Surface plasmon microcavity for resonant transmission through a slit in a gold film.

We demonstrate the use of a compact surface plasmon (SP) microcavity formed in a 300 nm thick gold film to resonantly enhance the optical transmission through a subwavelength slit. Focussed ion beam milling is used to create 200 nm deep SP microcavities, with widths between 800 nm to 1300 nm, each with a 180 nm slit in its center. The experimentally-measured TM polarized optical transmission has a wavelength-dependent peak that has similarities with finite-difference time-domain calculations in terms of peak-position and enhancement factors of peak transmission. The calculations show, by observing the near-field distributions, the interaction between the SP microcavity standing waves and the slit to create enhanced transmission. The SP microcavity demonstrated here is easily fabricated and may be optimized for future applications in surface-enhanced Raman scattering, nonlinear optics and surface plasmon resonance sensors.