Journal articles on the topic 'Non-Hermitian topological photonics'
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1
Zhao, Han, Xingdu Qiao, Tianwei Wu, Bikashkali Midya, Stefano Longhi, and Liang Feng. "Non-Hermitian topological light steering." Science 365, no. 6458 (September 12, 2019): 1163–66. http://dx.doi.org/10.1126/science.aay1064.
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Photonic topological insulators provide a route for disorder-immune light transport, which holds promise for practical applications. Flexible reconfiguration of topological light pathways can enable high-density photonics routing, thus sustaining the growing demand for data capacity. By strategically interfacing non-Hermitian and topological physics, we demonstrate arbitrary, robust light steering in reconfigurable non-Hermitian junctions, in which chiral topological states can propagate at an interface of the gain and loss domains. Our non-Hermitian–controlled topological state can enable the dynamic control of robust transmission links of light inside the bulk, fully using the entire footprint of a photonic topological insulator.
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Wang, Hongfei, Xiujuan Zhang, Jinguo Hua, Dangyuan Lei, Minghui Lu, and Yanfeng Chen. "Topological physics of non-Hermitian optics and photonics: a review." Journal of Optics 23, no. 12 (October 25, 2021): 123001. http://dx.doi.org/10.1088/2040-8986/ac2e15.
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Abstract The notion of non-Hermitian optics and photonics rooted in quantum mechanics and photonic systems has recently attracted considerable attention ushering in tremendous progress on theoretical foundations and photonic applications, benefiting from the flexibility of photonic platforms. In this review, we first introduce the non-Hermitian topological physics from the symmetry of matrices and complex energy spectra to the characteristics of Jordan normal forms, exceptional points, biorthogonal eigenvectors, Bloch/non-Bloch band theories, topological invariants and topological classifications. We further review diverse non-Hermitian system branches ranging from classical optics, quantum photonics to disordered systems, nonlinear dynamics and optomechanics according to various physical equivalences and experimental implementations. In particular, we include cold atoms in optical lattices in quantum photonics due to their operability at quantum regimes. Finally, we summarize recent progress and limitations in this emerging field, giving an outlook on possible future research directions in theoretical frameworks and engineering aspects.
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Ota, Yasutomo, Kenta Takata, Tomoki Ozawa, Alberto Amo, Zhetao Jia, Boubacar Kante, Masaya Notomi, Yasuhiko Arakawa, and Satoshi Iwamoto. "Active topological photonics." Nanophotonics 9, no. 3 (January 28, 2020): 547–67. http://dx.doi.org/10.1515/nanoph-2019-0376.
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AbstractTopological photonics emerged as a novel route to engineer the flow of light. Topologically protected photonic edge modes, which are supported at the perimeters of topologically nontrivial insulating bulk structures, are of particular interest as they may enable low-loss optical waveguides immune to structural disorder. Very recently, there has been a sharp rise of interest in introducing gain materials into such topological photonic structures, primarily aiming at revolutionizing semiconductor lasers with the aid of physical mechanisms existing in topological physics. Examples of remarkable realizations are topological lasers with unidirectional light output under time-reversal symmetry breaking and topologically protected polariton and micro/nanocavity lasers. Moreover, the introduction of gain and loss provides a fascinating playground to explore novel topological phases, which are in close relevance to non-Hermitian and parity-time symmetric quantum physics and are, in general, difficult to access using fermionic condensed matter systems. Here, we review the cutting-edge research on active topological photonics, in which optical gain plays a pivotal role. We discuss recent realizations of topological lasers of various kinds, together with the underlying physics explaining the emergence of topological edge modes. In such demonstrations, the optical modes of the topological lasers are determined by the dielectric structures and support lasing oscillation with the help of optical gain. We also address recent research on topological photonic systems in which gain and loss, themselves, essentially influence topological properties of the bulk systems. We believe that active topological photonics provides powerful means to advance micro/nanophotonics systems for diverse applications and topological physics, itself, as well.
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Chen, Weijin, Qingdong Yang, Yuntian Chen, and Wei Liu. "Evolution and global charge conservation for polarization singularities emerging from non-Hermitian degeneracies." Proceedings of the National Academy of Sciences 118, no. 12 (March 15, 2021): e2019578118. http://dx.doi.org/10.1073/pnas.2019578118.
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Core concepts in singular optics, especially the polarization singularities, have rapidly penetrated the surging fields of topological and non-Hermitian photonics. For open photonic structures with non-Hermitian degeneracies in particular, polarization singularities would inevitably encounter another sweeping concept of Berry phase. Several investigations have discussed, in an inexplicit way, connections between both concepts, hinting at that nonzero topological charges for far-field polarizations on a loop are inextricably linked to its nontrivial Berry phase when degeneracies are enclosed. In this work, we reexamine the seminal photonic crystal slab that supports the fundamental two-level non-Hermitian degeneracies. Regardless of the invariance of nontrivial Berry phase (concerning near-field Bloch modes defined on the momentum torus) for different loops enclosing both degeneracies, we demonstrate that the associated far polarization fields (defined on the momentum sphere) exhibit topologically inequivalent patterns that are characterized by variant topological charges, including even the trivial scenario of zero charge. Moreover, the charge carried by the Fermi arc actually is not well defined, which could be different on opposite bands. It is further revealed that for both bands, the seemingly complex evolutions of polarizations are bounded by the global charge conservation, with extra points of circular polarizations playing indispensable roles. This indicates that although not directly associated with any local charges, the invariant Berry phase is directly linked to the globally conserved charge, physical principles underlying which have all been further clarified by a two-level Hamiltonian with an extra chirality term. Our work can potentially trigger extra explorations beyond photonics connecting Berry phase and singularities.
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Yang, Frank, Ciril S. Prasad, Weijian Li, Rosemary Lach, Henry O. Everitt, and Gururaj V. Naik. "Non-Hermitian metasurface with non-trivial topology." Nanophotonics 11, no. 6 (February 2, 2022): 1159–65. http://dx.doi.org/10.1515/nanoph-2021-0731.
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Abstract The synergy between topology and non-Hermiticity in photonics holds immense potential for next-generation optical devices that are robust against defects. However, most demonstrations of non-Hermitian and topological photonics have been limited to super-wavelength scales due to increased radiative losses at the deep-subwavelength scale. By carefully designing radiative losses at the nanoscale, we demonstrate a non-Hermitian plasmonic–dielectric metasurface in the visible with non-trivial topology. The metasurface is based on a fourth order passive parity-time symmetric system. The designed device exhibits an exceptional concentric ring in its momentum space and is described by a Hamiltonian with a non-Hermitian Z 3 ${\mathbb{Z}}_{3}$ topological invariant of V = −1. Fabricated devices are characterized using Fourier-space imaging for single-shot k-space measurements. Our results demonstrate a way to combine topology and non-Hermitian nanophotonics for designing robust devices with novel functionalities.
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Parto, Midya, Yuzhou G. N. Liu, Babak Bahari, Mercedeh Khajavikhan, and Demetrios N. Christodoulides. "Non-Hermitian and topological photonics: optics at an exceptional point." Nanophotonics 10, no. 1 (October 2, 2020): 403–23. http://dx.doi.org/10.1515/nanoph-2020-0434.
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AbstractIn the past few years, concepts from non-Hermitian (NH) physics, originally developed within the context of quantum field theories, have been successfully deployed over a wide range of physical settings where wave dynamics are known to play a key role. In optics, a special class of NH Hamiltonians – which respects parity-time symmetry – has been intensely pursued along several fronts. What makes this family of systems so intriguing is the prospect of phase transitions and NH singularities that can in turn lead to a plethora of counterintuitive phenomena. Quite recently, these ideas have permeated several other fields of science and technology in a quest to achieve new behaviors and functionalities in nonconservative environments that would have otherwise been impossible in standard Hermitian arrangements. Here, we provide an overview of recent advancements in these emerging fields, with emphasis on photonic NH platforms, exceptional point dynamics, and the very promising interplay between non-Hermiticity and topological physics.
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Miri, Mohammad-Ali, and Andrea Alù. "Exceptional points in optics and photonics." Science 363, no. 6422 (January 3, 2019): eaar7709. http://dx.doi.org/10.1126/science.aar7709.
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Exceptional points are branch point singularities in the parameter space of a system at which two or more eigenvalues, and their corresponding eigenvectors, coalesce and become degenerate. Such peculiar degeneracies are distinct features of non-Hermitian systems, which do not obey conservation laws because they exchange energy with the surrounding environment. Non-Hermiticity has been of great interest in recent years, particularly in connection with the quantum mechanical notion of parity-time symmetry, after the realization that Hamiltonians satisfying this special symmetry can exhibit entirely real spectra. These concepts have become of particular interest in photonics because optical gain and loss can be integrated and controlled with high resolution in nanoscale structures, realizing an ideal playground for non-Hermitian physics, parity-time symmetry, and exceptional points. As we control dissipation and amplification in a nanophotonic system, the emergence of exceptional point singularities dramatically alters their overall response, leading to a range of exotic optical functionalities associated with abrupt phase transitions in the eigenvalue spectrum. These concepts enable ultrasensitive measurements, superior manipulation of the modal content of multimode lasers, and adiabatic control of topological energy transfer for mode and polarization conversion. Non-Hermitian degeneracies have also been exploited in exotic laser systems, new nonlinear optics schemes, and exotic scattering features in open systems. Here we review the opportunities offered by exceptional point physics in photonics, discuss recent developments in theoretical and experimental research based on photonic exceptional points, and examine future opportunities in this area from basic science to applied technology.
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Zeng, Chao, Zhiwei Guo, Kejia Zhu, Caifu Fan, Guo Li, Jun Jiang, Yunhui Li, et al. "Efficient and stable wireless power transfer based on the non-Hermitian physics." Chinese Physics B 31, no. 1 (January 1, 2022): 010307. http://dx.doi.org/10.1088/1674-1056/ac3815.
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As one of the most attractive non-radiative power transfer mechanisms without cables, efficient magnetic resonance wireless power transfer (WPT) in the near field has been extensively developed in recent years, and promoted a variety of practical applications, such as mobile phones, medical implant devices and electric vehicles. However, the physical mechanism behind some key limitations of the resonance WPT, such as frequency splitting and size-dependent efficiency, is not very clear under the widely used circuit model. Here, we review the recently developed efficient and stable resonance WPT based on non-Hermitian physics, which starts from a completely different avenue (utilizing loss and gain) to introduce novel functionalities to the resonance WPT. From the perspective of non-Hermitian photonics, the coherent and incoherent effects compete and coexist in the WPT system, and the weak stable of energy transfer mainly comes from the broken phase associated with the phase transition of parity–time symmetry. Based on this basic physical framework, some optimization schemes are proposed, including using nonlinear effect, using bound states in the continuum, or resorting to the system with high-order parity-time symmetry. Moreover, the combination of non-Hermitian physics and topological photonics in multi-coil system also provides a versatile platform for long-range robust WPT with topological protection. Therefore, the non-Hermitian physics can not only exactly predict the main results of current WPT systems, but also provide new ways to solve the difficulties of previous designs.
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Wang, Bo, Tian Chen, and Xiangdong Zhang. "Topological Photonics: Observation of Novel Robust Edge States in Dissipative Non‐Hermitian Quantum Walks (Laser Photonics Rev. 14(7)/2020)." Laser & Photonics Reviews 14, no. 7 (July 2020): 2070041. http://dx.doi.org/10.1002/lpor.202070041.
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Novitsky, Denis V., and Andrey V. Novitsky. "Bound States in the Continuum versus Fano Resonances: Topological Argument." Photonics 9, no. 11 (November 20, 2022): 880. http://dx.doi.org/10.3390/photonics9110880.
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There is a recent surge of interest to the bound states in the continuum (BICs) due to their ability to provide high-quality resonances in open photonic systems. They are usually observed in perturbed systems possessing Fano resonances in their spectra. We argue that, generally speaking, the Fano resonances should not be considered as a proxy for BICs (as it is often done) due to their fundamentally different topological properties. This difference is illustrated with the non-Hermitian layered structure supporting both topologically nontrivial quasi-BIC and topologically trivial Fano resonances. Non-Hermiticity can also be a source of additional topological features of these resonant responses. Moreover, the lasing mode associated with BIC in this structure also possesses nonzero topological charge that can be useful for producing unconventional states of light. This paper contributes to the discussion of BIC physics and raises new questions concerning topological properties of non-Hermitian systems.
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Xia, Shiqi, Dimitrios Kaltsas, Daohong Song, Ioannis Komis, Jingjun Xu, Alexander Szameit, Hrvoje Buljan, Konstantinos G. Makris, and Zhigang Chen. "Nonlinear tuning of PT symmetry and non-Hermitian topological states." Science 372, no. 6537 (April 1, 2021): 72–76. http://dx.doi.org/10.1126/science.abf6873.
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Topology, parity-time (PT) symmetry, and nonlinearity are at the origin of many fundamental phenomena in complex systems across the natural sciences, but their mutual interplay remains unexplored. We established a nonlinear non-Hermitian topological platform for active tuning of PT symmetry and topological states. We found that the loss in a topological defect potential in a non-Hermitian photonic lattice can be tuned solely by nonlinearity, enabling the transition between PT-symmetric and non–PT-symmetric regimes and the maneuvering of topological zero modes. The interaction between two apparently antagonistic effects is revealed: the sensitivity close to exceptional points and the robustness of non-Hermitian topological states. Our scheme using single-channel control of global PT symmetry and topology via local nonlinearity may provide opportunities for unconventional light manipulation and device applications.
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Weidemann, Sebastian, Mark Kremer, Tobias Helbig, Tobias Hofmann, Alexander Stegmaier, Martin Greiter, Ronny Thomale, and Alexander Szameit. "Topological funneling of light." Science 368, no. 6488 (March 26, 2020): 311–14. http://dx.doi.org/10.1126/science.aaz8727.
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Dissipation is a general feature of non-Hermitian systems. But rather than being an unavoidable nuisance, non-Hermiticity can be precisely controlled and hence used for sophisticated applications, such as optical sensors with enhanced sensitivity. In our work, we implement a non-Hermitian photonic mesh lattice by tailoring the anisotropy of the nearest-neighbor coupling. The appearance of an interface results in a complete collapse of the entire eigenmode spectrum, leading to an exponential localization of all modes at the interface. As a consequence, any light field within the lattice travels toward this interface, irrespective of its shape and input position. On the basis of this topological phenomenon, called the “non-Hermitian skin effect,” we demonstrate a highly efficient funnel for light.
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Jiang, Jiapei, Bei Yan, Yuchen Peng, Jianlan Xie, Aoqian Shi, and Jianjun Liu. "Multiband topological states in non-Hermitian photonic crystals." Optics Letters 47, no. 2 (January 14, 2022): 437. http://dx.doi.org/10.1364/ol.449733.
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Zhou, Xingping, Jing Wu, and Yongfeng Wu. "Topological corner states in non-Hermitian photonic crystals." Optics Communications 466 (July 2020): 125653. http://dx.doi.org/10.1016/j.optcom.2020.125653.
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Dong, Junhua, Qingmei Hu, Chang-Yin Ji, Bingsuo Zou, and Yongyou Zhang. "Exceptional points in a topological waveguide-cavity coupled system." New Journal of Physics 23, no. 11 (November 1, 2021): 113025. http://dx.doi.org/10.1088/1367-2630/ac3441.
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Abstract Exceptional points (EPs) as branch singularities describe peculiar degeneracies of non-Hermitian systems, widely studied in topological and non-topological optical architectures with introducing gain or loss technically. This work focuses on the EPs in a topological waveguide (TW)-cavity coupled structure, where there is no need to introduce practical gain or loss. The topological cavity contains two degenerate counter-propagation topological whispering gallery modes, whose coupling with the TW leads to the effective gain and loss, responsible for the EP. Such a photonic architecture is designed practically by crystal-symmetry-protected topological photonic insulators based on air rods in conventional dielectric materials. The relevant EP reveals the breaking of the parity-time symmetry, reflected by the change of the transmission-dip number in the optical transmission spectra of the system. Achieving EPs in topological photonic systems possibly opens a new avenue toward robust optical devices with exceptional-point-based unique properties and functionalities.
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Weidemann, Sebastian, Mark Kremer, Stefano Longhi, and Alexander Szameit. "Topological triple phase transition in non-Hermitian Floquet quasicrystals." Nature 601, no. 7893 (January 19, 2022): 354–59. http://dx.doi.org/10.1038/s41586-021-04253-0.
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AbstractPhase transitions connect different states of matter and are often concomitant with the spontaneous breaking of symmetries. An important category of phase transitions is mobility transitions, among which is the well known Anderson localization1, where increasing the randomness induces a metal–insulator transition. The introduction of topology in condensed-matter physics2–4 lead to the discovery of topological phase transitions and materials as topological insulators5. Phase transitions in the symmetry of non-Hermitian systems describe the transition to on-average conserved energy6 and new topological phases7–9. Bulk conductivity, topology and non-Hermitian symmetry breaking seemingly emerge from different physics and, thus, may appear as separable phenomena. However, in non-Hermitian quasicrystals, such transitions can be mutually interlinked by forming a triple phase transition10. Here we report the experimental observation of a triple phase transition, where changing a single parameter simultaneously gives rise to a localization (metal–insulator), a topological and parity–time symmetry-breaking (energy) phase transition. The physics is manifested in a temporally driven (Floquet) dissipative quasicrystal. We implement our ideas via photonic quantum walks in coupled optical fibre loops11. Our study highlights the intertwinement of topology, symmetry breaking and mobility phase transitions in non-Hermitian quasicrystalline synthetic matter. Our results may be applied in phase-change devices, in which the bulk and edge transport and the energy or particle exchange with the environment can be predicted and controlled.
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Ke, Shaolin, Dong Zhao, Jie Fu, Qing Liao, Bing Wang, and Peixiang Lu. "Topological Edge Modes in Non-Hermitian Photonic Aharonov-Bohm Cages." IEEE Journal of Selected Topics in Quantum Electronics 26, no. 6 (November 2020): 1–8. http://dx.doi.org/10.1109/jstqe.2020.3010586.
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Geng, Linlin, Weixuan Zhang, Xiangdong Zhang, and Xiaoming Zhou. "Topological mode switching in modulated structures with dynamic encircling of an exceptional point." Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences 477, no. 2245 (January 2021): 20200766. http://dx.doi.org/10.1098/rspa.2020.0766.
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Exceptional points are special degeneracies occurring in non-Hermitian systems at which both eigenfrequencies and eigenmodes coalesce simultaneously. Fascinating phenomena, including topological, non-reciprocal and chiral energy transfer between normal modes, have been envisioned in optical and photonic systems with the exceptional point dynamically encircled in the parameter space. However, it has remained an open question of whether and how topological mode switching relying on exceptional points could be achieved in mechanical systems. The present paper studies a two-mode mechanical system with an exceptional point and implements the dynamic encircling of such a point using dynamic modulation mechanisms with time-driven elasticity and viscosity. Topological mode switching with robustness against the input state and loop trajectories has been demonstrated numerically. It is found that the dynamical encircling of an exceptional point with the starting point near the symmetric phase leads to chiral mode transfer controlled mainly by the encircling direction, while non-chiral dynamics is observed for the starting point near the broken phase. Analyses also show that minor energy input is required in the process of encircling the exceptional point, demonstrating the intrinsically motivated behaviour of topological mode switching.
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Liu, Hang, Sheng Meng, and Feng Liu. "Non-Hermitian topological states in 2D line-graph lattices: evolving triple exceptional points on reciprocal line graphs." New Journal of Physics 23, no. 12 (December 1, 2021): 123038. http://dx.doi.org/10.1088/1367-2630/ac40cb.
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Abstract Non-Hermitian (NH) topological states, such as the doubly-degenerate nodes dubbed as exceptional points (EPs) in Bloch band structure of 2D lattices driven by gain and loss, have attracted much recent interest. We demonstrate theoretically that in the three-site edge-centered lattices, i.e. the so-called line-graph lattices, such as kagome lattice which is a line graph of hexagonal lattice, there exist three types of triply-degenerate EPs evolving intriguingly on another set of line graphs in the reciprocal space. A single TEP (STEP) with ±1/3 topological charge moves faithfully along the edges of reciprocal line graphs with varying gain and loss, while two STEPs merge distinctively into one unconventional orthogonal double TEP (DTEP) with ±2/3 charge at the vertices, which is characterized with two ordinary self-orthogonal eigenfunctions but one surprising ‘orthogonal’ eigenfunction. Differently, in a modified line-graph lattice with an off-edge-center site, the ordinary coalesced state of DTEPs emerges with three identical self-orthogonal eigenfunctions. Such NH states and their evolution can be generally realized in various artificial systems, such as photonic and sonic crystals, where light and sonic vortex beams with different fractional twisting can be found. Our findings shed new light on fundamental understanding of gapless topological states in NH systems in terms of creation and evolution of high-order EPs, and open up new research directions to further link line graph and flow network theory coupled with topological physics, especially under non-equilibrium gain/loss conditions.
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Passler, Nikolai C., Xiang Ni, Guangwei Hu, Joseph R. Matson, Giulia Carini, Martin Wolf, Mathias Schubert, et al. "Hyperbolic shear polaritons in low-symmetry crystals." Nature 602, no. 7898 (February 23, 2022): 595–600. http://dx.doi.org/10.1038/s41586-021-04328-y.
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AbstractThe lattice symmetry of a crystal is one of the most important factors in determining its physical properties. Particularly, low-symmetry crystals offer powerful opportunities to control light propagation, polarization and phase1–4. Materials featuring extreme optical anisotropy can support a hyperbolic response, enabling coupled light–matter interactions, also known as polaritons, with highly directional propagation and compression of light to deeply sub-wavelength scales5. Here we show that monoclinic crystals can support hyperbolic shear polaritons, a new polariton class arising in the mid-infrared to far-infrared due to shear phenomena in the dielectric response. This feature emerges in materials in which the dielectric tensor cannot be diagonalized, that is, in low-symmetry monoclinic and triclinic crystals in which several oscillators with non-orthogonal relative orientations contribute to the optical response6,7. Hyperbolic shear polaritons complement previous observations of hyperbolic phonon polaritons in orthorhombic1,3,4 and hexagonal8,9 crystal systems, unveiling new features, such as the continuous evolution of their propagation direction with frequency, tilted wavefronts and asymmetric responses. The interplay between diagonal loss and off-diagonal shear phenomena in the dielectric response of these materials has implications for new forms of non-Hermitian and topological photonic states. We anticipate that our results will motivate new directions for polariton physics in low-symmetry materials, which include geological minerals10, many common oxides11 and organic crystals12, greatly expanding the material base and extending design opportunities for compact photonic devices.
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Yan, Qiuchen, Boheng Zhao, Rong Zhou, Rui Ma, Qinghong Lyu, Saisai Chu, Xiaoyong Hu, and Qihuang Gong. "Advances and applications on non-Hermitian topological photonics." Nanophotonics, March 9, 2023. http://dx.doi.org/10.1515/nanoph-2022-0775.
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Abstract Non-Hermitian photonics and topological photonics, as new research fields in optics, have attracted much attention in recent years, accompanying by a great deal of new physical concepts and novel effects emerging. The two fields are gradually crossed during the development process and the non-Hermitian topological photonics was born. Non-Hermitian topological photonics not only constantly produces various novel physical effects, but also shows great potential in optical device applications. It becomes an important part of the modern physics and optics, penetrating into different research fields. On one hand, photonics system can introduce artificially-constructed gain and loss to study non-Hermitian physics. Photonics platform is an important methods and ways to verify novel physical phenomena and promote the development of non-Hermitian physics. On the other hand, the non-Hermitian topological photonics provides a new dimension for manipulating topological states. Active and dissipate materials are common in photonic systems; therefore, by using light pump and dissipation of photonic systems, it is expected to promote further development of topological photonics in device applications. In this review article, we focus on the recent advances and applications on non-Hermitian topological photonics, including the non-Hermitian topological phase transition and skin effect, as well as the applications emerging prosperously in reconfigurable, nonlinear and quantum optical systems. The possible future research directions of non-Hermitian topological photonics are also discussed at the end. Non-Hermitian topological photonics can have great potential in technological revolution and have the capacity of leading the development of both physics and technology industry.
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Liu, Hui, Pengtao Lai, Haonan Wang, Hua Cheng, Jianguo Tian, and Shuqi Chen. "Topological phases and non-Hermitian topology in photonic artificial microstructures." Nanophotonics, February 16, 2023. http://dx.doi.org/10.1515/nanoph-2022-0778.
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Abstract In the past few decades, the discovery of topological matter states has ushered in a new era in topological physics, providing a robust framework for strategically controlling the transport of particles or waves. Topological photonics, in particular, has sparked considerable research due to its ability to construct and manipulate photonic topological states via photonic artificial microstructures. Although the concept of topology originates from condensed matter, topological photonics has given rise to new fundamental ideas and a range of potential applications that may lead to revolutionary technologies. Here, we review recent developments in topological photonics, with a focus on the realization and application of several emerging research areas in photonic artificial microstructures. We highlight the research trend, spanning from the photonic counterpart of topological insulator phases, through topological semimetal phases, to other emerging non-Hermitian topologies.
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Nasari, Hadiseh, Georgios Pyrialakos, Demetrios Christodoulides, and Mercedeh Khajavikhan. "Non-Hermitian topological photonics." Optical Materials Express, January 31, 2023. http://dx.doi.org/10.1364/ome.483361.
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Peng, Jie, Ruo-Yang Zhang, Shiqi Jia, Wei Liu, and Shubo Wang. "Topological near fields generated by topological structures." Science Advances 8, no. 41 (October 14, 2022). http://dx.doi.org/10.1126/sciadv.abq0910.
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The central idea of metamaterials and metaoptics is that, besides their base materials, the geometry of structures offers a broad extra dimension to explore for exotic functionalities. Here, we discover that the topology of structures fundamentally dictates the topological properties of optical fields and offers a new dimension to exploit for optical functionalities that are irrelevant to specific material constituents or structural geometries. We find that the nontrivial topology of metal structures ensures the birth of polarization singularities (PSs) in the near field with rich morphologies and intriguing spatial evolutions including merging, bifurcation, and topological transition. By mapping the PSs to non-Hermitian exceptional points and using homotopy theory, we extract the core invariant that governs the topological classification of the PSs and the conservation law that regulates their spatial evolutions. The results bridge singular optics, topological photonics, and non-Hermitian physics, with potential applications in chiral sensing, chiral quantum optics, and beyond photonics in other wave systems.
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Chen, Zhigang, and Mordechai Segev. "Highlighting photonics: looking into the next decade." eLight 1, no. 1 (June 8, 2021). http://dx.doi.org/10.1186/s43593-021-00002-y.
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AbstractLet there be light–to change the world we want to be! Over the past several decades, and ever since the birth of the first laser, mankind has witnessed the development of the science of light, as light-based technologies have revolutionarily changed our lives. Needless to say, photonics has now penetrated into many aspects of science and technology, turning into an important and dynamically changing field of increasing interdisciplinary interest. In this inaugural issue of eLight, we highlight a few emerging trends in photonics that we think are likely to have major impact at least in the upcoming decade, spanning from integrated quantum photonics and quantum computing, through topological/non-Hermitian photonics and topological insulator lasers, to AI-empowered nanophotonics and photonic machine learning. This Perspective is by no means an attempt to summarize all the latest advances in photonics, yet we wish our subjective vision could fuel inspiration and foster excitement in scientific research especially for young researchers who love the science of light.
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Chen Yang, Zhang Tian-Yang, Guo Guang-Can, and Ren Xi-Feng. "Integrated photonic quantum simulation." Acta Physica Sinica, 2022, 0. http://dx.doi.org/10.7498/aps.71.20221938.
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Using a controllable quantum system to study another complicated or hard-to-control quantum system, quantum simulation provides a valuable tool to explore complex unknown quantum systems, which cannot be simulated on classical computers due to the exponential explosion of the Hilbert space. Among different kinds of physical realizations of quantum simulation, integrated optical systems have emerged as appropriate platforms in recent years, due to the advantages of flexible control, weak decoherence and lack of interaction in optical systems. In this review, we attempt to introduce some of the basic models used for quantum simulation in integrated photonic systems. The structure of this review article is shown as follows. Section 2 introduces the commonly used material platforms for integrated quantum simulation, including the silicon-based, lithium niobate-based integrated circuits and the femtosecond laser direct writing optical waveguides. Several integrated optical platforms such as the coupled waveguide arrays, photonic crystals, coupled resonator arrays and multiport interferometers are introduced. In section 3, we focus on the analog quantum simulation in the integrated photonic platform, including Anderson localization of light in disordered systems, various kinds of topological insulators, nonlinear and non-Hermitian systems. More concretely, section 3.1 is devoted to integrated photonic realizations of disordered and quasi-periodic systems. In section 3.2, we review integrated photonic realizations of the topological insulators with and without time-reversal symmetry, including Floquet topological insulators, quantum spin hall system, anomalous quantum hall system, valley hall system, Su-Schrieffer-Heeger (SSH) model and photonic topological Anderson insulators. Besides, topological insulator lasers and topologically protected quantum photon sources are briefly reviewed. The nonlinear and non-Hermitian integrated optical systems are reviewed in section 3.3. In section 4, we introduce integrated digital quantum simulations based on the multiport interferometers, including the discrete-time quantum random walk, boson sampling and molecular simulation. In section 5, we summarize the content of the article and outlook on the future perspectives of the integrated photonic quantum simulation. We believe that integrated photonic platforms will continue to provide an excellent platform for quantum simulation. More practical applications will be found based on this system, combining the fields of topological photonics, laser technologies, quantum information, nonlinear and non-Hermitian physics.
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Xu, Guoqiang, Wei Li, Xue Zhou, Huagen Li, Ying Li, Shanhui Fan, Shuang Zhang, Demetrios N. Christodoulides, and Cheng-Wei Qiu. "Observation of Weyl exceptional rings in thermal diffusion." Proceedings of the National Academy of Sciences 119, no. 15 (April 4, 2022). http://dx.doi.org/10.1073/pnas.2110018119.
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Significance Thermal diffusion is dissipative and strongly related to non-Hermitian physics. At the same time, non-Hermitian Weyl systems have spurred tremendous interest across photonics and acoustics. This correlation has been long ignored and hence shed little light upon the question of whether the Weyl exceptional ring (WER) in thermal diffusion could exist. Intuitively, thermal diffusion provides no real parameter dimensions, thus prohibiting a topological nature and WER. This work breaks this perception by imitating synthetic dimensions via two spatiotemporal advection pairs. The WER is achieved in thermal diffusive systems. Both surface-like and bulk states are demonstrated by coupling two WERs with opposite topological charges. These findings extend topological notions to diffusions and motivate investigation of non-Hermitian diffusive and dissipative control.
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Kim, Jungmin, Dayeong Lee, Sunkyu Yu, and Namkyoo Park. "Unidirectional scattering with spatial homogeneity using correlated photonic time disorder." Nature Physics, February 20, 2023. http://dx.doi.org/10.1038/s41567-023-01962-3.
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AbstractRecently, there has been increasing interest in the temporal degree of freedom in photonics due to its analogy with spatial axes, causality and open-system characteristics. In particular, the temporal analogues of photonic crystals have allowed the design of momentum gaps and their extension to topological and non-Hermitian photonics. Although recent studies have also revealed the effect of broken discrete time-translational symmetry in view of the temporal analogy of spatial Anderson localization, the broad intermediate regime between time order and time uncorrelated disorder has not been examined. Here we theoretically investigate the inverse design of photonic time disorder to achieve optical functionalities in spatially homogeneous platforms. By developing the structure factor and order metric using causal Green’s functions for disorder in the time domain, we propose an engineered time scatterer, which provides unidirectional scattering with controlled scattering amplitudes. We also show that the order-to-disorder transition in the time domain allows the manipulation of scattering bandwidths, which makes resonance-free temporal colour filtering possible. Our work could advance optical functionalities without spatial patterning.
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Kim, Ha-Reem, Min-Soo Hwang, Daria Smirnova, Kwang-Yong Jeong, Yuri Kivshar, and Hong-Gyu Park. "Multipolar lasing modes from topological corner states." Nature Communications 11, no. 1 (November 13, 2020). http://dx.doi.org/10.1038/s41467-020-19609-9.
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AbstractTopological photonics provides a fundamental framework for robust manipulation of light, including directional transport and localization with built-in immunity to disorder. Combined with an optical gain, active topological cavities hold special promise for a design of light-emitting devices. Most studies to date have focused on lasing at topological edges of finite systems or domain walls. Recently discovered higher-order topological phases enable strong high-quality confinement of light at the corners. Here, we demonstrate lasing action of corner states in nanophotonic topological structures. We identify several multipole corner modes with distinct emission profiles via hyperspectral imaging and discern signatures of non-Hermitian radiative coupling of leaky topological states. In addition, depending on the pump position in a large-size cavity, we generate selectively lasing from either edge or corner states within the topological bandgap. Our studies provide the direct observation of multipolar lasing and engineered collective resonances in active topological nanostructures.
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Galda, Alexey, and Valerii M. Vinokur. "Exceptional points in classical spin dynamics." Scientific Reports 9, no. 1 (November 25, 2019). http://dx.doi.org/10.1038/s41598-019-53455-0.
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AbstractNon-conservative physical systems admit a special kind of spectral degeneracy, known as exceptional point (EP), at which eigenvalues and eigenvectors of the corresponding non-Hermitian Hamiltonian coalesce. Dynamical parametric encircling of the EP can lead to non-adiabatic evolution associated with a state flip, a sharp transition between the resonant modes. Physical consequences of the dynamical encircling of EPs in open dissipative systems have been explored in optics and photonics. Building on the recent progress in understanding the parity-time ($${\mathscr{P}}{\mathscr{T}}$$PT)-symmetric dynamics in spin systems, we use topological properties of EPs to implement chiral non-reciprocal transmission of a spin through the material with non-uniform magnetization, like helical magnet. We consider an exemplary system, spin-torque-driven single spin described by the time-dependent non-Hermitian Hamiltonian. We show that encircling individual EPs in a parameter space results in non-reciprocal spin dynamics and find the range of optimal protocol parameters for high-efficiency asymmetric spin filter based on this effect. Our findings offer a platform for non-reciprocal spin devices for spintronics and magnonics.
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Galiffi, Emanuele, Paloma A. Huidobro, and J. B. Pendry. "An Archimedes' screw for light." Nature Communications 13, no. 1 (May 9, 2022). http://dx.doi.org/10.1038/s41467-022-30079-z.
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AbstractAn Archimedes’ Screw captures water, feeding energy into it by lifting it to a higher level. We introduce the first instance of an optical Archimedes’ Screw, and demonstrate how this system is capable of capturing light, dragging it and amplifying it. We unveil new exact analytic solutions to Maxwell’s Equations for a wide family of chiral space-time media, and show their potential to achieve chirally selective amplification within widely tunable parity-time-broken phases. Our work, which may be readily implemented via pump-probe experiments with circularly polarized beams, opens a new direction in the physics of time-varying media by merging the rising field of space-time metamaterials and that of chiral systems, and offers a new playground for topological and non-Hermitian photonics, with potential applications to chiral spectroscopy and sensing.
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Xie, Boyang, Hui Liu, Haonan Wang, Hua Cheng, Jianguo Tian, and Shuqi Chen. "A Review of Topological Semimetal Phases in Photonic Artificial Microstructures." Frontiers in Physics 9 (November 16, 2021). http://dx.doi.org/10.3389/fphy.2021.771481.
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In the past few years, the concept of topological matter has inspired considerable research in broad areas of physics. In particular, photonic artificial microstructures like photonic crystals and metamaterials provide a unique platform to investigate topologically non-trivial physics in spin-1 electromagnetic fields. Three-dimensional (3D) topological semimetal band structures, which carry non-trivial topological charges, are fundamental to 3D topological physics. Here, we review recent progress in understanding 3D photonic topological semimetal phases and various approaches for realizing them, especially with photonic crystals or metamaterials. We review topological gapless band structures and topological surface states aroused from the non-trivial bulk topology. Weyl points, 3D Dirac points, nodal lines, and nodal surfaces of different types are discussed. We also demonstrate their application in coupling spin-polarized electromagnetic waves, anomalous reflection, vortex beams generation, bulk transport, and non-Hermitian effects.
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Wang, Xinyue, Wen Zhao, Sayed Elshahat, and Cuicui Lu. "Topological rainbow trapping based on gradual valley photonic crystals." Frontiers in Physics 11 (February 20, 2023). http://dx.doi.org/10.3389/fphy.2023.1141997.
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Valley photonic crystals (PCs) play a crucial role in controlling light flow and realizing robust nanophotonic devices. In this study, rotated gradient valley PCs are proposed to realize topological rainbow trapping. A topological rainbow is observed despite the presence of pillars of different shapes, which indicates the remarkable universality of the design. Then, the loss is introduced to explore the topological rainbow trapping of the non-Hermitian valley PC. For the step-angle structure, the same or different losses can be applied, which does not affect the formed topological rainbow trapping. For a single-angle structure, the applied progressive loss can also achieve rainbow trapping. The rainbow is robust and topologically protected in both Hermitian and non-Hermitian cases, which is confirmed by the introduction of perturbations and defects. The proposed method in the current study presents an intriguing step for light control and potential applications in optical buffering and frequency routing.
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Lee, Ki Young, Kwang Wook Yoo, Youngsun Choi, Gunpyo Kim, Sangmo Cheon, Jae Woong Yoon, and Seok Ho Song. "Topological guided-mode resonances at non-Hermitian nanophotonic interfaces." Nanophotonics, April 2, 2021. http://dx.doi.org/10.1515/nanoph-2021-0024.
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Abstract The topological properties of photonic microstructures are of great interest because of their experimental feasibility for fundamental study and potential applications. Here, we show that robust guided-mode-resonance states exist in photonic domain-wall structures whenever the complex photonic band structures involve certain topological correlations in general. Using the non-Hermitian photonic analogy of the one-dimensional Dirac equation, we derive essential conditions for photonic Jackiw-Rebbi-state resonances taking advantage of unique spatial confinement and spot-like spectral features which are remarkably robust against random parametric errors. Therefore, the proposed resonance configuration potentially provides a powerful method to create compact and stable photonic resonators for various applications in practice.
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Prudêncio, Filipa R., and Mário G. Silveirinha. "First principles calculation of topological invariants of non-Hermitian photonic crystals." Communications Physics 3, no. 1 (December 2020). http://dx.doi.org/10.1038/s42005-020-00482-3.
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AbstractTopological photonic systems have recently emerged as an exciting new paradigm to guide light without back-reflections. The Chern topological numbers of a photonic platform are usually written in terms of the Berry curvature, which depends on the normal modes of the system. Here, we use a gauge invariant Green’s function method to determine from first principles the topological invariants of photonic crystals. The proposed formalism does not require the calculation of the photonic band-structure, and can be easily implemented using the operators obtained with a standard plane-wave expansion. Furthermore, it is shown that the theory can be readily applied to the classification of topological phases of non-Hermitian photonic crystals with lossy or gainy materials, e.g., parity-time symmetric photonic crystals.
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Lin, Quan, Tianyu Li, Lei Xiao, Kunkun Wang, Wei Yi, and Peng Xue. "Observation of non-Hermitian topological Anderson insulator in quantum dynamics." Nature Communications 13, no. 1 (June 9, 2022). http://dx.doi.org/10.1038/s41467-022-30938-9.
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AbstractDisorder and non-Hermiticity dramatically impact the topological and localization properties of a quantum system, giving rise to intriguing quantum states of matter. The rich interplay of disorder, non-Hermiticity, and topology is epitomized by the recently proposed non-Hermitian topological Anderson insulator that hosts a plethora of exotic phenomena. Here we experimentally simulate the non-Hermitian topological Anderson insulator using disordered photonic quantum walks, and characterize its localization and topological properties. In particular, we focus on the competition between Anderson localization induced by random disorder, and the non-Hermitian skin effect under which all eigenstates are squeezed toward the boundary. The two distinct localization mechanisms prompt a non-monotonous change in profile of the Lyapunov exponent, which we experimentally reveal through dynamic observables. We then probe the disorder-induced topological phase transitions, and demonstrate their biorthogonal criticality. Our experiment further advances the frontier of synthetic topology in open systems.
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Silveirinha, Mário G. "Topological theory of non-Hermitian photonic systems." Physical Review B 99, no. 12 (March 29, 2019). http://dx.doi.org/10.1103/physrevb.99.125155.
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Król, M., I. Septembre, P. Oliwa, M. Kędziora, K. Łempicka-Mirek, M. Muszyński, R. Mazur, et al. "Annihilation of exceptional points from different Dirac valleys in a 2D photonic system." Nature Communications 13, no. 1 (September 12, 2022). http://dx.doi.org/10.1038/s41467-022-33001-9.
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AbstractTopological physics relies on Hamiltonian’s eigenstate singularities carrying topological charges, such as Dirac points, and – in non-Hermitian systems – exceptional points (EPs), lines or surfaces. So far, the reported non-Hermitian topological transitions were related to the creation of a pair of EPs connected by a Fermi arc out of a single Dirac point by increasing non-Hermiticity. Such EPs can annihilate by reducing non-Hermiticity. Here, we demonstrate experimentally that an increase of non-Hermiticity can lead to the annihilation of EPs issued from different Dirac points (valleys). The studied platform is a liquid crystal microcavity with voltage-controlled birefringence and TE-TM photonic spin-orbit-coupling. Non-Hermiticity is provided by polarization-dependent losses. By increasing the non-Hermiticity degree, we control the position of the EPs. After the intervalley annihilation, the system becomes free of any band singularity. Our results open the field of non-Hermitian valley-physics and illustrate connections between Hermitian topology and non-Hermitian phase transitions.
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Chen, Menglin L. N., Li Jun Jiang, Shuang Zhang, Ran Zhao, Zhihao Lan, and Wei E. I. Sha. "Comparative study of Hermitian and non-Hermitian topological dielectric photonic crystals." Physical Review A 104, no. 3 (September 1, 2021). http://dx.doi.org/10.1103/physreva.104.033501.
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Gao, He, Haoran Xue, Zhongming Gu, Tuo Liu, Jie Zhu, and Baile Zhang. "Non-Hermitian route to higher-order topology in an acoustic crystal." Nature Communications 12, no. 1 (March 25, 2021). http://dx.doi.org/10.1038/s41467-021-22223-y.
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AbstractTopological phases of matter are classified based on their Hermitian Hamiltonians, whose real-valued dispersions together with orthogonal eigenstates form nontrivial topology. In the recently discovered higher-order topological insulators (TIs), the bulk topology can even exhibit hierarchical features, leading to topological corner states, as demonstrated in many photonic and acoustic artificial materials. Naturally, the intrinsic loss in these artificial materials has been omitted in the topology definition, due to its non-Hermitian nature; in practice, the presence of loss is generally considered harmful to the topological corner states. Here, we report the experimental realization of a higher-order TI in an acoustic crystal, whose nontrivial topology is induced by deliberately introduced losses. With local acoustic measurements, we identify a topological bulk bandgap that is populated with gapped edge states and in-gap corner states, as the hallmark signatures of hierarchical higher-order topology. Our work establishes the non-Hermitian route to higher-order topology, and paves the way to exploring various exotic non-Hermiticity-induced topological phases.
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Roszeitis, Karla, Markas Sudzius, Alexander Palatnik, Rebekka Koch, Jan Carl Budich, and Karl Leo. "Coherence onset in PT-symmetric organic microcavities: towards directional propagation of light." Journal of the European Optical Society-Rapid Publications, July 28, 2022. http://dx.doi.org/10.1051/jeos/2022006.
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For the investigation of non-Hermitian effects and physics under parity-time (PT) symmetry, photonic systems are ideal model systems for both experimental and theoretical research. We investigate a fundamental building block of a potential photonic device, consisting of coupled organic microcavities. The coupled system contains cavities with gain and loss and respects parity-time symmetry, leading to non-Hermitian terms in the corresponding Hamiltonian. Experimentally, two coupled cavities are realized and driven optically using pulsed laser excitation up to the lasing regime. We show that above the lasing threshold, when coherence evolves, the coupled-cavity system starts to operate asymmetrically, generating more light on one side of the device, being characteristic of non-Hermitian PT-symmetric systems. Calculations and simulations on a Su-Schrieffer-Heeger (SSH) chain composed of these PT-symmetric unit cells show the emergence of non-trivial topological features.
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Xue, Peng, Xingze Qiu, Kunkun Wang, Barry C. Sanders, and Wei Yi. "Observation of dark edge states in parity-time-symmetric quantum dynamics." National Science Review, January 10, 2023. http://dx.doi.org/10.1093/nsr/nwad005.
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Abstract Topological edge states arise in non-Hermitian parity-time ($\mathcal {PT}$)-symmetric systems, and manifest themselves as bright or dark edge states, depending on the imaginary components of their eigenenergies. As the spatial probabilities of dark edge states are suppressed during the non-unitary dynamics, it is a challenge to observe them experimentally. Here we report the experimental detection of dark edge states in photonic quantum walks with spontaneously broken $\mathcal {PT}$ symmetry, thus providing a complete description of the topological phenomena therein. We experimentally confirm that the global Berry phase in $\mathcal {PT}$-symmetric quantum-walk dynamics unambiguously defines topological invariants of the system in both the $\mathcal {PT}$-symmetry-unbroken and broken regimes. Our results establish a unified framework for characterizing topology in $\mathcal {PT}$-symmetric quantum-walk dynamics, and provide a useful method to observe topological phenomena in $\mathcal {PT}$-symmetric non-Hermitian systems in general.
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Zhang, Xintong, Ke Xu, Chunmin Liu, Xiaoxiao Song, Bowen Hou, Rui Yu, Hao Zhang, Dan Li, and Jing Li. "Gauge-dependent topology in non-reciprocal hopping systems with pseudo-Hermitian symmetry." Communications Physics 4, no. 1 (July 20, 2021). http://dx.doi.org/10.1038/s42005-021-00668-3.
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AbstractEnergy conservation is not valid in non-Hermitian systems with gain/loss or non-reciprocity, which leads to various extraordinary resonant characteristics. Compared with Hermitian systems, the intersection of non-Hermitian physics and topology generates new phases that have not been observed in condensed-matter systems before. Here, utilizing the designed two-dimensional periodical model with non-reciprocal hopping terms, we show how to obtain both the ellipse-like or hyperbolic-like spectral degeneracy, the topological boundary modes and the bulk-boundary correspondence by the protection of time-reversal symmetry and pseudo-Hermitian symmetry. Notably, the boundary modes and bulk-boundary correspondence can simultaneously appear only for specific selection of the primitive cell, and we explored the analytical solution to verify such gauge-dependent topological behaviors. Our topolectrical circuit simulation provides a flexible approach to confirm the designed properties and clarify the crucial role of pseudo-Hermiticity on the stability of a practical system. In a broader view, our findings can be compared to other platforms such as meta-surface or photonic crystals, for the purpose on the control of resonant frequency and localization properties.
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Manna, Sourav, and Bitan Roy. "Inner skin effects on non-Hermitian topological fractals." Communications Physics 6, no. 1 (January 16, 2023). http://dx.doi.org/10.1038/s42005-023-01130-2.
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AbstractNon-Hermitian (NH) crystals, quasicrystals, and amorphous network display an accumulation of a macroscopic number of states near one of its specific interfaces with vacuum, such as edge, surface, hinge, or corner. This phenomenon is known as the NH skin effect, which can only be observed with open boundary condition. In this regard self-similar fractals, manifesting inner boundaries in the interior of the system, harbor a novel phenomenon, the inner skin effect (ISE). Then the NH skin effect appears at the inner boundaries of the fractal lattice with periodic boundary condition. We showcase this observation by implementing prominent models for NH insulators and superconductors on representative planar Sierpinski carpet fractal lattices. They accommodate both first-order and second-order ISEs at inner edges and corners, respectively, for charged as well as neutral Majorana fermions. Furthermore, over extended parameter regimes ISEs are tied with nontrivial bulk topological invariants, yielding intrinsic ISEs. With the recent success in engineering NH topological phases on highly tunable metamaterial platforms, such as photonic and phononic lattices, as well as topolectric circuits, the proposed ISEs can be observed experimentally at least on fractal metamaterials with periodic boundary condition.
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Meng, Ya. "Topological Corner States in Non-Unitary Coinless Discrete-Time Quantum Walks." Frontiers in Physics 10 (May 3, 2022). http://dx.doi.org/10.3389/fphy.2022.861125.
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The discrete-time quantum walk provides a versatile platform for exploring abundant topological phenomena due to its intrinsic spin-orbit coupling. In this work, we study the non-Hermitian second-order topology in a two-dimensional non-unitary coinless discrete-time quantum walk, which is realizable in the three-dimensional photonic waveguides. By adding the non-unitary gain-loss substep operators into the one-step operator of the coinless discrete-time quantum walk, we find the appearance of the four-degenerate zero-dimensional corner states at ReE = 0 when the gain-loss parameter of the system is larger than a critical value. This intriguing phenomenon originates from the nontrivial second-order topology of the system, which can be characterized by a second-order topological invariant of polarizations. Finally, we show that the exotic corner states can be observed experimentally through the probability distributions during the multistep non-unitary coinless discrete-time quantum walks. Our work potentially pave the way for exploring exotic non-Hermitian higher-order topological states of matter in coinless discrete-time quantum walks.
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Zhang, Li, Yihao Yang, Yong Ge, Yi-Jun Guan, Qiaolu Chen, Qinghui Yan, Fujia Chen, et al. "Acoustic non-Hermitian skin effect from twisted winding topology." Nature Communications 12, no. 1 (November 2, 2021). http://dx.doi.org/10.1038/s41467-021-26619-8.
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AbstractThe recently discovered non-Hermitian skin effect (NHSE) manifests the breakdown of current classification of topological phases in energy-nonconservative systems, and necessitates the introduction of non-Hermitian band topology. So far, all NHSE observations are based on one type of non-Hermitian band topology, in which the complex energy spectrum winds along a closed loop. As recently characterized along a synthetic dimension on a photonic platform, non-Hermitian band topology can exhibit almost arbitrary windings in momentum space, but their actual phenomena in real physical systems remain unclear. Here, we report the experimental realization of NHSE in a one-dimensional (1D) non-reciprocal acoustic crystal. With direct acoustic measurement, we demonstrate that a twisted winding, whose topology consists of two oppositely oriented loops in contact rather than a single loop, will dramatically change the NHSE, following previous predictions of unique features such as the bipolar localization and the Bloch point for a Bloch-wave-like extended state. This work reveals previously unnoticed features of NHSE, and provides the observation of physical phenomena originating from complex non-Hermitian winding topology.
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Jiang, Xunya, Langlang Xiong, and Yufu Liu. "The evolution of topological singularities between real- and complex-frequency domains and the engineering of photonic bands for Hermitian and non-Hermitian photonic crystals." New Journal of Physics, December 13, 2022. http://dx.doi.org/10.1088/1367-2630/acab4d.
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Abstract Singularity annihilation, generation, and evolving (SAGE) lead to the topological phase transition (TPT) in electronic, photonic and acoustic systems. Traditionally the singularity study of Hermitian systems is only focused on the real frequency domain (RFD). In this work, we systematically investigate the complicated SAGE in complex frequency domain (CFD) for 1D Hermitian and non-Hermitian systems and a more general picture is revealed. First, we study the abnormal phenomenon that one singularity evolves from the first band to the zero frequency and then into the pure imaginary frequency for Hermitian 1D PhCs. New results, e.g., the general condition for the singularity at zero frequency, the stricter definition of the Zak phase of first band and the phenomenon that more singularities are pushed from first band into the imaginary frequency, are found. Second, a general evolving picture of SAGE in CFD for Hermitian systems is constructed. Complicated processes of singularities in CFD are observed, such as the SAGE not only on the real frequency axis but also on the imaginary frequency axis, the closed evolving loops for singularities which connected imaginary-frequency axis and real-frequency axis. Third, when gain or absorption is introduced in, the SAGE on a tilted axis is also observed. The phenomenon of one singularity moving back to real frequency axis for non-Hermitian systems means that the stable states with resonance could be realized. Such complicated and general singularity evolving picture in CFD opens a new window for the studies of TPT and the rich new topological phenomena could be expected. Besides the theoretical importance, the evolution of singularity can also be used to engineer the band properties of PhCs. Some novel applications, such as the super-broadband sub-wavelength high-transmission layered structure and the broadband deep-sub-wavelength absorber, are proposed.
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Wang, Wenhui, Wenlong Gao, Leifeng Cao, Yuanjiang Xiang, and Shuang Zhang. "Photonic topological fermi nodal disk in non-Hermitian magnetic plasma." Light: Science & Applications 9, no. 1 (March 11, 2020). http://dx.doi.org/10.1038/s41377-020-0274-3.
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Li, Yandong, Chongxiao Fan, Xiaoyong Hu, Yutian Ao, Cuicui Lu, C. T. Chan, Dante M. Kennes, and Qihuang Gong. "Effective Hamiltonian for Photonic Topological Insulator with Non-Hermitian Domain Walls." Physical Review Letters 129, no. 5 (July 28, 2022). http://dx.doi.org/10.1103/physrevlett.129.053903.
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Zhao, Wen, Yanji Zheng, and Cuicui Lu. "Topological rainbow trapping based on non-Hermitian twisted piecing photonic crystals." Photonics Research, October 11, 2022. http://dx.doi.org/10.1364/prj.470354.
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