Cavity-Defined van der Waals Plasmonic Nanolasers Enabled by Out-Of-Plane Dipole Coupling.

Plasmonic nanolasers enable deeply subwavelength coherent light sources but are often limited by rigid cavity designs and demanding gain conditions. Here, a cavity-defined plasmonic nanolaser platform is realized through van der Waals integration of layered Indium Selenide (InSe) gain media with lithographically engineered Au cavities separated by an ultrathin hexagonal boron nitride (hBN) spacer. Owing to the direct bandgap and predominantly out-of-plane (OP) emission dipole of InSe, efficient coupling to surface plasmon polariton (SPP) modes is achieved. Clear near-infrared (NIR) lasing characteristics, including a well-defined threshold, pronounced linewidth narrowing, and a drastic reduction of carrier lifetime, are observed under optical pumping. A plasmonic lasing regime below the optical-mode cutoff is identified, with lasing sustained for InSe thicknesses down to 79 nm, well beyond the limit of conventional photonic-mode lasers. Spatially resolved spectroscopy and numerical simulations reveal that the lasing feedback is governed by cavity-defined SPP modes. In addition, deterministic control of lasing behavior is achieved by engineering Au cavities with diverse geometries, establishing a versatile and general plasmonic nanolaser architecture.