Angular Goos-Hänchen Shift Sensor Using a Gold Film Enhanced by Surface Plasmon Resonance

We demonstrate the surface plasmon resonance (SPR)-enhanced angular Goos-Hänchen (GH) shift. Typical SPR-enhanced GH shift measurements make use of loosely collimated beams, which enhances only the spatial GH shift (Δ). Unlike this scheme, we focused the incident beam to a small beam waist to induce enhancement in the angular GH shift (Θ). Although this makes Δ negligible, the enhancement of Θ is much larger than the decrease in Δ. In order to excite surface plasmons, we employ a Kretschmann configuration using a simple gold (Au) film on a substrate. We show that although the efficiency of surface plasmon excitation is decreased by the focused geometry, a significantly large Θ was induced. With the simultaneous measurement of reflectivity for SPR and the beam shift for the GH shift used in this work, we experimentally show the potential of measuring enhanced Θ toward sensing application when the Au film is exposed to local environmental changes even in the simplest thin film structure.