Cavity quantum electrodynamic analysis of spasing in nanospherical dimers

We present a detailed cavity quantum electrodynamic model of a nanospherical dimer and use it to analyze bright hybrid spasing modes there. Using an approximate numerical scheme, we model the complete spasing system as an open quantum system under the Lindblad dissipator formalism. We show that while in general the longitudinal dimer setups display higher intensity spasing as compared with transverse dimers, the latter actually consistently lead to output with higher coherence. Intriguingly, and somewhat counterintuitively, we find that transverse dimers only reach peak output at an intermediate dimer separation at which field confinement is not the strongest. We show that this is due to low radiative decay rates of transverse dimers with significant dimer gaps. We also find that transverse dimers outperform longitudinal dimers in terms of output intensity in a weakly pumped sparse gain medium made of dimers with relatively large separations. Moreover, in all the configurations considered, we find that the second-order coherence of the spasing output shows a peaked behavior just before the threshold, suggesting that the coherence is a useful indicator of spasing. Even though the scheme we describe is focused on dimers, owing to the generic form of the analysis presented, it can be easily extended to investigate spasing in the bright modes of multiple coupled plasmon sources.