  The possibility for controlling the probe-field optical gain and absorption
switching and photon conversion by a surface-plasmon-polariton near field is
explored for a quantum dot above the surface of a metal. In contrast to the
linear response in the weak-coupling regime, the calculated spectra show an
induced optical gain and a triply-split spontaneous emission peak resulting
from the interference between the surface-plasmon field and the probe or
self-emitted light field in such a strongly-coupled nonlinear system. Our
result on the control of the mediated photon-photon interaction, very similar
to the `gate' control in an optical transistor, may be experimentally
observable and applied to ultra-fast intrachip/interchip optical interconnects,
improvement in the performance of fiber-optic communication networks and
developments of optical digital computers and quantum communications.
