Special Issue on the ‘Topological photonics and beyond: novel concepts and recent advances’
In the past decade, research on topological photonics has grown exponentially. Tremendous progress has been made in implementing topological phases of light using different material platforms such as metamaterials, photonic crystals, waveguide arrays and coupled cavities. Exemplary successes include demonstrations of protected edge states and corner states and the discovery of non-Hermitian topological phases. During the summer of 2019, two international workshops on related topics were held in Tianjin, China and Daejeon, Korea, where pioneers in the topological photonics field brainstormed together and gave birth to this special issue - Topological photonics and beyond: novel concepts and recent advances.
This Special Issue is co-Editored by Prof. Zhigang Chen from Nankai University and San Francisco State University, Prof. Hrvoje Buljan from University of Zagreb and Nankai University, and Prof. Daniel Leykam from Institute for Basic Science.
In total, we have 20 published papers listed below,
Hao Lin and Ling Lu have done numerical calculations to reveal how optical fibers composed of topological photonic crystals could allow improved versatility and control over the modes and polarizations of the light they transmit. The authors base their novel method to transmit pure “single mode” light, over a large frequency range, using a topological feature known as a “Dirac-vortex”. This theoretical work suggests that fibers could be made from silica using existing stack-and-draw or 3-D printing technologies.
Shuqi Chen and his colleagues have shown that the surfaces of a sonic crystal can conduct sound by supporting three-dimensional (3D) Dirac points – four-fold degenerate band crossings that possess topological charge. The team used 3D printing to build a sonic crystal from hexagonal unit cells, linked by tube structures that enable hopping between acoustic ‘atoms’. When chiral hopping is introduced, it allowed Dirac points to split into so-called Weyl points, enabling the transition of exotic surface states and interface states.
Mordechai Segev and his colleagues studied theoretically and experimentally what happens when waves are incident at the interface between two photonic systems made from the same material, with the only thing making them different being their artificial gauge fields. The team formulated the generalized laws of refraction and reflection at such “gauge interfaces”, and demonstrated the concepts with micro-printed waveguides arrays with different tilt angles. The research demonstrates that several interfaces between regions with differing gauge fields could be used to develop novel photonic devices.
C.T. Chan, Yuntian Chen and their colleagues studied a specific type of layered photonic crystals with properties that vary in different directions. They identified unique arrangements of the energy bands, called nexus points, around which new optical phenomena can emerge. Their work offers new insights into the hidden symmetry of Maxwell’s equations, which describe the propagation of light.
Yi Yang, Bo Zhen, John D. Joannopoulos and Marin Soljačić, have studied two non-Abelian generalizations of the Hofstadter model, which describes the behavior of two-dimensional non-interacting electrons in a non-Abelian magnetic field. They proved the genuine non-Abelian conditions and analyze the topological properties of the models. This work could lead to a new way of manipulating light in photonic devices.
Alexander Cerjan and his colleagues studied adiabatic pumping, also known as Thouless pumping, in a one-dimensional array of laser-written evanescently-coupled waveguides in borosilicate glass. Nearly quantized topological transport of light was observed, even in the presence of disorder which was introduced by a variation in the spacing between neighboring waveguides. The adiabatic pumping was achieved by dynamically modulating the refractive index of the waveguide array (where temporal modulation was mapped to spatial variations).
Daohong Song, Zhigang Chen, Hrvoje Buljan and their colleagues proposed a laser-written Su-Schrieffer-Heeger lattice in a nonlinear optical crystal of SBN:61, and demonstrated the coupling of light into a non-trivial interface state, which is otherwise not accessible due to topological protection. The team further developed a general theoretical framework for interpreting the mode-coupling dynamics in nonlinear topological systems.
Yuanjiang Xiang, Tie Jun Cui, Shuang Zhang and their colleagues used electrical components to mimic the atoms in higher-order topological insulators, which feature zero-dimensional corner states topologically protected by three anticommuting reflection symmetries of the bulk lattice.
Henning Schomerus, Matthieu Bellec, Fabrice Mortessagne and their colleagues demonstrated how the application of a deformation in a photonic analogue of graphene results in the formation of pseudo-Landau levels. The work shows that the material’s edge and bulk properties and sub-lattice polarisation are directly linked and provides a unifying principle that connects a wide variety of topological phenomena.
Omar Jamadi, Alberto Amo and their colleagues reported the realization of a synthetic magnetic field for photons and polaritons in a honeycomb lattice of coupled semiconductor micropillars. A strong synthetic field is induced in both the s and p orbital bands by engineering a uniaxial hopping gradient in the lattice, giving rise to the formation of Landau levels at the Dirac points. The team provided direct evidence of the sublattice symmetry breaking of the lowest-order Landau level wavefunction, and opened a novel way of engineering propagating edge states in photonic lattices.
Moti Segev and his colleagues found theoretically that photonic topological insulators can also exist in fractal lattices, comprising only edges—with no bulk at all, breaking the current knowledge that all topological insulators have a bulk surrounded by edges, such fractal lattices could be potentially fabricated by femtosecond laser writing technology.
Yuri Kivshar, Hong-Gyu Park and their colleagues created a triangle-shaped cavity with room-temperature lasing behavior governed by the band topology, which brings new opportunities for integrated nanophotonics.
Vladimir Shalaev, Alexandra Boltasseva and their colleagues developed a plasmonic-enhanced graphene photodetector with a 25-fold increase in photocurrent generation compared to conventional graphene devices. Further enhancement can be achieved by applying a direct current bias, which paves the way towards compact, ultra-broadband and ultrafast photodetectors.
Shanhui Fan and his colleagues designed high-dimensional topological insulators made from arrays of ring-shaped resonators that each pair of rings acts like a ‘photonic molecule’, generating complex isolated quantum states that could be switched on and off. This work provides extra dimensions to topological insulators and could enable novel quantum states to be manipulated.
Andrey Sukhorukov, Kai Wang and his colleagues developed a framework for multidimensional lattices wrapped in a tube using light travelling in a single nonlinear waveguide. Further, they experimentally demonstrated the synthetic dimensions in a nonlinear fibre by implementing light interactions with precisely tailored optical pumps. This work allows deeper exploration of topological effects and shows potential practical applications in quantum communication and information processing.
Zhengyou Li, Liping Ye and his colleagues reported an experimental observation of Dirac points that are enforced completely by the crystal symmetry. This nonsymmorphic three-dimensional phononic crystal hosts four spiral topological surface states, in which the surface states of opposite helicities intersect gaplessly along certain momentum lines. This work opens new opportunities for elusive (pseudo) studying and acoustic applications.
Nader Engheta and Victor Pacheco-Peña developed an approach called “temporal aiming”, by which that beam-steering of electromagnetic waves can be accomplished by temporally changing the permittivity of metamaterials between isotropic to anisotropic values.
Junsunk Rho and his colleagues reviewed the recent progress in 2D, 3D and higher-order topological photonics and the efforts towards practical applications for topological photonics.
Yidong Chong, Baile Zhang and their colleagues investigated how disorder impacts the formation of topological edge states. The team experimentally observed the bulk mobility gap and the unidirectional propagation of topological edge states, which is robust against defects and disorder. By gradually deforming the amorphous lattice into a liquid-like lattice through the glass transition, they observed the closing of the mobility gap and the disappearance of the topological edge states, which illustrate the key role of short-range order in the formation of topological edge states.
