  Plasmonic nanolasers have received a substantial interest for their promising
applications in integrated photonics, optical sensing, and biomedical imaging.
To date, a room-temperature plasmonic nanolaser, submicron in all dimensions,
remains elusive in the visible regime due to high metallic losses. Here, we
demonstrate single-particle lasing around 2.3 eV with full-submicron, cesium
lead bromide perovskite (CsPbBr3) crystals atop polymer-coated gold substrates
at room temperature. With a large number (~100) of devices in total, we
systematically study the lasing action of plasmonic test and photonic control
groups. The achieved smallest plasmonic laser was 0.56 micrometer x 0.58
micrometer x 0.32 micrometer in size, ten-fold smaller than that of our
smallest photonic laser. Key elements to efficient plasmonic lasing are
identified as enhanced optical gain by the Purcell effect, long carrier
diffusivity, a large spontaneous emission factor, and a high group index. Our
results shed light on three-dimensional miniaturization of plasmonic lasers.
