Dirac-buffer-layer valley hall photonic crystal cavities for group velocity engineering and power scaling with terahertz quantum cascade lasers

Abstract

Topological photonic crystals based on the valley Hall (VH) effect enable robust light confinement immune to fabrication disorders, which facilitate various applications for on-chip routing and topological lasers. However, the practical performance of electrically pumped topological lasers remains constrained by limited mode volumes, high injection currents, and vulnerability to specific defects. Targeting terahertz quantum cascade lasers (QCLs), we systematically simulate ABA'-type VH photonic crystal heterostructure cavities in which a gapless Dirac-point-degenerate lattice with different hole shapes is inserted at the interface between conventional VH domains. Through band-structure, edge-state, cavity eigenmode, and robustness analyses covering different D_Dirac (Dirac lattice number) and air-hole geometries, we establish valuable design rules for Dirac-buffer-layer VH cavities for QCLs. First, the Dirac buffer layer serves as a transverse mode filter that enables single-mode operation by increasing the contrast of the cavity-mode Q factors and reducing the number of high-Q modes within the bandgap. Second, the Dirac layer is helpful for power scaling, which enhances the robustness of the topological cavity and facilitates a more uniform mode field distribution, improving current injection efficiency. Most importantly, the geometry of the Dirac lattice is critical for group velocity engineering: triangular holes at D_Dirac = 8 realize an almost flat-band edge state in a large K space, while D_Dirac = 5 triangular holes cavity retains 94.8% of its pristine Q factor under deliberate defects. These results establish valuable design strategies for topological cavities in terahertz QCLs and can be extended to high-performance electrically pumped topological lasers at other wavelengths.