Loss-driven topological transitions in lasing from quasi-BIC in plasmonic lattices

Plasmonic lattices of metal nanoparticles have emerged as an effective platform for strong light-matter coupling, lasing, and Bose-Einstein condensation. However, the full potential of complex unit cell structures has not been exploited. On the other hand, bound states in continuum (BICs) have attracted attention, as they provide topologically protected optical modes with diverging quality factors. We show that nanoparticle lattices with complex unit cells enable lasing in quasi-BIC modes with a exceedingly high quality Q factor. By combining theory with polarization-resolved measurements of the emission, we show that the lasing mode has a non-trivial polarization winding, which changes when tuning the scale of the unit cell. By a theoretical analysis we identify the lasing modes as quasi bound states in continuum (quasi-BICs) of topological charges of zero, one or two. A T-matrix simulation of the structure reveals that the mode Q factors depend on the scale of the unit cell, with highest-Q modes favored by lasing. The system thus shows a loss-driven transition between lasing in modes of trivial and high-order topological charge.