Recently, a research team led by Professor Hui Liu from the School of Physics at Nanjing University and Professor Meng Xiao from the School of Physics at Wuhan University has proposed a novel mechanism of generating topological degeneracies by twisting between two metasurfaces. Generally, degeneracies are protected by certain symmetries of the system considered, once these symmetries are broken, degeneracies would be lifted, resulting in gapped phases. However, this work observed an opposite mechanism in a twisted bilayer photonic lattice, in which two bands are gapped in a mirror-symmetric situation, while degeneracies (band touching) emerge if the mirror symmetry is broken by a twist. Furthermore, the position of the degeneracies is mediated by the twist angle, forming a nodal chain structure in the synthetic dimension consist of twist angle and two inplane wave vectors. Above results are all examined both in theory and in experiment. The twisted lattice supports birefringent waves as the degeneracies considered as the optic axis, thus providing potential applications in integrated photonics. This work was published in the journal Physical Review Letters[Phys. Rev. Lett. 134, 106601 (2025)], entitled ‘Topological degeneracy induced by twisting’.

Publication link: http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.134.106601