Low-contrast grating induced high-Q modes of laser cavity

Bound states in the continuum (BICs) remain localized even though they coexist with a continuous spectrum of radiating waves that can carry energy away. These modes can be almost perfectly localized in the structure, making lasers working at BIC or quasi-BIC have an ultrahigh quality factor (Q) and hence low threshold. Lowcontrast gratings (LCGs) have better mode selectivity than high-contrast gratings and promise higher single-mode output power for LCG-based vertical-cavity surface-emitting lasers. A quasi-BIC (i.e. supercavity mode) with a Q factor of 9.2 × 105 is obtained in the LCG, and a simplified three-layer slot laser with a Q factor of 9.66 × 106 is constructed. Further, a law of using the period of a grating to control resonant wavelength and using etched depth and width to control Q factor can be used for designing a high-Q structure at a specified wavelength. The calculated Q factor is optimized systematically by changing various parameters, and the highest Q factor obtained reaches 2.81 × 107 . The results of all these analyses are instructive to the design of grating-based low threshold electrically injected surface-emitting lasers or other high-Q devices.