  Semiconductor nanowire lasers can be subject to modifications of their lasing
threshold resulting from a variation of their environment. A promising choice
is to use metallic substrates to gain access to low-volume
Surface-Plasmon-Polariton (SPP) modes. We introduce a simple, yet
quantitatively precise model that can serve to describe mode competition in
nanowire lasers on metallic substrates. We show that an aluminum substrate can
decrease the lasing threshold for ZnO nanowire lasers while for a silver
substrate, the threshold increases compared with a dielectric substrate.
Generalizing from these findings, we make predictions describing the
interaction between planar metals and semiconductor nanowires, which allow to
guide future improvements of highly-integrated laser sources.
