  We theoretically propose directional, low-threshold plasmon lasing in both
the near-infrared and visible wavelengths by utilizing spatially coherent
surface plasmon polaritons on a meta-surface. The gain strength required for
threshold lasing can be tuned down to a large extent through compatible
structural parameters. Our calculations show that no more than 65 cm$^{-1}$ at
193.5 THz (1.55 $\mu$m) or 267 cm$^{-1}$at 474THz (0.633 $\mu$m) of gain
coefficient is sufficient to compensate for the dissipation of metal films for
threshold lasing; these values are smaller than any reported studies at the
same frequencies. These findings present a planar solid-state route for plasmon
lasing that is highly efficient and spatially coherent.
