image: When a sphere transforms into a torus, the geometrical parameters continuously change whereas topological parameters undergo an abrupt and discontinuous change. This definition of topological equivalence explains why a physical phenomenon that is characterized by topology does not change under continuous deformations view more
Credit: by Minkyung Kim, Zubin Jacob, and Junsuk Rho
Topology, which is a branch of mathematics, is rising as a universal notion in physics, from condensed matter physics to classical wave systems. Topology has been received particularly warmly in photonics and has spawned a new field called topological photonics. Over the past decade, topological photonics has undergone surprisingly rapid development. Researchers in topological photonics have realized physical phenomena that have only been predicted so far in other fields and have even discovered new phenomena for the first time ever.
In a new review article published in Light: Science & Applications, a team of scientists, led by Professor Junsuk Rho from POSTECH, South Korea have reviewed the recent progress of topological photonics. In the review article, they focus on three-dimensional photonic topological phases and the latest findings that have yet to be reviewed. Starting by emphasizing the importance the implementation of topology in photonics, they provide the general concepts of topological band theory and two-dimensional phases and introduce the numerous approaches to realizing 3D photonic topological phases using photonic crystals and metamaterials. The recently emerging fields including, but not limited to, layer pseudospin, Weyl degeneracies with nonzero dimension, Maxwellian spin waves, and higher-order topological photonics are all reviewed. Finally, their perspectives of the outlook of topological photonics are given.
The team describe the power of topology as:
"A physical phenomenon that is characterized by topology is not affected by any continuous deformation. Then light can flow through a topological device by even if there are defects or impurities."
"The merger of topology and photonics is mutually beneficial to each other. Photonics serves as a concrete platform to test and realize theories of topological physics, and topology enables robust control of electromagnetic waves even in imperfect devices. The advances in topological photonics may realize some fascinating applications that have been impossible in conventional photonics such as lossless waveguides." They added.
"As in the past decade, the advances of topological photonics will continue. The scope of the topological photonics is diversifying, covering nonlinearity, non-Hermiticity, and higher-dimensions and will be further broadened by being combined with interdisciplinary fields." The scientists forecast.
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