  Active core-shell nanoresonators were designed in order to achieve large
near-field enhancement, large power-outflow and minimal spaser threshold in the
pump E-field strength. Gain-metal-dielectric (GMD) and gain-metal-gain (GMG)
nanoresonator compositions were optimized with corresponding objective
functions. The average local E-field, power-outflow and extinction
cross-section were mapped above the pump E-field strength and dye concentration
parameter plane with the criterion that the local E-field is smaller than the
damage threshold of the nanoresonator. Regions, corresponding to the maxima in
the average local E-field, the highest power-outflow, or to the zero-crossing
of the extinction cross-section, were selected for detailed studies. The
spectral distribution of the near-field enhancement, optical cross-sections,
optical responses, quantum efficiencies, as well as the polar angle
distribution of the far-field radiated power and the local charge distribution
of dominant modes were inspected. Based on the results the GMD nanoresonator
composition is proposed to maximize local E-field in near-field amplifiers, to
maximize power-outflow in far-field-emitting lasers and to minimize threshold
E-field in spasers. Comparing the complete characteristics, both compositions
are suitable for different operation regions, the GMG is proposed as near-field
amplifier and far-field out-coupling nanolaser, whereas the GMD is
unambiguously preferable to achieve optimal spaser properties.
