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Topological insulators are among this year's top achievements in photonics

ITMO University

Optics & Photonics News recognised a recent study on three-dimensional topological insulators as one of the most promising advances in photonics this year. These structures are capable of controlling light without any losses caused by absorption and material defects, which shows a great potential for applications in optical computers, communication networks, antennas and lasers.

Topological insulators are unique structures which conduct electric current along the surface only, while remaining insulating inside. A peculiar protected state forms at the surface where electrons in the current become stable to all external influences or material defects. The current thus runs through the material in one direction without reflection and loss.

Although scientists have been studying electronic topological insulators for several decades now, there were no such structures for electromagnetic waves until 2015. Physicists from the ITMO University, City University of New York and the Australian National University first managed to make a substantial progress in the research of compact all-dielectric topologically protected photonic structures, providing full and unprecedented control over light localisation. The progress on electromagnetic topological insulators promises major technological advances in the near future.

The research began with a one-dimensional structure, which was basically a chain of nanodisks of the subwavelength size. Topologically protected states at the edges of this structure made it possible to control the spatial distribution of the electromagnetic field at the nanoscale. This opened up the potential to transfer an optical signal with no reflection and defect-induced scattering in two-dimensions. Professor Alexander Khanikaev from the City University of New York suggested how it could be done theoretically. Prof. Khanikaev supervised experimental realization of this idea conducted by a team led by Alexey Slobozhanyuk, a PhD student of the ITMO University and the Australian National University.

Initially, the experimental structure was based on metallic metamaterials, yet it was only an initial success. "Although the signal propagated linearly avoiding reflection at acute angles, metals could not be used for optics due to strong absorption," says Alexey Slobozhanyuk. "Later we performed our first successful experiment with dielectric metamaterials. However, two-dimensional systems didn't provide a full control over light propagation in any direction. Now we know that this is possible even in three dimensions."

Further research confirmed the existence of three-dimensional photonic topological insulators with a considerable practical potential. This finding was published in Nature Photonics early this year. Such insulators may be used for stable optical communication lines where the signal can propagate not just linearly, but in different ways, avoiding reflection and energy loss. This can make Internet connection faster as well as serve as a basis for high-power optical computers and lasers. "We've generalized the formalism of solid-state physics and now use it in electromagnetic systems," explains the leader of the project Prof. Alexander Khanikaev. "Using three-dimensional insulators based on dielectric metamaterials we can make light waves behave in ways previously considered impossible."

The international team already started experimenting with three-dimensional insulators. Professor Yuri Kivshar, who is working at both the ITMO University and the Australian National University, says: "It is much more difficult to conduct experiments with a three-dimensional structure, but Alexey Slobozhanyuk with his young colleagues is working hard on this. This is the second time research involving Alexey is recognised by Optics & Photonics News. Special December issues of this journal traditionally highlight about 30 of the best works published in the field of optics and photonics during the year, selected by a specially appointed editorial team. This year, the editors stressed the importance of these results placing an image of a three-dimensional topological insulator on the magazine cover."

"Optics and Photonic News is a members' magazine of the Optical Society of America, and it is very prestigious among scientists," adds Alexey Slobozhanyuk. "When your work is recognized in such a way, you know how important and useful your efforts are".



"Topological Photonics Goes Three-Dimensional" Alexey Slobozhanyuk, Yuri S. Kivshar, Alexander B. Khanikaev. Optics & Photonic News Dec. 1, 2017

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