加而增加,达到饱和后缓慢下降,强场诱发的二次电子数目更多导致本底沉积功率增高;电离频率随频率减小而增加,达到饱和后缓慢下降,频率太高会抑制次级电子倍增;因此,低频强场下击穿压力较大;反射引发表面电场下降及磁场增加效应,降低表面场强虽使表面击穿压力下降,但磁场的增加会导致二次电子倍增起振时间缩短,且会增加器件内部击穿风险;圆极化相对线极化诱导二次电子数目更多、本底沉积功率更高,击穿风险增加;短脉冲产生电子、离子总数少,平均能量低,沉积功率低,击穿风险低于长脉冲;脉冲上升时间的缩短和延长,只会提前或推后击穿时间,并不会改善击穿压力;材料二次电子发射率的增加会给击穿造成巨大压力,表面光滑度对击穿过程影响不大;电离频率和电子平均能量随释气压强增加均先增加后减小,低气压二次电子倍增占优,高气压碰撞电离占优。     

Calculation of effective material parameters by field averaging over lattices with non-negligible unit cell size

The Pendry et?al. field averaging method for calculation of effective material parameters is reviewed and its limits explored. The method is then extended so that it can accurately calculate the effective material parameters of lattices where the unit cell size is appreciable but still quasistatic (d??0.1?? ₀). The new algorithm is verified by calculating the effective material properties of periodically placed particles suspended in free space, as the unit cell size becomes appreciable. Results of our proposed formulation are then compared with the Pendry et?al. and conventional volumetric averaging algorithms.