Loss-driven topological transitions in lasing

We experimentally observe lasing in a hexamer plasmonic lattice and find that when tuning the scale of the unit cell, the polarization winding of the emission changes. 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 quality(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.