Topological quantum cascade laser with Dirac-scaled valley edge modes.

Topological lasers, born from the fusion of topological physics and lasing theory, offer an unprecedented mechanism to achieve coherent lasing. By utilizing topologically protected boundary states, they exhibit robustness against defects and imperfections. However, this mechanism faces a fundamental limitation: while relying on the bulk of a photonic topological insulator for topological protection, the lasing modes, defined as boundary states, must be evanescently confined to the boundary, leading to severely limited mode volume. Here, using the platform of topological quantum cascade lasers previously demonstrated with valley edge states, we demonstrate a substantial expansion of the lasing mode volume under topological protection. Unlike conventional designs, we introduce an additional bulk region featuring gapless Dirac points at the interface between topologically distinct domains. This Dirac region enables the topological lasing mode to uniformly extend across the entire region, dramatically increasing the lasing mode volume. The scaled-up mode volume enhances the robustness of the laser against defects of comparable size, outperforming unscaled designs. Our results resolve a fundamental challenge in topological laser design and also contribute to the development of cutting-edge quantum cascade lasers with enhanced efficiency and robustness.