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Relationship between group velocity and symmetry of photonic crystals

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Chinese Physics B 2010年第5期19卷 363-368页

作者：叶卫民罗章袁晓东曾淳 College of Optoelectronic Science and Engineering National University of Defense Technology

Group velocity （GV） of eigenmode is a crucial parameter to explain the extraordinary phenomena about light propagation in photonic crystals （PhCs）. To study relationships between group velocity and symmetry of PhCs, a new general expression of CV in PhCs made up of non-dispersive material is introduced. Based on this, the CVs of eigenmodes of PhCs, especially those of degenerate eigenmodes at highly symmetric points in the first Brillouin zone, are discussed. Some interesting results are obtained. For example, the summation of degenerate eigenmodes＇ CVs is invariant under the operations of wave vector K-group MK. In addition, some numerical results are presented to verify them.

关键词： group velocity symmetry photonic crystals degenerate eigenmode

Application of the Eu^(3+) Ion for Site symmetry Determination

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Journal of Rare Earths 1996年第3期14卷 173-180页

作者： Binnemans K Gorller-Walrand C (University of Leuven,Department of Chemistry, Coordnation Chemistry Division,Celesti jnenlaan 200F, B-3001 Heverlee, Belgium) University of Leuven Department of Chemistry Coordination Chemistry Division Celestijnenlaan 200F B-3001 Heverlee Belgium

A scheme has been developed to determine the site symmetry of rare earth compounds on the basis of the number and polarization of the peaks in the Eu3+ absorption spectrum for the magnetic dipole allowed 5D1←7Fo transition and the electric dipole allowed 5D←7Fo, 5D2←7Fo and 5L6 ←7F0 transitions. The Eu3+ ion is chosen as probing ion because of its non-degenerate ground state 7F0 and the well-separated 2s+1LJ multiplets. For the luminescence spectrum, an adapted scheme is presented. In this way it is possible to discriminate between all non-centrosymmetric crystallographic point groups. For rare earth compounds in solutions the scheme has to be adapted, because no polarised spectra can be recorded, only the number of spectral peaks can be counted.

关键词： Europium Lanthanides Crystal field Spectroscopy Optical absorption Luminescence symmetry

Hidden-symmetry-enforced nexus points of nodal lines in layer-stacked dielectric photonic crystals

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Light(Science & Applications) 2020年第1期9卷 315-324页

作者： Zhongfei Xiong Ruo-Yang Zhang Rui Yu C.T.Chan Yuntian Chen School of Optical and Electronic Information Huazhong University of Science and TechnologyWuhan 430074China Department of Physics The Hong Kong University of Science and TechnologyClear Water BayHong KongChina School of Physics and Technology Wuhan UniversityWuhan 430072China Wuhan National Laboratory of Optoelectronics Huazhong University of Science and TechnologyWuhan 430074China

It was recently demonstrated that the connectivities of bands emerging from zero frequency in dielectric photonic crystals are distinct from their electronic counterparts with the same space *** discover that in an AB-layerstacked photonic crystal composed of anisotropic dielectrics,the unique photonic band connectivity leads to a new kind of symmetry-enforced triply degenerate points at the nexuses of two nodal rings and a Kramers-like nodal *** emergence and intersection of the line nodes are guaranteed by a generalized 1/4-period screw rotation symmetry of Maxwell’s *** bands with a constant kz and iso-frequency surfaces near a nexus point both disperse as a spin-1 Dirac-like cone,giving rise to exotic transport features of light at the nexus *** show that spin-1 conical diffraction occurs at the nexus point,which can be used to manipulate the charges of optical *** work reveals that Maxwell’s equations can have hidden symmetries induced by the fractional periodicity of the material tensor components and hence paves the way to finding novel topological nodal structures unique to photonic systems.

关键词： symmetry nodal dielectric

Computing Classification of Interacting Fermionic symmetry-Protected Topological Phases Using Topological Invariants

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Chinese Physics Letters 2021年第12期38卷 38-43页

作者： Yunqing Ouyang Qing-Rui Wang Zheng-Cheng Gu Yang Qi State Key Laboratory of Surface Physics Fudan UniversityShanghai 200433China Center for Field Theory and Particle Physics Department of PhysicsFudan UniversityShanghai 200433China Department of Physics Yale UniversityNew HavenCT 06511USA Department of Physics The Chinese University of Hong KongShatinNew TerritoriesHong KongChina Collaborative Innovation Center of Advanced Microstructures Nanjing 210093China Yau Mathematical Sciences Center Tsinghua UniversityBeijing 100084China

In recent years,great success has been achieved on the classification of symmetry-protected topological(SPT)phases for interacting fermion systems by using generalized cohomology ***,the explicit calculation of generalized cohomology theory is extremely hard due to the difficulty of computing obstruction *** on the physical picture of topological invariants and mathematical techniques in homotopy algebra,we develop an algorithm to resolve this hard *** is well known that cochains in the cohomology of the symmetry group,which are used to enumerate the SPT phases,can be expressed equivalently in different linear bases,known as the *** expressing the cochains in a reduced resolution containing much fewer basis than the choice commonly used in previous studies,the computational cost is drastically *** particular,it reduces the computational cost for infinite discrete symmetry groups,like the wallpaper groups and space groups,from infinity to *** examples,we compute the classification of two-dimensional interacting fermionic SPT phases,for all 17 wallpaper symmetry groups.

关键词： theory symmetry topological

Polarimetric parity-time symmetry in a photonic system

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Light(Science & Applications) 2020年第1期9卷 390-397页

作者： Lingzhi Li Yuan Cao Yanyan Zhi Jiejun Zhang Yuting Zou Xinhuan Feng Bai-Ou Guan Jianping Yao Guangdong Provincial Key Laboratory of Optical Fiber Sensing and Communications Institute of Photonics TechnologyJinan UniversityGuangzhou 511442China Microwave Photonics Research Laboratory School of Electrical Engineering and Computer ScienceUniversity of OttawaOttawaON K1N 6N5Canada

Parity-time(PT)symmetry has attracted intensive research interest in recent *** symmetry is conventionally implemented between two spatially distributed subspaces with identical localized eigenfrequencies and complementary gain and loss *** implementation is *** this paper,we propose and demonstrate that PT symmetry can be implemented between two subspaces in a single spatial unit based on optical polarimetric *** controlling the polarization states of light in the single spatial unit,the localized eigenfrequencies,gain,loss,and coupling coefficients of two polarimetric loops can be tuned,leading to PT symmetry *** a demonstration,a fiber ring laser based on this concept supporting stable and single-mode lasing without using an ultranarrow bandpass filter is implemented.

关键词： polarization symmetry Polar

Inversion/Mirror symmetry-Protected Dirac Cones in Distorted Ruby Lattices

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Chinese Physics Letters 2020年第12期37卷 66-73页

作者： Lei Sun Xiaoming Zhang Han Gao Jian Liu Feng Liu Mingwen Zhao School of Physics and State Key Laboratory of Crystal Materials Shandong UniversityJinan 250100China Department of Materials Science and Engineering University of UtahSalt Lake CityUtah 84112USA Collaborative Innovation Center of Light Manipulations and Applications Shandong Normal UniversityJinan 250358China

The exotic electronic band structures of Ruby and Star lattices,characterized by Dirac cone and nontrivial topology,offer a unique platform for the study of two-dimensional(2D)Dirac *** general,an ideal isotropic Dirac cone is protected by time reversal symmetry and inversion,so that its robustness against lattice distortion is not only of fundamental interest but also crucial to practical *** we systematically investigate the robustness of Dirac cone in a Ruby lattice against four typical lattice distortions that break the inversion and/or mirror symmetry in the transition from Ruby to *** a tight-binding approach,we show that the isotropic Dirac cones and their related topological features remain intact in the rotationally distorted lattices that preserve the inversion symmetry(i-Ruby lattice)or the in-plane mirror symmetry(m-Ruby lattice).On the other hand,the Dirac cones are gapped in the a-and b-Ruby lattices that break both these lattice symmetries or ***,a rotational unitary matrix is identified to transform the original into the distorted *** symmetry-protected Dirac cones were also verified in photonic crystal *** robust Dirac cones revealed in the non-mirror symmetric i-Ruby and non-centrosymmetric m-Ruby lattices provide a general guidance for the design of 2D Dirac materials.

关键词： Dirac symmetry mirror

High-dimensional neural network potentials for magnetic systems using spin-dependent atom-centered symmetry functions

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npj Computational Materials 2021年第1期7卷 1541-1551页

作者： Marco Eckhoff Jörg Behler Universität Göttingen Institut für Physikalische ChemieTheoretische ChemieTammannstrasse 637077 GöttingenGermany Universität Göttingen International Center for Advanced Studies of Energy Conversion(ICASEC)Tammannstrasse 637077 GöttingenGermany

Machine learning potentials have emerged as a powerful tool to extend the time and length scales of first-principles quality ***,most machine learning potentials cannot distinguish different electronic spin arrangements and thus are not applicable to materials in different magnetic *** we propose spin-dependent atom-centered symmetry functions as a type of descriptor taking the atomic spin degrees of freedom into *** used as an input for a high-dimensional neural network potential(HDNNP),accurate potential energy surfaces of multicomponent systems can be constructed,describing multiple collinear magnetic *** demonstrate the performance of these magnetic HDNNPs for the case of manganese oxide,*** method predicts the magnetically distorted rhombohedral structure in excellent agreement with density functional theory and *** efficiency allows to determine the Néel temperature considering structural fluctuations,entropic effects,and *** method is general and is expected to be useful also for other types of systems such as oligonuclear transition metal complexes.

关键词： magnetic symmetry neural

Sorting radar signal from symmetry clustering perspective

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Journal of Systems Engineering and Electronics 2017年第4期28卷 690-696页

作者： Mohaned Giess Shokrallah Ahmed Bin Tang School of Electronic Engineering University of Electronic Science and Technology of China Chengdu 611731 China

The main function of electronic support measure system is to detect threating signals in order to take countermeasures against them. To accomplish this objective, a process of associating each interleaved pulse with its emitter must be done. This process is termed sorting or de-interleaving. A novel point symmetry based radar sorting (PSBRS) algorithm is addressed. In order to deal with all kinds of radar signals, the symmetry measure distance is used to cluster pulses instead of the conventional Euclidean distance. The reference points of the symmetrical clusters are initialized by the alternative fuzzy c-means (AFCM) algorithm to ameliorate the effects of noise and the false sorting. Besides, the density filtering (DF) algorithm is proposed to discard the noise pulses or clutter. The performance of the algorithm is evaluated under the effects of noise and missing pulses. It has been observed that the PSBRS algorithm can cope with a large number of noise pulses and it is completely independent of missing pulses. Finally, PSBRS is compared with some benchmark algorithms, and the simulation results reveal the feasibility and efficiency of the algorithm.

关键词： sorting radar pulse symmetry alternative fuzzy c-means noise missing pulse

PT symmetry Induced Rings of Lasing Threshold Modes Embedded with Discrete Bound States in the Continuum

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Chinese Physics Letters 2021年第8期38卷 50-56页

作者： Qianju Song Shiwei Dai Dezhuan Han Z.Q.Zhang C.T.Chan Jian Zi College of Physics Chongqing UniversityChongqing 401331China Department of Physics Hong Kong University of Science and TechnologyClear Water BayHong KongChina Department of Physics Key Laboratory of Micro-and Nano-Photonic Structures(MOE)and State Key Laboratory of Surface PhysicsFudan UniversityShanghai 200433China

It is well known that spatial symmetry in a photonic crystal(PhC)slab is capable of creating bound states in the continuum(BICs),which can be characterized by topological charges of polarization ***,we show that when a PT-symmetric perturbation is introduced into the PhC slab,a new type of BICs(pt-BICs)will arise from each ordinary BIC together with the creation of rings of lasing threshold modes with pt-BICs embedded in these *** from ordinary BICs,the Q-factor divergence rate of a pt-BIC is reduced and anisotropic in momentum ***,pt-BICs can even appear at off-high symmetry lines of the Brillouin *** pt-BICs also carry topological charges and can be created or annihilated with the total charge conserved.A unified picture on pt-BICs and the associated lasing threshold modes is given based on the temporal coupled mode *** findings reveal the new physics arising from the interplay between PT symmetry and BIC in PhC slabs.

关键词： polarization symmetry lasing