Abstract

We present a semi-classical analytic model for spherical core-shell surface plasmon lasers (spasers) and dielectric nanolasers. Within this model, we drop the widely used quasi-static approximation of the electromagnetic field in favor of fully electromagnetic Mie theory. This allows for precise incorporation of realistic gain relaxation rates that so far have been massively underestimated in spaser theory. Based on this, we obtain a quantitative understanding of the threshold characteristics that limit efficient spaser devices. Specifically, our model suggests how the threshold of spasers can be significantly reduced by introducing an emitter-free spacing layer between the gain medium and the metal core. We further apply our theory on dielectric nanospheres of high refractive index, e.g., silicon, that are nanoscopic in all spatial dimensions to propose a fully dielectric nanolaser complementary to the spaser. As for the spaser, the silicon nanoparticles act as passive cavities (without intrinsic gain) that are decorated with a thin film of organic gain media. As recently shown resonance frequencies of silicon nanoparticles can be tuned over the entire visible range and bright and dark modes can be addressed [1,2]. The small intrinsic losses in silicon yield relatively high quality factors and low non-radiative decay rates of emitters close to the cavity, both will lead to low thresholds. As we show in this work, the dielectric nanolaser exhibits certain advantages relative to spasers, such as reduced laser threshold, short switch-on times, size and design flexibility. The dielectric nanolaser is compatible with standard lithographic fabrication approaches and its relative simple design may allow for easy testing and realization of the concept. The silicon nanolaser might find applications in nanooptics and metamaterials as is can support so-called magnetic modes in contrast to spasers. Our model can be extended to complex plasmonic and dielectric nanostructures using the recently introduced concept of quasi-normal modes and generalized Purcell-factor [3,4]. Fig. 1 Left: Scheme of the model including a spherical silicon nanocavity that supports Mie-eigenmodes, e.g., a magnetic dipole mode as shown by the intensity plot in false color. Dipolar emitters, pictured by the arrows on the surface of the sphere act as gain-medium. Right: Laser-input-output curve (photons in cavity vs. pump rate) of a comparative study between core-shell spasers and silicon nanocavity based nanolasers.

© 2015 IEEE