As part of my Marie-Curie Action TEBLa, I am thrilled to investigate the role of topology in lasing effect of nanoscale photonic systems.

In our new letter Loss-driven topological transition in lasing just published on Physical Review Letters, we study the lasing action of plasmonic topological BIC modes of topological charge 0, 1 and 2. We evidence a topological transition in the lasing mode as the scale of the unit cell is varied, which is an unexplored method of investigating topological phases. Our experimental findings are corroborated by theoretical calculations with the T-matrix approach, which show that the energies of the modes cross repeatedly in the complex energy plane. In fact, not only the frequency of the modes, but also the losses are affected as the scale of the unit cell is varied. This change results in a different nature of the mode loss such that, close to the BIC condition, Ohmic losses get small while radiative losses are suppressed. The Q-factor of a specific mode increase, predicting the lasing mode transition in the different regimes.

The transitions we observe are distinct from the equilibrium case where the topological nature of the ground state is relevant; here, the losses determine the topological charge of the lasing mode. Our work shed the light on the interplay between losses and topological transitions, as well as the relationship between the radiation topology and the conventional topological band theory, which are critical problems in the very lively avenue of research in non-Hermitian and out-of-equilibrium topological photonics.

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