Twist Engineering of Photonic Crystal Cavities for Ultralow‐Threshold Continuous‐Wave WS ₂ Nanolasers at Room Temperature

ABSTRACT 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 SiN x 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 WS 2 with the twisted lattice SiN x nanocavity, we experimentally achieve a record‐low lasing threshold of 0.03 W/cm 2 at room temperature. Our work establishes a versatile platform for advanced two‐dimensional (2D) semiconductor light sources.