SPECIAL TOPIC—Optical metamaterials
DOI: 10.7498/aps.69.150101
超构材料 (metamaterials) 自 21 世纪初被提出以来, 已经经历了 20 年的发展, 目前, 它仍然是非常活跃的前沿领域. 超构材料的研究覆盖非常广泛,涉及光、电磁波、太赫兹波、声波、热、力学和弹性波等. 超构材料的基本思想是, 利用人工结构单元作为人造原子, 来构造宏观连续的介质, 通过结构单元的设计, 来调控介质的材料参数, 实现对波的传播性质的控制. 最早的超构材料结构单元是 1999 年英国科学家 John Pendry 提出的金属开口环结构, 它可以与电磁场相互作用, 产生磁共振, 从而实现负磁导率系数. 后来, 人们又提出了很多其他的人工结构单元, 实现各种应用. 超构材料最早的应用是负折射材料, 后来又有零折射率材料、双曲色散材料等. 早期设计的超构材料在空间上是均匀的, 后来人们又进一步提出变换光学设计方法, 利用空间不均匀分布的超构材料, 可以实现对电磁波在空间传播路径的任意调控. 变换光学器件可以实现一些非常新奇的应用, 包括隐身斗篷、幻想光学、麦克斯韦鱼眼透镜, 甚至可以模拟黑洞的引力场捕获光子. 除了三维的超构材料之外, 美国科学家 Federico Capasso 提出超构表面的结构, 利用结构单元对电磁波的散射相位以及相位的梯度分布, 可以实现广义的折射以及对电磁波波前的调控, 从而可以获得各种结构光场. 最近,研究者还通过结构单元设计调控超构材料的色散能带, 实现具有拓扑特性的光子传播模, 可以克服缺陷的散射实现单向的传输. 在更新的应用领域, 超构材料还被拓展用于实现信息超构材料和空间光子计算等.
应《物理学报》编辑部的邀请, 我们邀请了部分活跃在研究光学超构材料的第一线的中青年科学家, 组织了本期的专题. 专题文章包括如下几个方面: 在超构材料的等效介质模型及其应用方面,罗杰和赖耘教授报道了一种新的赝局域等效介质理论, 冯一军教授报道了通过掺杂近零介质的方式实现光场增强, 江海涛教授介绍了双曲超构材料的带隙调控效应, 刘仿和黄翊东教授介绍了双曲超构材料中切伦科夫辐射效应; 在超构材料变换光学方面, 郑斌和陈红胜教授综述了电磁隐身技术的进展, 陈焕阳教授报道了一种通过模拟黑洞实现完美吸收器, 刘辉教授综述了变换光学芯片上类比引力的研究进展; 在超构表面方面, 仇旻教授介绍了基于相变材料材料调控超构表面的进展, 孙树林和周磊教授介绍了等离激元超构表面实现波前调控的进展; 在拓扑光学方面, 陈子亭教授综述了传输线网络的拓扑性质, 宋道红、许京军和陈志刚教授介绍了平带光子结构的局域与拓扑性质, 伍瑞新教授报道了磁性光子晶体中的拓扑效应, 胡传灯和侯波教授报道了超构材料中的外尔点的数值设计, 韩德专和石磊教授综述了二维等离激元的束缚态与拓扑能带性质, 陈云天和徐竞教授综述了耦合波导体系中的互易与手征特性等; 在超构材料的新兴应用方面, 崔铁军教授综述了信息超构材料在微波成像与无线通信方面的应用, 马云贵教授介绍了超构材料的空域模拟光学计算的进展.本专题从不同的角度描述了超构材料的理论与实验方面的进展, 反映了此领域的一些现状, 希望对读者了解此前沿课题有所帮助.
2020, 69 (15): 150301.
doi: 10.7498/aps.69.20200258
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Transmission line is a common kind of one-dimensional waveguide. In addition to being widely used in engineering, the transmission lines can be used in proof-of-principle experiments in basic scientific research. For example, the wave equations governing the transmission line and quantum wire are equivalent, so transmission lines are widely used in the research of quantum graphs. The transmission line network equations are similar to the equations of zero-energy tight binding model, so the transmission line network can also be used to study some physical properties predicted by the theories based on tight binding model, and examples include Anderson localization, band dispersions, topological properties, etc. According to the transmission line network equations, we review some applications of transmission lines in the research fields mentioned above. We will discuss Anderson localization in one-, two-, and three-dimensional networks, the band structures of periodic and quasiperiodic networks, and the angular moment-dependent topological transport in transmission line network. We introduce the methods and results in detail to show the potential of transmission lines in basic scientific research.
2020, 69 (15): 157301.
doi: 10.7498/aps.69.20200193
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Due to its special optical properties the surface plasmon polariton (SPP) has been applied to many fields such as chemistry, biology, communication, nano energy. The more in-depth researches on plasmonic band structures can conduce to understanding more the properties of plasmonic micro- and nano-structures. In this review, we first introduce some metal structures which have plasmonic band structures. Then, we review some unique properties of plasmonic band structures including bound state in the continuum, waveguide, complete band gap, topology, etc. Based on the above properties, the plasmonic applications are introduced. Finally, we briefly introduce the band structures of graphene-based plasmonics and its applications.
2020, 69 (15): 154104.
doi: 10.7498/aps.69.20200976
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With the development of science and technology, the invisibility has gradually moved from a simple and plain visual deception trick to a precise and systematic modern technology system. By designing appropriate electromagnetic parameters, the novel electromagnetic wave cloaking technology is able to control the propagation and scattering of electromagnetic wave, thereby reducing the detectability of the cloaked object. The electromagnetic parameters of these novel cloaking devices can be realized by using the artificially designed nanostructures, or by combining the medium that already exists in nature. In this review, according to a detailed introduction of the research progress of novel electromagnetic wave cloaking, we discuss the difficulties and challenges in this field, and give an outlook on the future development.
2020, 69 (15): 157804.
doi: 10.7498/aps.69.20200614
Abstract +
Surface plasmon polaritons (SPPs) have found many important applications in on-chip signal transportation, enhanced nonlinear/Raman effect, biological/chemical sensing, super resolution imaging, etc. In these applications, the near-field propagation and far-field scattering of SPPs play a vital role. However, there has been strong desire to understand these physical effects. In this paper, we first briefly review the history and progress of SPPs. Next, we mainly focus on the near-field propagation and far-field scattering of SPPs, including their fundamental theories and practical applications. Finally, we review several different approaches to manipulating the near-field wavefronts of SPPs. These researches offer us a more in-depth understanding and the ability to more strongly control the scattering characteristics of SPPs, which may promote the scientific researches and practical applications of SPPs in the future.
2020, 69 (15): 157803.
doi: 10.7498/aps.69.20200283
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Spatial analog optical computing devices possess the capability of high-throughput, real-time and low-energy information processing. Optical metamaterials, which are ultracompact in structure and possess powerful ability to control the light, can be utilized to establish miniatured and integrated spatial analog optical computing devices. The methods of designing the spatial analog optical computing devices could be mainly classified as two kinds—4F system method and Green’s function method. The 4F system method requires two Fourier transform lenses and a spatial frequency filter, where the actual computing procedure is performed in the spatial domain. The 4F system is usually bulky and complicated. The Green’s function method directly leverages the nonlocal response of the carefully tailored optical materials to implement analog computing procedure in the spatial frequency domain and its structure is compact without extra Fourier transform components. Research advances in spatial analog optical computing devices by using these two methods for the last few years are introduced in this paper. These researches could be classified as differentiators, integrators, equation solvers and spatial frequency filters according to the standard of computing functions. The approaches to designing these devices are further demonstrated. Then, computing devices which could realize spatial analog first-order difference by use of the spin-orbit interaction proposed recently are introduced. Finally, application fields and study prospects of spatial analog optical computing devices are discussed and summarized.
2020, 69 (15): 157802.
doi: 10.7498/aps.69.20200183
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Optical metamaterial is a kind of artificially designed microstructured material. Its occurrence breaks the localization of traditional material design thinking and provides a new paradigm for artificially controlling electromagnetic waves on a micro-nano scale, especially realizes optical properties beyond conventional materials in nature. Furthermore, metamaterial has the ability to couple electromagnetic waves into the sub-wavelength regime, meeting the high-speed development of modern science and technology, which puts forward new requirements for high performance, miniaturization and integration of optical components. Therefore, optical chips based on metamaterials bring many encouraging applications such as in perfect imaging that breaks through the diffraction limit, multifunctional integrated optics, etc. In addition, metamaterial photonic chips can also simulate some phenomena in general relativity, especially exploring some phenomena that have not been experimentally proven. This review paper briefly introduces the study of analogical gravitation based on different kinds of photonic chips on the basis of metamaterials. In the end, there present the summary and outlook about the current development, advantages and challenges of this field.
2020, 69 (15): 154101.
doi: 10.7498/aps.69.20200147
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Field enhancement is an interesting and important topic in electromagnetic study. Electromagnetic field concentration and enhancement devices have wide applications in high directional antenna design, laser ignition, optical control, etc. At present, there are usually two ways of implementing the field enhancement, one is to use the artificial electromagnetic materials to realize the radiation direction control and energy concentration, which is more suitable for the applications at microwave or lower frequencies, and the other is to use the materials with high permittivity or high permeability. However, the latter is extremely sensitive to the position and characteristic of the radiation source and the cross-sectional area of the material, and depends heavily on the value of the relative permittivity or the relative permeability of the material. Therefore, both methods cannot fully meet the application requirements of creating high field intensity in optical band, such as laser ignition, etc. In this paper, based on the theory of photonic crystal doping, the strong electromagnetic field enhancement has been successfully realized by epsilon-near-zero medium filled with ordinary dielectric dopant. We first make the comprehensive theoretical analysis of the field enhancement in the structure of epsilon-near-zero medium with dielectric dopant. The method of calculating the central magnetic field in the doped medium is then rigorously derived, and the formula for the ratio of the central magnetic field in the doped medium to the external radiation field is deduced. We find that the optimal magnetic field enhancement occurs only when the proposed structure is equivalent to an epsilon-mu-near-zero medium. Subsequently, under the above condition, various parameters (radius of the cylindrical dopant, number of sources, etc.) are studied to analyze the magnetic field enhancement performance inside the doped medium. The theoretical analysis and simulation results show that the proposed structure can significantly enhance the magnetic field which is applicable in a broad frequency band from microwave to optical region, and meets the application requirements of providing high field intensity. Finally, as a practical realization example, an ultraviolet ignition device working at 270 nm is designed, which presents an efficient and alternative way of developing electromagnetic (optical) devices for producing strong field enhancement.
2020, 69 (15): 158101.
doi: 10.7498/aps.69.20200246
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2020, 69 (15): 154102.
doi: 10.7498/aps.69.20200198
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The topological transitions in two-dimensional photonic crystals (PCs) originate from the opening-closing-reopening of the bandgap, accompanied with the band order inversion. The topological bandgap in magnetic PC can be created by applying a bias magnetic field or deforming the geometry structure of the PC. In this paper, we demonstrate that the direction of the bias magnetic field also plays a key role in modifying the band structure in a two-dimensional magnetic PC. The results show that by reversing the direction of the bias magnetic field, the eigenstates with the same parity may exchange their orders in the band structure. We investigate this type of band order exchange in the applications of constructing topological edge states and its influence on the properties of edge states. We find, for example, reversing the direction of the bias magnetic field can create two almost degenerated topological edge modes, which propagate in the same direction but have opposite orbital angular momenta. The edge modes and their characteristics can be determined by the schematics of the band orders for the photonic crystals on the two sides of the boundary. The relative relationship of the band orders determines the emergence of the topological edge states, the number of edge states, and edge modes’ properties such as the orbital angular momentum and group velocity. Also, it affects the transmission efficiency of the electromagnetic wave on the boundary. The direction effect of the bias magnetic field on band order exchange presented in this paper provides us with a new way to change the feature of topological edge states and helps us to better understand the influence of band order on topological phases of photonic crystals. It may have potential applications, such as in pseudo-spin splitter and reflection-free one-way optical switch.
2020, 69 (15): 154202.
doi: 10.7498/aps.69.20200453
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Due to their superior ability to control light, metasurfaces, ultrathin two-dimensional metamaterials composed of subwavelength nanostructures, have attracted great attention in recent years. Exploring geometric and material freedom in designing elementary nanostructures and their ambient environment of metasurfaces enables versatile optical devices, such as planar metalenses, holographic imaging and thermal radiators. With phase-change materials (PCMs) such as GeSbTe and VO2 integrated into metasurfaces, the optical functionalities of metasurfaces can be flexibly tuned by exploiting the phase transitions of PCMs induced by external stimuli, thereby opening up new directions and perspectives for dynamic tunable metamasurfaces. In this article, we review the recent progress of tunable metasurfaces based on PCMs, analyze their underlying working mechanisms and highlight their important applications. We conclude this review by bringing our perspectives on challenges and future directions in this field.
2020, 69 (15): 154103.
doi: 10.7498/aps.69.20200260
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Cherenkov radiation (CR) is an electromagnetic radiation emitted by charged particles traveling through a dielectric medium at a speed faster than the phase velocity of light. CR plays an important role in the fields of particle detection, biomedicine and electromagnetic-radiation source. Recently, metamaterials demonstrate their novel mechanical, acoustic, and optical properties by delicately designing the structures and materials. In metamaterials, the electromagnetic properties, such as wave propagation, coupling, and radiation, could be flexibly manipulated. Thus, it is expected that the combination of vacuum electronics and micro- & nano-photonics would result in numerous novel phenomena and effects by having free electrons interacting with metamaterials. In this paper, we firstly review the concept and generation mechanism of CR. Then, recent research advances in the CR generation by using different types of metamaterials are reviewed, including threshold-less CR in hyperbolic metamaterials, reverse CR in negative metamaterials, CR lasing based on high Q-factor metamaterials and Smith-Purcell radiation manipulation with metasurfaces. The unique characteristics and interesting mechanisms of CR based on these metamaterials are elaborated. The research and development of interaction between free electrons and various metamaterials open up possibilities for realizing novel integrated free-electron devices.
2020, 69 (15): 154205.
doi: 10.7498/aps.69.20200084
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Behaviours of light in materials strongly depend on the topological structure of the iso-frequency surface (IFS). The usual materials, of which the unit cell of photonic crystal is made up, are dielectrics, whose IFSs have the same closed topological structure. As a simplest photonic crystal, one-dimensional photonic crystal (1DPC) has attracted intensive attention due to its simple fabrication technique as well as numerous applications. However, in a conventional all-dielectric 1DPC, photonic band gaps (PBGs) for both transverse magnetic (TM) and transverse electric (TE) polarizations will shift toward short wavelengths (i.e. blueshift) as incident angle increases. The underlying physical reason is that the propagating phase in isotropic dielectric will decrease as incident angle increases. The blueshift property of band gap for TM and TE polarization will limit the band width of omnidirectional band gap and the range of operating incident angles in some PBG-based applications, including near-perfect absorption, polarization selection and sensitive refractive index sensing. However, for TM polarization, the propagating phase in a hyperbolic metamaterial (HMM) will increase with incident angle increasing. This special phase property of HMM provides us with a way to flexibly tune the angle-dependent property of band gap in periodic compound structure composed of alternative HMM with open IFS and dielectric with close IFS. In this review, we realize zeroshift (i.e. angle-independent) band gaps as well as redshift band gaps in 1DPCs containing HMMs, which can be utilized to realize near-perfect absorption, sensitive refractive index sensing and polarization selection working in a wide range of incident angles.
2020, 69 (15): 154206.
doi: 10.7498/aps.69.20200194
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Mode coupling is a common phenomenon in waveguides. The mode coupling among different guided modes in fiber-optic communication can cause crosstalk, and the mode coupling of guided mode and radiated mode can reduce the power of the guided mode. Application of mode coupling can guide the design of optical devices such as couplers and beam splitters with specific functions, which have been widely used in fiber optic communication and fiber sensing. So it is important to analyze how waveguide modes are coupled. The coupled-mode theory is a common method of studying mode coupling in waveguides. It provides not only an intuitive picture of how the photonic modes are hybridized, but also a quantitative assessment of how the hybridization among those relevant modes evolves. In recent years, non-Hermitian waveguides, represented by parity-time symmetrical structures, have become a research hotspot. However the conventional coupled-mode theory no longer works in this case. In this review, we briefly summarize the development history of coupled-mode theory and introduce the representative work in reciprocal waveguide coupled-mode theory in detail. Then the relationship among several coupled-mode theories is analyzed and their applications are briefly introduced.
2020, 69 (15): 154207.
doi: 10.7498/aps.69.20200384
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In recent years, flatband systems have aroused considerable interest in different branches of physics, from condensed-matter physics to engineered flatband structures such as in ultracold atoms, various metamaterials, electronic materials, and photonic waveguide arrays. Flatband localization, as an important phenomenon in solid state physics, is of broad interest in the exploration of many fundamental physics of many-body systems. We briefly review the recent experimental advances in light localization in engineered flatband lattices, with the emphasis on the optical induction technique of various photonic lattices and unconventional flatband states. The photonic lattices, established by various optical induction techniques, include quasi-one-dimensional diamond lattices and two-dimensional super-honeycomb, Lieb and Kagome lattices. Nontrivial flatband line states, independent of linear superpositions of conventional compact localized states, are demonstrated in photonic Lieb and super-honeycomb lattices, and they can be considered as an indirect illustration of the non-contractible loop states. Furthermore, we discuss alternative approaches to directly observing the non-contractible loop states in photonic Kagome lattices. These robust loop states are direct manifestation of real-space topology in such flatband systems. In this paper we do not intend to comprehensively account the vast flatband literature, but we briefly review the relevant work on photonic lattices mainly from our group. We hope that the mentioned concepts and techniques can be further explored and developed for subsequent applications in other structured photonic media such as photonic crystals, metamaterials, and other synthetic nanophotonic materials.
2020, 69 (15): 154201.
doi: 10.7498/aps.69.20200110
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The perfectly matched layer plays a key role in electromagnetic simulations, and it makes the infinite space look like a finite space, so that the electromagnetic waves propagating to the boundary seem like their propagations to the infinity. The inner perfectly matched layer has a similar concept, usually in the form of a cylinder or sphere placed inside the physical field. It makes the electromagnetic field matched at the boundary, so that the electromagnetic waves propagate on its convex surface as if they were propagating to an infinite distance, without any scattering. In addition to the perfectly matched layer, planar absorbers can be realized in a variety of ways, such as spatial Kramers-Kronig relations, photonic crystals, metamaterials, etc. On the other hand, the inner cylindrical or spherical absorbers are generally perfect absorbers, electromagnetic “black hole”, etc. Transformation optics always arouse great research interests. For its property of controlling propagation of electromagnetic waves arbitrarily under coordinate mappings, transformation optics has a wide range of applications and has also been used as a theoretical tool for designing absorbers. However, to the authors’ knowledge, there is no effective method to achieve perfect absorption of inner absorbers with no reflections and independence of incident angle or wave frequency. In this paper, transformation optics theory is used to design an inner perfectly matched layer whose material parameters are obtained by a radial coordinate transformation of the complex plane. Through investigating the electromagnetic wave patterns and the two-dimensional far-field diagrams, we intuitively compare and analyse one by one the absorption characteristics of the matched and mismatched perfect absorber, electromagnetic “black hole” and the inner perfectly matched layer. It is found that the matched perfect absorber has better absorption property than mismatched one and electromagnetic “black hole”. In the electromagnetic “black hole” there appear a lot of scatterings. While our inner perfectly matched layer demonstrates the best effectiveness of absorption with no back scattering. It can be used as an absorbing kernel in electromagnetic simulations and relevant experiments.
2020, 69 (15): 154203.
doi: 10.7498/aps.69.20200196
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Effective medium theory is of great importance for using the artificial microstructure materials to extend the optical parameters. In this article, we develop a new kind of effective medium theory for artificial microstructures with nonlocal effects, like photonic crystals, which we name the pseudo-local effective medium theory. The optical properties of the pseudo-local effective medium are described by effective local permittivity ${\overleftrightarrow \varepsilon ^{\rm{p}}}\left( \omega \right)$ and permeability ${\overleftrightarrow \mu ^{\rm{p}}}\left( \omega \right)$ , together with an additional wave vector ${{{k}}_a}$ . We find that the pseudo-local medium exhibits a unique blend of local and nonlocal characteristics. On the surface normal to ${{{k}}_a}$ , the pseudo-local medium is optically equivalent to its local medium counterpart. While on the surface parallel to ${{{k}}_a}$ , the abnormal wave phenomena induced by inherent nonlocality, such as negative refraction and total reflection, may occur. Furthermore, it is found that a $\text{π}$ phase shift is added to transmission wave through the pseudo-local medium composed of odd number of unit cells under all incident angles. Based on this unique feature, an all-angle phase grating is proposed. Our work opens a route towards the advanced optical devices based on the pseudo-local effective media.
2020, 69 (15): 154204.
doi: 10.7498/aps.69.20200195
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Weyl semimetal has the massless and chiral low-energy electronic excitation charateristic, and its quasi-particle behavior can be described by Weyl equation, and may lead to appealing transport properties, such as Fermi arc surface state, negative magnetic resistance, chiral Landau level, etc. By analogous with Weyl semimetal, one has realized Weyl point degeneracy of electromagnetic wave in an ideal Weyl metamaterial. In this article, by breaking the mirror symmetry of the saddle-shaped meta-atom structure, we theoretically investigate chirality-dependent split and shift effect of Weyl point frequencies which would otherwise be identical. The frequency shift can be tuned by the symmetry-broken intensity. Finally, we study the Fermi arc surface state connecting two Weyl points on $\left\langle {001} \right\rangle $ crystal surface.
