Reconsidering the design of planar plasmonic lasers: gain, gap layers, and mode competition

TL;DR

This study investigates how design parameters like gain layer thickness and gap layer refractive index influence the lasing behavior of planar metallic plasmonic lasers, combining experiments with theoretical models to guide future device design.

Contribution

It provides a comprehensive experimental and theoretical analysis of planar plasmonic lasers, revealing the impact of gain layer thickness and gap index on lasing modes and mode competition.

Findings

Gain-layer thickness determines lasing mode nature.

High refractive index gap layers enhance plasmonic lasing.

Plasmonic and photonic modes can compete, enabling active mode switching.

Abstract

Because surface plasmons can be confined below the diffraction limit, metallic lasers that support plasmonic modes can provide miniaturized sources of electromagnetic waves. Such devices often exploit a multilayer design, in which a semiconductor gain layer is placed near a metallic interface with a gap layer in between. However, despite many experimental demonstrations, key considerations for these planar metallic lasers remain understudied, leading to incorrect conclusions about the optimal design. Here, we pursue a detailed experimental and theoretical study of planar metallic lasers to explore the effect of design parameters on the lasing behavior. We print semiconductor nanoplatelets as a gain layer of controllable thickness onto alumina-coated silver films with integrated planar Fabry-P\'erot 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 detailed rate-equation model, which consider both plasmonic and photonic modes. We show that the nature of the lasing mode is dictated by the gain-layer thickness. Moreover, by explicitly treating gain in our waveguide model, we find that, contrary to conventional wisdom, a gap layer with high refractive index is advantageous for plasmonic lasing. Additionally, our rate-equation model reveals a regime where plasmonic and photonic modes compete within the same device, raising the possibility of facile, active mode switching. These findings provide guidance for future designs of metallic lasers and could lead to on-chip lasers with controlled photonic and plasmonic output, switchable at high speeds.