Twist Engineering of Photonic Crystal Cavities for Ultralow-Threshold Continuous-Wave WS2 Nanolasers at Room Temperature.

Owing to their dangling-bond-free surfaces and strong excitonic effects, monolayer transition-metal dichalcogenides (TMDs) hold promise for ultra-low-threshold heterogeneously integrated nanolasers. However, despite extensive demonstrations of TMD-based nanolasers, further reduction of the lasing threshold is hindered by the challenge of simultaneously achieving an ultra-small mode volume and an ultra-high quality (Q) factor in conventional optical cavities. Moreover, the dielectric interfaces of these cavities induce strong dielectric screening and defect-assisted nonradiative exciton recombination, both of which severely suppress exciton emission in monolayer TMDs. Here, we overcome these fundamental limitations by employing the air modes in a twisted lattice nanocavity. By twisting two finite-sized hexagonal photonic crystal structures in a single layer of SiNx thin film, we introduce a radial, quasi-continuous gradient in the air-filling fraction, thereby forming a radially graded bandgap. This graded bandgap acts as concentric mirrors that tightly confine Bloch modes at the K-point, yielding air modes with extreme field confinement in the air and ultra-high Q factors. By integrating a monolayer WS2 with the twisted lattice SiNx nanocavity, we experimentally achieve a record-low lasing threshold of 0.03 W/cm2 at room temperature. Our work establishes a versatile platform for advanced two-dimensional (2D) semiconductor light sources.