Blue Lasing at Room Temperature Based on a Quasi-Bound State in the Continuum

We demonstrate room-temperature lasing in the blue spectral range, 440 to 460 nm, where optical gain is provided by InGaN quantum wells coupled to quasi-bound states in the continuum hosted by a monolithic GaN-based subwavelength grating. A low-refractive-index nanoporous GaN layer obtained by electrochemical etching, placed beneath the structure, suppresses the coupling of the optical modes to the substrate. The gratings are designed by theoretical modeling and fabricated with sub-10 nm precision using e-beam lithography and dry etching. The presence of quasi-bound states in the continuum is confirmed by angle-resolved reflectivity and observation of a polarization vortex. The structures ensure a low threshold (0.16 mJ cm^-2 pulse^-1), narrow spectral bandwidth (below 0.04 nm), and strongly linearly polarized lasing emission. The efficient tunability of the lasing wavelength is achieved by adjusting the grating geometry parameters. This work opens new opportunities for compact and tunable monolithic GaN-based photonic devices.