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Helical Edge States in Parity-time Symmetric Photonic Crystals
Speaker Prof. Ruey-Lin Chern, National Taiwan University
Date 18 March 2019 (Monday)
Time 11:00 to 12:30
Venue Room 4334 (Lift 3), HKUST

We investigate the helical edge states in parity-time (PT) symmetric photonic crystals, with the balanced gain and loss on a triangular lattice with C6v symmetry. A two-level non-Hermitian model that incorporates the gain and loss in the tight-binding approximation was employed to describe the dispersion of the PT symmetric system. In the unbroken PT phase, the topological property is determined by the Hermitian part of the effective Hamiltonian, which is in a similar form as the Bernevig-Hughes-Zhang model. The helical edge states with real eigenvalues can exist in the common band gap for two topologically distinct lattices. In the broken PT phase, the non-Hermitian perturbation deforms the dispersion by merging the frequency bands into complex conjugate pairs and forming the exceptional contours that feature the PT phase transition. In this situation, the band gap closes and the edge states are mixed with the bulk states.