Sliver spherical nanoshells coated gain-assisted ellipsoidal silica core for low-threshold surface plasmon amplification

We have proposed an ellipsoidal gain-assisted silica core coated with a spherical Ag nanoshell for the stable low-threshold spaser generation. The results show that the surface plasmon can be amplified greatly by increasing the optical gain to a critical value (gain threshold). By varying the ellipticities of the ellipsoidal dielectric core nanoparticles (NPs), the gain threshold of the silica can drop 32.7% compared to that of a spherical core-shell particle. The physical mechanism of the lower gain threshold has been explained and discussed in detail by investigating the quality factor (QF) and the localized field distributions associated with the laser mode. Furthermore, we have also investigated the influence of the ellipticities of the gain-assist silica core on the level of gain threshold. With increasing the ellipticities of the silica core, the level of gain threshold decreases accordingly, and the corresponding super-resonance wavelength also shows a red shift.