Topological photonics is a rapidly emerging field of research in which geometrical and topological ideas are exploited to design and control the behavior of light. Drawing inspiration from the discovery of the quantum Hall effects and topological insulators in condensed matter, recent advances have shown how to engineer analogous effects also for photons, leading to remarkable phenomena such as the robust unidirectional propagation of light, which hold great promise for applications. Thanks to the flexibility and diversity of photonics systems, this field is also opening up new opportunities to realize exotic topological models and to probe and exploit topological effects in new ways. This article reviews experimental and theoretical developments in topological photonics across a wide range of experimental platforms, including photonic crystals, waveguides, metamaterials, cavities, optomechanics, silicon photonics, and circuit QED. A discussion of how changing the dimensionality and symmetries of photonics systems has allowed for the realization of different topological phases is offered, and progress in understanding the interplay of topology with non-Hermitian effects, such as dissipation, is reviewed. As an exciting perspective, topological photonics can be combined with optical nonlinearities, leading toward new collective phenomena and novel strongly correlated states of light, such as an analog of the fractional quantum Hall effect.

Topological Photonics

Current experimental realizations of the quantum anomalous Hall phase in both electronic and photonic systems have been limited to a Chern number of one. In photonics, this corresponds to a single-mode one-way edge waveguide. Here, we predict quantum anomalous Hall phases in photonic crystals with large Chern numbers of 2, 3, and 4. These new topological phases were found by simultaneously gapping multiple Dirac and quadratic points. We demonstrate a continuously tunable power splitter as a possible application of multimode one-way waveguides. All our findings are readily realizable at microwave frequencies.

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Multimode One-Way Waveguides of Large Chern Numbers

We demonstrate that two-dimensional atomic emitter arrays with subwavelength spacing constitute topologically protected quantum optical systems where the photon propagation is robust against large imperfections while losses associated with free space emission are strongly suppressed. Breaking time-reversal symmetry with a magnetic field results in gapped photonic bands with nontrivial Chern numbers and topologically protected, long-lived edge states. Due to the inherent nonlinearity of constituent emitters, such systems provide a platform for exploring quantum optical analogs of interacting topological systems.

https://link.aps.org/accepted/10.1103/PhysRevLett.119.023603

Topological Quantum Optics in Two-Dimensional Atomic Arrays

There has recently been a surge of interest in the physics and applications of broadband ultraslow waves in nanoscale structures operating below the diffraction limit. They range from light waves or surface plasmons in nanoplasmonic devices to sound waves in acoustic-metamaterial waveguides, as well as fermions and phonon polaritons in graphene and van der Waals crystals and heterostructures. We review the underlying physics of these structures, which upend traditional wave-slowing approaches based on resonances or on periodic configurations above the diffraction limit. Light can now be tightly focused on the nanoscale at intensities up to ~1000 times larger than the output of incumbent near-field scanning optical microscopes, while exhibiting greatly boosted density of states and strong wave-matter interactions. We elucidate the general methodology by which broadband and, simultaneously, large wave decelerations, well below the diffraction limit, can be obtained in the above interdisciplinary fields. We also highlight a range of applications for renewable energy, biosensing, quantum optics, high-density magnetic data storage, and nanoscale chemical mapping.

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Ultraslow waves on the nanoscale

Topological photonics is an emerging field of research, which is inspired by the discovery of topological insulators. The introduction of topology endows traditional photonic systems with brand new properties, including unidirectional propagating edge states and robustness against impurities or defect without backscattering. With the further development of theoretical research on topological photonics, many new photonic devices have been proposed and experimentally demonstrated, showing its broad prospects in constructing high-performance integrated photonic chips. Topology, as a new research perspective in photonics, is expected to bring about remarkable changes to the field of integrated photonic devices and optical interconnection. This review summarizes the fundamental realization principles of topology in photonic systems, comparison of topology in photonics and electronics, applications of topological photonics in integrated photonic devices and related optical effects. Finally, a brief outlook on the challenges and future development direction in the pursuit of the application of topological photonics in integrated photonic devices is provided.

Applications of Topological Photonics in Integrated Photonic Devices

We demonstrate that one-way electromagnetic modes could be sustained by the edge of a gyromagnetic photonic crystal slab of triangular lattice under an external dc magnetic field. The applied magnetic field breaks the time-reversal symmetry of the three-dimensional system, and thus the original degeneracy point in k space, at which two dispersion surfaces intersect, is lifted, resulting in a photonic band gap below the light cone. At this band gap, the one-way mode is localized horizontally to the slab edge, while confined by the index contrast in the vertical direction.

One-way edge mode in a gyromagnetic photonic crystal slab

The concept of a “trapped rainbow” has generated considerable interest for optical data storage and processing. It aims to trap different frequency components of the wave packet at different positions permanently. However, all the previously proposed structures cannot truly achieve this effect, due to the difficulties in suppressing the reflection caused by strong intermodal coupling and distinguishing different frequency components simultaneously. In this article, we found a physical mechanism to achieve a truly “trapped rainbow” storage of electromagnetic wave. We utilize nonreciprocal waveguides under a tapered magnetic field to achieve this and such a trapping effect is stable even under fabrication disorders. We also observe hot spots and relatively long duration time of the trapped wave around critical positions through frequency domain and time domain simulations. The physical mechanism we found has a variety of potential applications ranging from wave harvesting and storage to nonlinearity enhancement.

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Truly trapped rainbow by utilizing nonreciprocal waveguides

We theoretically analyze surface magnetoplasmon modes in a compact circular cavity made of magneto-optical material under a static magnetic field. Such a cavity provides two different methods for the surface wave to circulate in a unidirectional manner around the cavity, which offers more freedom, both in the one-way direction and in the frequency range, for designing nonreciprocal photonic components. We also show the interaction between this one-way cavity and waveguides through the example of a circulator, which lays the fundamental groundwork for potential nonreciprocal devices.

One-way surface magnetoplasmon cavity and its application for nonreciprocal devices

A passive DC magnetic concentrator is designed with transformation optics (TO) and realized by meta-materials. The passive DC magnetic concentrator based on space compression transformation can greatly enhance the magnetic field in a free space region and can be used for e.g., improving the sensitivity of magnetic sensors and increasing the efficiency of wireless energy transmission. The magnetic property of the medium obtained by TO is extremely anisotropic. To solve this, we use magnetic metamaterials made of alternated high-permeability ferromagnetic (HPF) materials and high-temperature superconductor (HTS) materials. We optimize our structure by conducting simulations using the finite element method (FEM) and experimentally demonstrate a strong, 4.74-time enhancement of the DC magnetic field by our meta-material magnetic concentrator. We also demonstrate that a simplified structure with only HPF materials working at room temperature can still gives 3.84-time enhancement of the DC magnetic field. The experimental results are in good agreement with the numerical simulations based on FEM.

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Experimental Realization of Strong DC Magnetic Enhancement with Transformation Optics

The interaction between two (parallel) one-way waveguides formed by photonic crystals is investigated theoretically. It is shown that when the two waveguides support modes propagating in opposite directions, they can effectively interact with each other only within a narrow guiding region where their propagation constants are nearly zero. In this coupling window, the waveguides are contra-directionally coupled and the efficiency grows monotonously with the coupling length, reaching 100% as the coupling length is large enough. When the one-way waveguides support the modes propagating in the same direction, they may be efficiently coupled through the whole guiding regime, and their coupling exhibits the same behavior as the conventional uniform waveguides. The coupling between the one-way waveguides is first analyzed with the coupled-mode approximation and then verified by rigorous numerical simulation.

Kexin Liu

Papers

One-way edge mode in a gyromagnetic photonic crystal slab

Truly trapped rainbow by utilizing nonreciprocal waveguides

One-way surface magnetoplasmon cavity and its application for nonreciprocal devices

Experimental Realization of Strong DC Magnetic Enhancement with Transformation Optics

Interaction Between Two One-Way Waveguides