In contrast to an object moving along a straight line under no external force, a spinning object moves along a curved trajectory. This spin-orbit coupling indicates that a spin of an object is related to its trajectory. The spin-orbit coupling also exists in electrons in natural materials and has been considered as a fundamental ingredient in realizing three-dimensional (3D) topological insulators.

Korean researchers at Pohang University of Science and Technology (POSTECH) in collaboration with researchers in Nanyang Technological University report the experimental demonstration of a 3D photonic topological insulator that does not require the spin-orbit coupling. This study is reported in the Journal Nature Communications in June.

A main feature of a topological insulator is the topological surface states that appear at the boundaries of the insulator. However, as photonic system has no electronic spin states, 3D topological insulators realized in photonics exhibit topological surface states along an internal boundaries between two photonic systems, not on their external surface.

The research team led by Prof. Junsuk Rho proposed a 3D photonic topological insulator that supports the topological surface states on its external boundary by fully abolishing the spin-orbit coupling. The researchers designed and demonstrated the self-guided topological surface states and their quadratic dispersion in the microwave regime.

Prof. Rho said that “We demonstrated that 3D topological insulators can be realized in the absence of spin-orbit coupling. This work opens new routes to the topological manipulation of light at the outer surface and enables cladding-free photonic devices”.

[Reference] Minkyung Kim, Zihao Wang, Yihao Yang, Hau Tian Teo, Junsuk Rho, Baile Zhang, Nature Communications

[Main Author] Minkyung Kim (Pohang University of Science and Techology), Junsuk Rho (Pohang University of Science and Techology)

* Contact : Professor Junsuk Rho (jsrho@postech.ac.kr)