Cathodoluminescence plasmon microscopy

Abstract

Surface plasmon polaritons (SPPs) are electromagnetic waves that are strongly coupled to the collective oscillation of free electrons at an interface between a dielectric and a metal. Strong confinement of the electromagnetic field and tunability of SPP dispersion allow two-dimensional optics. This thesis focuses on acquiring fundamental understanding of the generation and confinement of SPPs using electron beam irradiation. SPPs are generated using the focused electron beam of a scanning electron microscope. The electron beam acts as point source for circular SPP waves. The resulting emission is detected using a cathodoluminescence (CL) spectroscopy setup The CL emission in front of a grating patterned in an otherwise planar gold surface shows oscillations with distance from the grating. These oscillations are caused by the coherent interaction of transition radiation and SPPs scattered by the grating, which leads to interference in the far field. We present a detailed theoretical analysis that successfully explains the measured CL signal. A connection between the CL signal and the photonic local density of states associated to SPPs is established. The SPP damping was determined by measuring the decay of the CL intensity on a line scan perpendicular to gratings fabricated into the surface. We find that the propagation length for single-crystalline gold is in agreement with calculations based on dielectric constants while for poly-crystalline films the propagation length is reduced. Scattering of SPPs at grain boundaries is identified as additional loss mechanism. We have numerically studied the reflectivity for SPPs of single grooves structured into the surface. Using FDTD calculations we find that the reflectivity shows resonances that are related to localized groove modes. The groove reflectivity is the result of coupling of the incident plasmon wave to the localized modes that then reradiate to cause a reflected plasmon wave. Two parallel grooves structured into a single-crystalline gold surface by focused ion beam milling act as SPP Fabry-Perot resonators. SPPs excited between the grooves are reflected and form standing waves. The plasmonic cavity modes are spatially and spectrally resolved. The observed cavity quality factor depends strongly on groove depth as expected from the depth dependent reflectivity of the grooves. Two-dimensional confinement was observed for SPPs that are confined to rectangular plateaus bounded by grooves. We use CL spectroscopy to image the SPP modes which show pronounced maxima of high emission with the position depending on wavelength and plateau size. The measurements are reproduced using a two-dimensional image source model for the local density of states. Finally, we use CL spectroscopy to excite metal-insulator-metal (MIM) plasmons. Two parallel slits in the MIM stack act as Fabry-Perot resonators for MIM plasmons. CL measurements show that the excited MIM plasmon modes form standing waves in the cavities. From the observed spatial mode pattern we derive the MIM plasmon wavevector which agrees well with expected values from analytical dispersion relation calculations. We also present measurements of disc resonators bounded by circular slits. The observed MIM plasmon modes are confined to mode volumes as small as 0.58\lambda^3.