  Nanotechnology has revolutionized our ability to control matter at the
nanoscale, with metallic nanostructures offering remarkable versatility.
Metallic nanoparticles (MNs) find compelling applications when metal losses are
balanced by optical gain elements, including visible-frequency metamaterial
cloaking and sensitive bio-molecular sensors. Prior studies of the
electromagnetic response of these nano-systems have focused on single spherical
MNs within infinite gain media, facing non-uniformity challenges beyond
emission thresholds. To overcome this issue, we propose transitioning to
spherical nanoshells incorporating gain elements, facilitating field
homogeneity and offering a comprehensive spectral description using a hybrid
quasi-static model. This system enables us to describe the electromagnetic
behavior of the MN below as well as above the emission threshold.
Interestingly, we found that the steady-state emission of a gain enhanced
nanoshell is not the single line emission predicted by other authors, but
instead it involves a range of frequencies whose span is governed by physical
and morphological parameters that can be chosen at the stage of synthesis to
design an optimal emitter.
