  Because surface plasmons can be confined below the diffraction limit,
metallic lasers that support plasmonic modes can provide miniaturized sources
of electromagnetic waves. Such devices often exploit a multilayer design, in
which a semiconductor gain layer is placed near a metallic interface with a gap
layer in between. However, despite many experimental demonstrations, key
considerations for these planar metallic lasers remain understudied, leading to
incorrect conclusions about the optimal design. Here, we pursue a detailed
experimental and theoretical study of planar metallic lasers to explore the
effect of design parameters on the lasing behavior. We print semiconductor
nanoplatelets as a gain layer of controllable thickness onto alumina-coated
silver films with integrated planar Fabry-P\'erot cavities. Lasing behavior is
then monitored with spectrally and polarization-resolved far-field imaging. The
results are compared with a theoretical waveguide model and a detailed
rate-equation model, which consider both plasmonic and photonic modes. We show
that the nature of the lasing mode is dictated by the gain-layer thickness.
Moreover, by explicitly treating gain in our waveguide model, we find that,
contrary to conventional wisdom, a gap layer with high refractive index is
advantageous for plasmonic lasing. Additionally, our rate-equation model
reveals a regime where plasmonic and photonic modes compete within the same
device, raising the possibility of facile, active mode switching. These
findings provide guidance for future designs of metallic lasers and could lead
to on-chip lasers with controlled photonic and plasmonic output, switchable at
high speeds.
