Ultracompact nanolasers based on flat-band quasi-guided modes.

We propose an all-dielectric, self-confined cavity that supports a flat-band quasi-guided mode (QGM) to realize an ultracompact nanolaser. By setting a spacer layer between the grating and the waveguide layer, precise band engineering is achieved. Subsequently, introducing periodic perturbations folds the GM into the light cone, forming a flat-band QGM. Benefiting from the slow-light effect induced by the flat band and the inherently low radiation loss of the GM, both in-plane photon leakage and radiative loss are suppressed. Consequently, a cavity with a quality factor of approximately 2.4 × 103 is achieved within only 10 unit cells (5.7 λ). The flat-band QGM enables a single-mode C-band laser with a low threshold electric-field amplitude of 7.7 × 105 V/m, despite the minimal number of unit cells. Notably, the unique field distribution of the high-frequency flat-band mode also provides significant robustness against variations in grating depth. This flat-band-assisted design can be extended to other wavelength ranges, providing a path toward power-efficient, on-chip, and compact lasers.