Controllable deformations presented in periodic structures might result in the appearance of disclinations and set the new stages for the construction of higher-order topological insulators with varied distinct rotational symmetries, which were noticed until now only in linear regimes.
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To overcome this restriction, researchers explored the interplay of self-action effects and topology in the disclination structures with different discrete rotational symmetries and, for the first time, observed nonlinear topological dislocation states experimentally, where localization is regulated by their power.
Higher-order topological insulators are strange materials that can direct topologically protected states. Recently discovered disclination states also belong to the class of higher-order topological states but are restricted to the boundary of the disclination defect of the higher-order topological insulator and can be forecasted with the use of the bulk-disclination correspondence principle.
Until now, topological disclination states were noticed only in the linear regime, while the interplay among nonlinearity and topology in the systems with disclinations has never been experimentally examined.
In a new study published in Light Science & Application, a group of researchers, headed by Professor Yaroslav V. Kartashov from the Institute of Spectroscopy, Russian Academy of Sciences, Russia, and Professor Yiqi Zhang from Key Laboratory for Physical Electronics and Devices of the Ministry of Education & Shaanxi Key Lab of Information Photonic Technique, Xi’an Jiaotong University, China, have given reports on the first experimental observation of the nonlinear photonic disclination states in waveguide arrays with pentagonal or heptagonal disclination cores inscribed in transparent optical medium with the use of the fs-laser writing technique.
Using Gaussian input beams, nonlinear disclination states can be successfully excited when they are aimed into the waveguides that belong to the disclination core of the array. The input beam power regulates spatial localization. Edge and corner states in these structures with disclinations are also examined.
They are helpful for the improvement of nonlinear effects and for the realization of stable lasing—thanks to the compactness of disclination states. Moreover, in the design of different nonlinear topological functional devices, disclination lattices can be utilized. For instance, disclination lattices might be utilized for the realization of lasing in states with varied vorticity restricted by the structure’s discrete rotational symmetry. In addition, as such systems disclination states coexist with topological corner states, everyone can possibly notice switching between lasing in these two varied topological modes.
The disclination states that can be seen in aperiodic structures achieved owing to particular deformations of periodic arrays considerably vary from earlier considered higher-order insulator geometries, as systems with disclinations may have distinct rotational symmetries that are unsuited to crystalline symmetries and cannot be realized in regular higher-order insulators with periodic bulk.
The investigations on the interplay of nonlinear effects and topology in structures with disclinations and varied distinct rotational symmetries might set a new avenue for an in-depth study of the behavior and applications of higher-order topological states. The outcomes are summarized by researchers as follows:
“Nonlinear disclination states open new prospects for investigation of nonlinear effects in topological systems with disclinations and may inspire new ideas in developing compact optical functional devices.”
“The results are relevant for different areas of science, including Bose-Einstein and polariton condensates, where potentials with the disclinations can be created.”
Ren, B., et al. (2023). Observation of nonlinear disclination states. Light: Science & Applications. doi.org/10.1038/s41377-023-01235-x