Dual-mode lasing with vortex and linear polarization from a shallowly etched photonic crystal

This paper reports how a two-dimensional photonic crystal slab of ${\mathrm{TiO}}_{2}$ covered with liquid dye gain can generate lasing with vortex and linearly polarized output beams concurrently. The photonic crystal slab is formed by shallowly etching a ${\mathrm{TiO}}_{2}$ film, which can support multiple band-edge modes within the gain bandwidth of the chosen dye. We identify the lasing band edges by mapping out the band structure from correlated transmission and emission measurements. The experimental results are in good agreement with the simulated band structure. The vortex lasing is connected to a symmetry-protected bound state in the continuum mode, while the linearly polarized lasing is connected to a doubly degenerate symmetric band-edge mode. The involved band-edge modes are arising from hybridized first-order slab waveguide modes. The quality factor and spatial mode-gain overlap are analyzed to understand mode discrimination and gain competition in this lasing device.