Role of Gain in Fabry–Pérot Surface Plasmon Polariton Lasers

Abstract

Plasmonic lasers generate strongly confined electromagnetic fields over a narrow range of wavelengths. This is potentially useful for enhancing nonlinear effects, sensing chemical species, and providing on-chip sources of plasmons. By placing a semiconductor gain layer near a metallic interface with a gap layer in between, plasmonic lasers have been demonstrated. However, the role of gain in this common design has been understudied, leading to suboptimal choices. Here, we examine planar metallic lasers and explore the effect of gain on the lasing behavior. We print semiconductor nanoplatelets as a gain layer of controllable thickness onto alumina-coated silver films with integrated planar Fabry–Pérot cavities. Lasing behavior is then monitored with spectrally and polarization-resolved far-field imaging. The results are compared with a theoretical waveguide model and a rate-equation model, which consider both plasmonic and photonic modes and explicitly include losses and gain. We find that the nature of the lasing mode is dictated by the gain-layer thickness and, contrary to conventional wisdom, a gap layer with a high refractive index can be advantageous for plasmonic lasing in planar Fabry–Pérot cavities. Our rate-equation model also reveals a regime where plasmonic and photonic modes compete in an unintuitive way, potentially useful for facile, active mode switching. These results can guide future design of metallic lasers and could lead to on-chip lasers with controlled photonic and plasmonic output.