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Bound states in the continuum based on the total internal reflection of Bloch waves

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National Science Review 2023年第1期10卷 178-186页

作者： Peng Hu Chongwu Xie Qianju Song Ang Chen Hong Xiang Dezhuan Han Jian Zi College of physics Chongqing University state key laboratory of surface physics Key Laboratory of Microand Nano-Photonic Structures (Ministry of Education) and Department of Physics Fudan University Chongqing key laboratory for Strongly Coupled physics

A photonic-crystal slab can support bound states in the continuum(BICs) that have infinite lifetimes but are embedded into the continuous spectrum of optical modes in free space.The formation of BICs re quires a total internal reflection(TIR) condition at both interfaces between the slab and the free space.Here,we show that the TIR of Bloch waves can be directly obtained based on the generalized Fresnel equations proposed.If each of these Bloch waves picks up a phase with integer multiples of 2π for traveling a round trip,light can be perfectly guided in the slab,namely forming a BIC.A BIC solver with low computational complexity and fast convergence speed is developed,which can also work efficiently at high frequencies beyond the diffraction limit where multiple radiation channels exist.Two examples of multi-channel BICs are shown and their topological nature in momentum space is also revealed.Both can be attributed to the coincidence of the topological charges of far-field radiations from different radiation channels.The concept of the generalized TIR and the TIR-based BIC solver developed offer highly effective approaches for explorations of BICs that could have many potential applications in guided-wave optics and enhanced light-matter interactions.

关键词： bound states in the continuum total internal reflection Bloch waves generalized Fresnel equations topological charges

High-efficiency nonlocal reflection-type vortex beam generation based on bound states in the continuum

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National Science Review 2023年第5期10卷 201-207页

作者： Tongyu Li Jiajun Wang Wenjie Zhang Xinhao Wang Wenzhe Liu Lei Shi Jian Zi state key laboratory of surface physics Key Laboratory of Micro-and Nano-Photonic Structures (Ministry of Education) and Department of PhysicsFudan University Department of physics The Hong Kong University of Science and Technology

Momentum-space polarization vortices centered at symmetry-protected bound states in the continuum of a periodic structure,e.***.photonic crystal slab,provide a novel nonlocal approach to generate vortex beams.This approach enjoys a great convenience of no precise alignment requirements,although the generation efficiency of the nonlocal generators requires further optimization before the practical application.In this work,we propose a temporal-coupled-mode-theory-based guideline for high-efficiency nonlocal reflection-type vortex generator design.The conversion efficiency of the vortex beam is found to be limited by the ratio of the radiative loss to the intrinsic absorption in practical systems.To increase this ratio through mode selection and structure design,the photonic crystal slabs are theoretically designed and experimentally characterized,showing a maximum on-resonance conversion efficiency of up to 86%.Combining high efficiency with simple fabrication and no re quirement for precise alignment,reflection-type photonic crystal slabs could offer a new and competitive way to generate vortex beams flexibly.

关键词： photonic crystal slab vortex beam bound states in the continuum

Multiple control of thermoelectric dual-function metamaterials

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International Journal of Mechanical System Dynamics 2023年第2期3卷 127-135页

作者： Pengfei Zhuang Jiping Huang Department of physics State Key Laboratory of Surface PhysicsKey Laboratory of Micro and Nano Photonic Structures(MOE)Fudan UniversityShanghaiChina

Thermal metamaterials based on transformation theory offer a practical design for controlling heat flow by engineering spatial distributions of material parameters,implementing interesting functions such as cloaking,concentrating,and rotating.However,most existing designs are limited to serving a single target function within a given physical domain.Here,we analytically prove the form invariance of thermoelectric(TE)governing equations,ensuring precise controls of the thermal flux and electric current.Then,we propose a dual‐function metamaterial that can concentrate(or cloak)and rotate the TE field simultaneously.In addition,we introduce two practical control methods to realize corresponding functions:one is a temperature‐switching TE rotating concentrator cloak that can switch between cloaking and concentrating;the other is an electrically controlled TE rotating concentrator that can handle the temperature field precisely by adjusting external voltages.The theoretical predictions and finite‐element simulations agree well with each other.This work provides a unified framework for manipulating the direction and density of theTE field simultaneously and may contribute to the study of thermal management,such as thermal rectification and thermal diodes.

关键词： transformation thermotics thermoelectric effect dual-function thermal metamaterials thermal management

Breaking efficiency limit of thermal concentrators by conductivity couplings

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Science China(physics,Mechanics & Astronomy) 2022年第11期65卷 68-77页

作者： Pengfei Zhuang Liujun Xu Peng Tan Xiaoping Ouyang Jiping Huang Department of physics State Key Laboratory of Surface Physicsand Key Laboratory of Micro and Nano Photonic Structures(MOE)FudanUniversityShanghai200438China Department of physics and state key laboratory of surface physics Fudan UniversityShanghai 200438China School of Materials Scienceand Engineering Xiangtan UniversityXiangtan 411105China

The concentrating efficiency of a thermal concentrator can be reflected in the ratio of its interior to exterior temperature gradients,which, however, has an upper limit in existing schemes. Here, we manage to break this upper limit by considering the couplings of thermal conductivities and improve the concentrating efficiency of thermal concentrators. For this purpose, we first discuss a monolayer scheme with an isotropic thermal conductivity, which can break the upper limit but is still restricted by its geometric configuration. To go further, we explore another degree of freedom by considering the monolayer scheme with an anisotropic thermal conductivity or by adding the second shell with an isotropic thermal conductivity, thereby making the concentrating efficiency completely free from the geometric configuration. Nevertheless, apparent negative thermal conductivities are required, and we resort to external heat sources realizing the same effect without violating the second law of thermodynamics. Finite-element simulations are performed to confirm the theoretical predictions, and experimental suggestions are also provided to improve feasibility. These results may have potential applications for thermal camouflage and provide guidance to other diffusive systems such as static magnetic fields and dc current fields for achieving similar behaviors.

关键词： thermal concentrator concentrating efficiency conductivity coupling apparent negative conductivity

Diffusional inhomogeneity in cell cultures

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Chinese physics B 2018年第2期27卷 88-91页

作者：张佳政李娜陈唯 state key laboratory of surface physics and Department of physics Fudan University

Cell migrations in the cell cultures are found to follow non-Gaussian statistics. We recorded long-term cell migration patterns with more than six hundred cells located in 28 mm2. Our experimental data support the claim that an individual cell migration follows Gaussian statistics. Because the cell culture is inhomogeneous, the statistics of the cell culture exhibit a non-Gaussian distribution. We find that the normalized histogram of the diffusion velocity follows an exponential tail. A simple model is proposed based on the diffusional inhomogeneity to explain the exponential distribution of locomotion activity in this work. Using numerical calculation, we prove that our model is in great agreement with the experimental data.

关键词： random walks cell locomotion collective behavior

Experimental realization of non-Abelian permutations in a three-state non-Hermitian system

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National Science Review 2022年第11期9卷 79-86页

作者： Weiyuan Tang Kun Ding Guancong Ma Department of physics Hong Kong Baptist University Department of physics State Key Laboratory of Surface Physics and Key Laboratory of Micro and Nano Photonic Structures(Ministry of Education) Fudan University

Eigenstates of a non-Hermitian system exist on complex Riemannian manifolds, with multiple sheets connecting at branch cuts and exceptional points(EPs). These eigenstates can evolve across different sheets—a process that naturally corresponds to state permutation. Here, we report the first experimental realization of non-Abelian permutations in a three-state non-Hermitian system. Our approach relies on the stroboscopic encircling of two different exceptional arcs(EAs), which are smooth trajectories of order-2EPs appearing from the coalescence of two adjacent states. The non-Abelian characteristics are confirmed by encircling the EAs in opposite sequences. A total of five non-trivial permutations are experimentally realized, which together comprise a non-Abelian group. Our approach provides a reliable way of investigating non-Abelian state permutations and the related exotic winding effects in non-Hermitian systems.

关键词： non-Abelian permutation non-Hermitian physics topological physics acoustics

Asymmetric and symmetric meta-correlations in financial markets

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Chinese physics B 2016年第10期25卷 579-586页

作者：李晓辉沈翔瀛黄吉平 Department of physics and state key laboratory of surface physics Fudan University

In financial markets, the relation between fluctuations of stock prices and trading behaviors is complex. It is intriguing to quantify this kind of meta-correlation between market fluctuations and the synchronous behaviors. We refine the theoretical index leverage model proposed by Reigneron et al., to exactly quantify the meta-correlation under various levels of price fluctuations [Reigneron P A, Allez R and Bouchaud J P 2011 Physica A 390 3026]. The characteristics of meta-correlations in times of market losses, are found to be significantly different in Chinese and American financial markets. In addition,unlike the asymmetric results at the daily scale, the correlation behaviors are found to be symmetric at the high-frequency scale.

关键词： financial market collective behavior complex system asymmetry and symmetry

Spin detection and manipulation with scanning tunneling microscopy

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Chinese physics B 2018年第10期27卷 79-87页

作者：高春雷 state key laboratory of surface Science and Department of physics Fudan University

Over the past few decades, spin detection and manipulation at the atomic scale using scanning tunneling microcopy has matured, which has opened the possibility of realizing spin-based functional devices with single atoms and molecules.This article reviews the principle of spin polarized scanning tunneling microscopy and inelastic tunneling spectroscopy,which are used to measure the static spin structure and dynamic spin excitation, respectively. Recent progress will be presented, including complex spin structure, magnetization of single atoms and molecules, as well as spin excitation of single atoms, clusters, and molecules. Finally, progress in the use of spin polarized tunneling current to manipulate an atomic magnet is discussed.

关键词： spin-polarized STM inelastic tunneling spectroscopy atomic spin detection

Spin transport in a Zigzag normal/ferromagnetic graphene junction

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Chinese physics B 2015年第5期24卷 514-518页

作者：史豪晟 Vahram L.Grigoryan Department of physics and state key laboratory of surface physics Fudan University

We study magnetic proximity effect induced low-energy spin transport in the normal/ferromagnetic junction of a semi-infinite zigzag graphene nanoribbon. Due to the absence of a spin flip in a single interface, the spin transfer in this model can be described by the ＂two-spin channel＂ model. We identify each spin channel as either a perfect conducting or a non-conducting channel. This feature leads to spin filter in symmetric zigzag graphene nanoribbon and spin precession in antisymmetric zigzag graphene nanoribbon, and helps to directly determine the exchange-splitting intensity directly, even without an external auxiliary bias.

关键词： graphene junction spin filter spin transfer torque magnetic proximity effect