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 (ΔGH). Unlike this scheme, we focused the incident beam to a small beam waist to induce enhancement in the angular GH shift (ΘGH). Although this makes ΔGH negligible, the enhancement of ΘGH is much larger than the decrease in ΔGH. 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 ΘGH 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 ΘGH toward sensing application when the Au film is exposed to local environmental changes even in the simplest thin film structure.