Lasing from Doubly Degenerate Bound States in the Continuum

This paper reports dual-mode vortex lasing with a topological charge of -2 via doubly degenerate bound states in the continuum. We patterned a photonic crystal slab with hexagonal TiO2 nanoparticles arranged in a hexagonal lattice to support doubly degenerate bound states in the continuum at the Gamma point, and the finite array was truncated to have a square boundary. Because emission from bound states in the continuum at the Gamma point is forbidden, lasing emerges at off-Gamma points, where the double degeneracy is lifted, resulting in two lasing peaks close in wavelength. The coherence time of vortex lasing is about 1.4 ps under pulsed pumping with a width of 75 fs. Polarization- and angle-resolved spectral measurements reveal that the two lasing peaks show orthogonal polarization states.