  Spasers have been theoretically predicted and experimentally observed and
promise to deliver new exciting nanophotonic and biomedical applications. Here
we theoretically investigate ultrafast dynamical properties of spasers with
external plasmonic feedback. We consider a spaser both as a nanoscale source
and detector of plasmons which could be used to design novel nano-imaging and
sensing techniques. We show that, as with conventional lasers, spasers are
sensitive to external feedback. However, unlike the lasers, spasers have faster
relaxation dynamics which could be used to develop new ultrasensitive near
field imaging techniques. We investigate the dependence of spaser relaxation
oscillations on feedback parameters and show that quantum coherence can be used
to increase the sensitivity to feedback.
