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Recently, optical bound states in continuum in various passive photoniccrystals have been identified and similar structures incorporated with opticalgain have been reported to exhibit lasing. However, no explicit control overthe type of lasing BIC has been reported. In this work, we utilize all fourfundamental BICs related to the lowest energy Gamma-point of a square photoniccrystal lattice. We identify the associated topological charges fromexperimentally obtained dispersions, finite element method simulations, as wellas from spherical decomposition method based on the microscopic polarizationcurrents in the photonic crystal plane. By tailoring the periodicity and thehole diameter of the photonic crystal slab, we selectively bring each of thefour BIC resonances to a wavelength regime, where fluorescent IR702 moleculesoverlaid with the photonic crystal provide sufficient gain for the onset oflasing. We experimentally analyze all four observed lasing BICs by imagingtheir far-field polarization vortices and their associated topological charges.The results correspond excellently with the transmission results as well as thesimulation results in the absence of gain. Finally, we experimentally present acase where the lasing signal reveals the coexistence of two BICs with oppositetopological charges, resulting in a unique polarization pattern. We believe ourresults enable tailoring the properties, such as polarization winding andtopological charge of BICs, by a priori design and thus pave the way for a moregeneral utilization of their appealing properties.

Topological charge engineering in lasing bound states in continuum