Fabry-Perot resonators for surface plasmon polaritons probed by cathodoluminescence

Surface plasmon polariton Fabry-Perot resonators were made in single-crystalline gold by focused ion beam milling of two parallel 100 nm deep grooves. The plasmonic cavity modes were spatially and spectrally resolved using cathodoluminescence spectroscopy. Mode numbers up to n=10 were observed. The cavity quality factor Q depends strongly on groove depth; the highest Q=21 was found for groove depth of 100 nm at lambda=690 nm. The data are consistent with finite-difference time domain calculations that show that the wavelength of maximum reflectivity is strongly correlated with groove depth.