Plasmon lasers are a new class of coherent optical amplifiers that generate and sustain light well below its
diffraction limit [1-4]. Their intense, coherent and confined optical fields can enhance significantly light-matter
interactions and bring fundamentally new capabilities to bio-sensing, data storage, photolithography and optical
communications [5-11]. However, metallic plasmon laser cavities generally exhibit both high metal and radiation
losses, limiting the operation of plasmon lasers to cryogenic temperatures, where sufficient gain can be attained.
Here, we present room temperature semiconductor sub-diffraction limited laser by adopting total internal
reflection of surface plasmons to mitigate the radiation loss, while utilizing hybrid semiconductor-insulator-metal
nano-squares for strong confinement with low metal loss. High cavity quality factors, approaching 100, along with strong λ/20 mode confinement lead to enhancements of spontaneous emission rate by up to 18 times. By controlling the structural geometry we reduce the number of cavity modes to achieve single mode lasing.