Ultralow-Threshold Lasing in a Nanobeam Cavity Using Topological Interface State

Achieving ultralow-threshold topological lasing remains an important challenge for integrated photonics. We report an ultralow-threshold nanolaser based on a topological Su-Schrieffer-Heeger (SSH) nanobeam interface state with embedded semiconductor quantum dots. By removing the central hole at the SSH interface, the topological cavity mode is preserved, while the quality factor is improved and the small mode volume is maintained. The optimized field profile also enhances cavity-gain coupling, increasing the high-field fraction inside the semiconductor by ∼20%. Experimentally, optimized-interface nanobeam devices exhibit single-mode continuous-wave lasing with a threshold of approximately 640 nW. Under pulsed excitation, single-mode lasing is also observed with a peak incident pump threshold of approximately 62 μW. Power-dependent photon-correlation measurements reveal pronounced near-threshold bunching followed by gradual suppression at higher pump powers, indicating delayed coherence buildup above the threshold.