Guided polariton optics: A combined numerical, analytical, and experimental investigation of surface plasmon waveguides.

摘要

Metals possess unique optical properties distinct from the dielectric materials used today for integrated optics. The promise of "metal optics" is that these properties may one day be exploited to manipulate light at smaller length scales than feasible with dielectric structures. In this context, surface plasmon-polaritons (SPPs) have received considerable attention for their ability to guide electromagnetic energy at optical frequencies. Prior works on surface plasmon waveguides have highlighted their differences from conventional, diffraction-limited dielectric waveguides. It has been suggested that guided polariton modes are not diffraction limited, and specifically, that the surface plasmon modes supported by finite width metal stripes are inconsistent with a ray-optics interpretation of guided wave phenomena.; In contrast to previous studies, the work summarized in this thesis presents a physical interpretation for guided polariton optics that is consistent with conventional guided wave optics. While we demonstrate that the confinement of SPPs normal to a metal surface can produce deeply subwavelength optical modes, we prove that the lateral confinement for a surface plasmon mode along a finite interface is consistent with momentum conservation as described by physical optics.; Specifically, we have developed a numerical method to solve for the leaky and bounds modes of arbitrary geometry polariton waveguides. With this method, we demonstrate that published experimental results for the field profiles and propagation lengths of metal stripe waveguides are anticipated by their leaky modal solutions. Although the surface plasmon modes supported by stripes guide electromagnetic energy in three dimensions, we establish that such modes can be approximated by the solutions of two-dimensional dielectric slab waveguides. Leveraging this model, we suggest both an effective basis set for the guided polariton modes and an effective diffraction limit in the lateral dimension. To validate these findings, we have fabricated and characterized a variety of passive plasmonic devices using a photon scanning tunneling microscope. In good agreement with numerical simulations and analytical models, we present empirical evidence of guided polariton propagation, diffraction, and interference to support our physical interpretation.