  Recent years have witnessed growing interest in the development of
small-footprint lasers for potential applications in small-volume sensing and
on-chip optical communications. Surface-plasmons, electromagnetic modes
evanescently confined to metal-dielectric interfaces, offer an effective route
to achieving lasing at nanometer-scale dimensions when resonantly amplified in
contact with a gain-medium. Here, we achieve visible frequency ultra-narrow
linewidth lasing at room-temperature by leveraging surface plasmons propagating
in an open Fabry-Perot cavity formed by a flat metal surface coated with a
subwavelength-thick layer of optically-pumped gain medium and orthogonally
bound by a pair of flat metal sidewalls. Low perturbation
transmission-configuration sampling of the lasing plasmon mode is achieved via
an evanescently coupled recessed nanoslit, opening the way to
high-figure-of-merit refractive-index sensing of analytes interacting with the
open cavity.
