Surface plasmon propagation on thin metal films: theory and experiments

Surface plasmon-polaritons (SPPs) – electromagnetic waves propagating on the boundary between a metal and a dielectric and tightly bound to the interface – have been known for years. Recent progresses in microfabrication methods, near-field characterization techniques, and numerical methods have boosted the research interest in this field. New materials and fabrication techniques at the micro- and nano-scale have opened up new possibilities for SPP-based signal transmission in metal waveguides and the utilization SPPs for sensing applications. However, many fundamental phenomena related to SPP remain unclear or are not fully understood. This thesis addresses some of these fundamental questions related to SPP propagation and scattering. It utilizes modeling, microfabrication and near-field microscopy to further our understanding of SPP in a variety of physical systems. Different types of SPP waveguides are fabricated, including tapered waveguides and strip waveguides with interruptions. Propagation of SPP on these waveguides is studied with a heterodyne photon scanning tunneling microscope (PSTM), which resolves not only the intensity, but also the amplitude and phase information, and provides the full description of SPP waves. Effects such as SPP propagation, focusing, and interaction with an interruption in the waveguide are studied this way. Numerical methods are used extensively to study SPP propagation over irregular profiles, a situation which cannot be treated analytically. The influence of surface roughness on the propagation of SPP on corrugated thin metal films is studied in detail at the two wavelengths of λ = 785nm and λ = 1500nm. In many cases, the roughness reduces the propagation length, which is quite intuitive. However, an interesting situation where the roughness can increase the SPP propagation length is found for SPP propagating in the near-infrared. This phenomenon is explained with an effective medium theory. The resonant tunneling or SPPs over an interruption in the plasmonic waveguide is discovered for symmetrical and non-symmetrical geometries and its physical origin explained in details. Extremely long tunneling lengths are observed for the symmetrical geometry. Furthermore, the reflection and interaction of SPPs with the edge of the metal film are investigated numerically for a corrugated edge, which corresponds to a realistic experimental situation. This issue has raised some controversy in the scientific literature. It is found that surface irregularities at the metal edge can indeed change the reflectivity of the SPP modes; however, the reflectivity remains in the order of a few percents. Finally, the concept of long-range SPP mode cutoff is introduced and its application to realize optical switches is discussed. A practical implementation based on photoaddressable polymers is discussed, together with alternative routes that rely on V-groove plasmonic waveguides.


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