Time-reversal analysis for scatterer characterization

A new application of time-reversal processing of wave scattering data permits characterization of scatterers by analyzing the number and nature of the singular functions (or eigenfunctions) associated with individual scatterers when they have multiple contributions from monopole, dipole, and/or quadrupole scattering terms. We discuss acoustic, elastic, and electromagnetic scattering problems for low frequencies. Specific examples for electromagnetic scattering from one of a number of small conducting spheres show that each sphere can have up to six distinct time-reversal eigenfunctions associated with it.     

基于时间反演技术的电磁器件端口场与内部场转换方法

随着电磁器件的集成化,器件搭载的模块、实现的功能愈发多样.各模块间的耦合难以忽略,设计难度陡然增加,传统设计方法逐渐力不从心,迫切需要寻找一种新的电磁综合设计方法.本文利用时间反演电磁波的时空同步聚焦特性,探索了将时间反演技术应用于器件设计的可能性.首先,基于通用的器件逆设计流程,利用时间反演技术、并矢格林函数及电磁学的基本原理,提出了将器件端口场分布转换为内部场分布的方法,并证明由端口期望场的时间反演场在空间某一位置获得的连续等效源的共轭分布可在端口处产生与期望场接近的场分布.且在单点频逆设计过程中,只需知道端口电场或磁场的切向分量即可完成端口场与内部场的转换.同时,借助格林函数的互易性对本文所提理论做适当变换后,进行数值仿真验证,分析讨论了不同初始信息条件下该方法的适用性.仿真结果与理论相符,证明了理论的正确性,为将时间反演技术应用于电磁器件的逆设计提供了可能.     

Broken time reversal of light interaction with planar chiral nanostructures

We report unambiguous experimental evidence of broken time-reversal symmetry for the interaction of light with an artificial nonmagnetic material. Polarized color images of planar chiral gold-on-silicon nanostructures consisting of arrays of gammadions show intriguing and unusual symmetry: structures, which are geometrically mirror images, lose their mirror symmetry in polarized light. The symmetry of images can be described only in terms of antisymmetry (black-and-white symmetry) appropriate to a time-odd process. The effect results from a transverse chiral nonlocal electromagnetic response of the structure and has some striking resemblance with the expected features of light scattering on anyon matter.     

Theory of the time reversal cavity for electromagnetic fields

We derive a general expression of the electric dyadic Green function in a time-reversal cavity, based on vector diffraction theory in the frequency domain. Our theory gives a rigorous framework to time-reversal experiments using electromagnetic waves and suggests a methodology to design structures generating subwavelength focusing after time reversal.     

Time reversal of electromagnetic waves

We report the first experimental demonstration of time-reversal focusing with electromagnetic waves. An antenna transmits a 1-micros electromagnetic pulse at a central frequency of 2.45 GHz in a high-Q cavity. Another antenna records the strongly reverberated signal. The time-reversed wave is built and transmitted back by the same antenna acting now as a time-reversal mirror. The wave is found to converge to its initial source and is compressed in time. The quality of focusing is determined by the frequency bandwidth and the spectral correlations of the field within the cavity.     

An angular correlation test of time-reversal invariance

A test of time-reversal invariance in electromagnetic transitions is proposed in which all possible two-gamma angular correlations in a triple cascade are measured. The results can be combined to yield a term proportional to cos η.     

Seeing time-reversal transmission characteristics through kinetic anti-ferromagnetic Ising chain

As an example of our new approach to complex near-field (NF) scattering of electromagnetic waves, the time-reversal (TR) transmission process on an NF current-element array is mapped to the statistical process on a kinetic Ising transmission chain. Equilibrium statistical mechanics and non-equilibrium Monte Carlo (MC) dynamics help us to find signal jamming, aging, annihilating, creating, and TR symmetry breaking on the chain with inevitable background noises; and these results are general in NF systems where complex electromagnetic scattering arises.     

时间反演脉冲电磁波在双负材料中传播特性研究

利用全时域电磁仿真技术, 对比研究了时间反演脉冲电磁波和脉冲电磁波透过Smith结构双负材料后的时域波形、脉宽压缩以及材料内部空间电场峰值强度分布等时域传播特性. 结果表明：时间反演脉冲电磁波在透过双负材料后, 在初始源激励处表现出良好的时间和空间聚焦特性. 更重要的是, 在双负材料内部, 观测到了电场峰值强度减弱、截面电场峰值强度趋向均衡分布等新型的物理现象. 这些物理现象对发展能够承受大功率新型的双负材料电子器件及其应用系统很有参考研究价值.     

The determination of the relative permittivity of periodic stratified media based on the iterative time-reversal method

In this paper, we present a new method to determine the relative permittivity of periodic stratified media using the iterative time-reversal method. Based on transmission line theory, the focal peak value of iterative time-reversal electro- magnetic waves, which contain information about the periodic stratified medium, is computed in pulse-echo mode. Using the relationship between the focal peak value and the relative permittivity of the periodic stratified medium, the relative permittivity can be obtained by measuring the focal peak value. Numerical simulations are conducted, and the results demonstrate the feasibility of the proposed approach to the measurement of the relative permittivity of a periodic stratified medium.     

Reflection-free one-way edge modes in a gyromagnetic photonic crystal

We point out that electromagnetic one-way edge modes analogous to quantum Hall edge states, originally predicted by Raghu and Haldane in 2D photonic crystals possessing Dirac point-derived band gaps, can appear in more general settings. We show that the TM modes in a gyromagnetic photonic crystal can be formally mapped to electronic wave functions in a periodic electromagnetic field, so that the only requirement for the existence of one-way edge modes is that the Chern number for all bands below a gap is nonzero. In a square-lattice yttrium-iron-garnet crystal operating at microwave frequencies, which lacks Dirac points, time-reversal breaking is strong enough that the effect should be easily observable. For realistic material parameters, the edge modes occupy a 10% band gap. Numerical simulations of a one-way waveguide incorporating this crystal show 100% transmission across strong defects.     

一种水平变化波导中匹配场定位的虚拟时反实现方法

在水平变化波导中,匹配场被动定位的计算量非常大,严重阻碍了其工程应用.本文提出了一种水平变化波导中匹配场定位的虚拟时反实现方法,其抛物方程模型计算网格总数远小于匹配场处理,从而大大减小计算代价.与匹配场处理不同,虚拟时反实现方法是一个利用介质互易性和叠加性的后向传输过程.通过在各水听器位置放置虚拟声源,并在搜索区域产生相应的模糊平面,对各个模糊平面进行相应加权求和,获得的定位模糊平面.利用地中海浅海实验数据验证了虚拟时反实现方法的快速性能.     

Telecommunication in a disordered environment with iterative time reversal

Abstract

We present a method to transmit digital information through a highly scattering medium in a MIMO-MU (multiple input multiple output multiple users) context. It is based on iterations of a time-reversal process, and permits us to focus short pulses, both spatially and temporally, from a base antenna to different users. This iterative technique is shown to be more efficient (lower inter-symbol interference and lower error rate) than classical time-reversal communication, while being computationally light and stable. Experiments are presented: digital information is conveyed from 15 transmitters to 15 receivers by ultrasonic waves propagating through a highly scattering slab. From a theoretical point of view, the iterative technique achieves the inverse filter of propagation in the subspace of non-null singular values of the time-reversal operator. We also investigate the influence of external additive noise, and show that the number of iterations can be optimized to give the lowest error rate.

(Some figures in this article are in colour only in the electronic version)     

Charge transfer between a superconductor and a hopping insulator

We develop a theory of the low-temperature charge transfer between a superconductor and a hopping insulator. We show that the charge transfer is governed by the coherent two-electron-Cooper pair conversion process time-reversal reflection, where electrons tunnel into a superconductor from the localized states in the hopping insulator located near the interface, and calculate the corresponding interface resistance. A specific feature of this problem is the interplay between the time-reversal reflection at the interface and transport through the percolation cluster. To allow for this interplay, we have generalized the connectivity criterion of the percolation theory to include surface effects. We show that the time-reversal interface resistance is accessible experimentally, and that in mesoscopic structures it can exceed the bulk hopping resistance.     

Quantum entropy production as a measure of irreversibility

We consider conservative quantum evolutions possibly interrupted by macroscopic measurements. When started in a nonequilibrium state, the resulting path-space measure is not time-reversal invariant and the weight of time-reversal breaking equals the exponential of the entropy production. The mean entropy production can then be expressed via a relative entropy on the level of histories. This gives a partial extension of the result for classical systems, that the entropy production is given by the source term of time-reversal breaking in the path-space measure.     

时间反演电磁波超分辨率特性

时间反演电磁波在特定的远场环境下具有超分辨率特性.结合时间反演技术构建具有超分辨率特性的模型进行仿真,对结果分析后筛选出能够实现超分辨率特性的关键因素,可为实用的小型化、多天线无线通信系统的设计提供指导. 关键词： 时间反演电磁波 时间反演技术 超分辨率     

Electric–magnetic duality of lattice systems with topological order

We investigate the duality structure of quantum lattice systems with topological order, a collective order also appearing in fractional quantum Hall systems. We define electromagnetic (EM) duality for all of Kitaev?s quantum double models based on discrete gauge theories with Abelian and non-Abelian groups, and identify its natural habitat as a new class of topological models based on Hopf algebras. We interpret these as extended string-net models, whereupon Levin and Wen?s string-nets, which describe all intrinsic topological orders on the lattice with parity and time-reversal invariance, arise as magnetic and electric projections of the extended models. We conjecture that all string-net models can be extended in an analogous way, using more general algebraic and tensor-categorical structures, such that EM duality continues to hold. We also identify this EM duality with an invertible domain wall. Physical applications include topology measurements in the form of pairs of dual tensor networks.     

Optimal performance at arbitrary power of minimally nonlinear irreversible thermoelectric generators with broken time-reversal symmetry

We investigate the performance at arbitrary power of minimally nonlinear irreversible thermoelectric generators (MNITGs) with broken time-reversal symmetry within linear irreversible thermodynamics, and the efficiency of MNITGs at arbitrary power is analytically derived. Furthermore, a universal bound on the efficiency of thermoelectric generators (TGs) with broken time-reversal symmetry and the arbitrary power is obtained. Some system-specific characteristics are discussed and uncovered. A large efficiency at arbitrary power can also be achieved via the cooperative mechanism between the system parameters. Our results indicate that the broken time-reversal symmetry provides the physically allowed degrees of freedom for tuning the performance of thermoelectric devices, and the physical trade-off region between the efficiency and the power output can also offer the appropriate space for optimizing the performance of TGs.     

Edge currents in superconductors with a broken time-reversal symmetry

We analyze edge currents and edge bands at the surface of a time-reversal symmetry breaking dx2-y2 + id(xy) superconductor. We show that the currents have large Friedel oscillations with two interfering frequencies: square root of 2kF from subgap states, and 2kF from the continuum. The results are based independently on a self-consistent slave-boson mean-field theory for the t-J model on a triangular lattice, and on a T-matrix scattering theory calculation. The shape of the edge-state band, as well as the particular frequency square root of 2kF of the Friedel oscillations, are attributes unique for the dx2-y2 + id(xy) case, and may be used as a fingerprint for its identification. Extensions to different time-reversal symmetry breaking superconductors can be achieved within the same approach.     

Test der Zeitumkehrinvarianz der elektromagnetischen Wechselwirkung am 73 keV-γ-Übergang von193Ir

In order to test the time-reversal invariance of the electromagnetic interaction of nucleons the phase angle η between theE2- andM1-matrix-elements of the mixed \(\frac{1}{2}^ + - \frac{3}{2}^ + \) -transition in¹⁹³Ir has been measured in a Mössbauer experiment. In a measurement which avoids possible systematic errors the sine of the phase angle which is proportional to the time-reversal odd part of the transition matrix element has been deduced. The value is sinη=M _odd/M _even=(1.6±2.4) · 10^?3. In accordance with the results of other experiments there is no evidence for a strong violation of time-reversal invariance in the electromagnetic interaction as postulated by Bernstein, Feinberg, and Lee.     

Measurement of wave chaotic eigenfunctions in the time-reversal symmetry-breaking crossover regime

We present experimental results on eigenfunctions of a wave chaotic system in the continuous crossover regime between time-reversal symmetric and time-reversal symmetry-broken states. The statistical properties of the eigenfunctions of a two-dimensional microwave resonator are analyzed as a function of an experimentally determined time-reversal symmetry-breaking parameter. We test four theories of one-point eigenfunction statistics and introduce a new theory relating the one-point and two-point statistical properties in the crossover regime. We also find a universal correlation between the one-point and two-point statistical parameters for the crossover eigenfunctions.