  We provide a numerical study showing that a bottom reflector is indispensable
to achieve unidirectional emission from a photonic-crystal (PhC) nanolaser.
First, we study a PhC slab nanocavity suspended over a flat mirror formed by a
dielectric or metal substrate. We find that the laser's vertical emission can
be enhanced by more than a factor of six compared with the device in the
absence of the mirror. Then, we study the situation where the PhC nanocavity is
in contact with a flat metal surface. The underlying metal substrate may serve
as both an electrical current pathway and a heat sink, which would help achieve
continuous-wave lasing operation at room-temperature. The design of the laser
emitting at 1.3 um reveals that relatively high cavity Q of over 1,000 is
achievable assuming room-temperature gold as a substrate. Furthermore,
linearly-polarized unidirectional vertical emission with the radiation
efficiency over 50 % can be achieved. Finally, we discuss how this hybrid
design relates to various plasmonic cavities and propose a useful quantitative
measure of the degree of the `plasmonic' character in a general metallic
nanocavity.
