  Active nanoplasmonics has recently led to the emergence of many promising
applications. One of them is spaser (surface plasmons amplification by
stimulated emission of radiation) that has been shown to generate coherent and
intense fields of selected surface plasmon modes that are strongly localized in
the nanoscale. We propose a novel nanospaser composed of metal
nanoparticles-graphene nanodisks hybrid plasmonic system as its resonator and a
quantum dots cascade stack as its gain medium. We derive the plasmonic fields
induced by pulsed excitation through the use of the effective medium theory.
Based on the density matrix approach and by solving the Lindblad quantum master
equation, we get ultrafast dynamics of the spaser associated with coherent
amplified plasmonic fields. The intensity of the latter is significantly
affected by the width of the metallic contact and the time duration of the
laser pulse used to launch the surface plasmons. The proposed nanospaser
operates in the mid-infrared region that has received much attention due to its
wide biomedical, chemical and telecommunication applications.
