Understanding plasmonic lasers: Fabrication, characterization, and design of plasmonic Fabry–Pérot lasers

Abstract

Plasmonic lasers are the plasmonic analog to conventional lasers. They coherently amplify surface plasmon polaritons (shortened to surface plasmons) instead of photons. Surface plasmons are electromagnetic waves that propagate at the interface between a metal and a dielectric. The free electrons in the metal contribute to this wave giving it partial electronic character. This allows surface plasmons to be confined to much smaller volumes than the minimum size of conventional light as dictated by the diffraction limit. Therefore, plasmonic lasers can be much smaller than their photonic counterparts. Moreover, they provide sources of coherent surface plasmons that can be used to feed optical circuitry. Consequently, optical components can be miniaturized, rendering them more efficient and cost-effective. In addition, due to their high field intensity, coherently amplified surface plasmons can be used to improve sensors and applications that rely on nonlinear effects. In this thesis, we thoroughly study the fabrication, characterization, and design of plasmonic lasers, enabling an in-depth understanding of these devices. First, we experimentally investigate a plasmonic laser that is based on a metallic cavity inside which a gain medium is deposited. The open-cavity design allows us to characterize the lasing behavior. We find that the thickness of the gain medium largely determines whether the metallic cavity lases in the plasmonic or photonic modes. A theoretical model gives insight into the underlying physics of these findings and allows us to make predictions for improved laser designs. Second, we examine the confinement factor, a metric for describing how geometrical aspects of waveguides that include a gain medium influence the amplification of waveguide modes. This discussion is particularly important, as ambiguous interpretations of the confinement factor are common in the literature, hindering optimization of optical gain in waveguides. We clarify these ambiguities and provide the necessary understanding to correctly employ the confinement factor for optimization of designs in nanophotonics and plasmonics. Third, we optimize the geometry of plasmonic Fabry–Pérot lasers to minimize their threshold gain. A plasmonic laser combines a lossy metal and a medium that exhibits gain. By tailoring the geometry of the waveguide inside the Fabry–Pérot cavity, the material contributions to the amplification of the waveguide mode can be tuned. Therefore, the gain required to reach the lasing threshold can be minimized by clever design choices. We find that the magnitude of the reflection losses significantly influences the optimal geometry and identify suitable design guidelines. In summary, this thesis provides an in-depth understanding of various aspects of plasmonic lasers. Besides the experimental study that involves fabrication and characterization of plasmonic lasers, it also gives physical insights through theoretical models. This knowledge can be used to improve the design of plasmonic lasers for practical applications.