Lasing from Doubly Degenerate Bound States in the\nContinuum

This paper reports dual-mode vortex\nlasing with a topological charge\nof −2 via doubly degenerate bound states in the continuum.\nWe patterned a photonic crystal slab with hexagonal TiO<sub>2</sub> nanoparticles arranged in a hexagonal lattice to support doubly\ndegenerate bound states in the continuum at the Γ point, and\nthe finite array was truncated to have a square boundary. Because\nemission from bound states in the continuum at the Γ point is\nforbidden, lasing emerges at off-Γ points, where the double\ndegeneracy is lifted, resulting in two lasing peaks close in wavelength.\nThe coherence time of vortex lasing is about 1.4 ps under pulsed pumping\nwith a width of 75 fs. Polarization- and angle-resolved spectral measurements\nreveal that the two lasing peaks show orthogonal polarization states.