Study on unidirectional-emitting photonic-crystal lasers based on topological photonics

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In 2022 year, first, I continued to improve the theory about the unidirectional guided resonances (UGRs). I established a new topological interpretation of UGRs in parameter space, which is consistent with the conventional topological charge picture, but may better clarify the physical origins and dynamic evolution of UGRs. Comparing to conventional topological charges picture, with this new interpretation, it's convenient to construct UGRs in parameter space, and their interplay with other radiation singularities in parameter space, such as exceptional points (EPs) and conventional band degeneracy is also discussed. Based on these, I updated the paper pre-printed in arXiv, and now it has been accepted by Physical Review Letter (Phys. Rev. Lett. 130, 056401)Second, I found that, this topological physical interpretation of UGRs actually can also be applied to other amplitudes singularities in photonic crystals. For example, it can be used to describe the Bound states in continuum (BICs). Following the same strategy, I established a universal picture of BICs in whole momentum space. It is found that the definition of BICs can be extended even outside the lightcone, and all tunable BICs are actually resulted from the interband coupling of band inversion, including the accidental BICs and F-W BICs.