  Nano-scale lasers harnessing metallic plasmons hold promise across physical
sciences and industrial applications. Plasmons are categorized as surface
plasmon polaritons (SPP) and localized surface plasmons (LSP). While SPP has
gained popularity for nano-lasers by fitting a few cycles of SPP waves into
resonators, achieving LSP lasing in single nanoparticles remains an elusive
goal. Here, we highlight the equivalence of LSP and SPP within resonant systems
and present lasers oscillating in the lowest-order LSP or, equivalently,
half-cycle SPP. This diffraction-limited dipolar emitter is realized through
strong coupling of plasmonic oscillation in gold and dielectric resonance in
high-gain III-V semiconductor in the near infrared away from surface plasmon
frequencies. The resulting single-mode stimulated emission peak exhibits
linewidth Q factors over 50 at room temperature, with wide tunability spanning
from 1190 to 1460 nm determined by resonator sizes ranging from 190 to 280 nm.
A semiconductor laser model elucidates the temporal and spectral buildup
dynamics under optical pumping. Notably, linewidth Q values surpassing 250 are
attained from higher-order, isolated laser particles within live biological
cells. These results offer fresh perspectives in nanophotonics and indicate
promising opportunities for multiplexed biological applications.
