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

Abstract

We demonstrate the surface plasmon resonance (SPR)-enhanced angular Goos−Hanchen (GH) shift. Typical SPRenhanced GH shift measurements make use of loosely collimated beams, which enhances only the spatial GH shift (Δ GH ). Unlike this scheme, we focused the incident beam to a small beam waist to induce enhancement in the angular GH shift (Θ GH ). Although this makes Δ GH negligible, the enhancement of Θ GH is much larger than the decrease in Δ GH . 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 GH 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 GH toward sensing application when the Au film is exposed to local environmental changes even in the simplest thin film structure.

Molecular Monolayer Sensing Using Surface Plasmon Resonance and Angular Goos-Hänchen Shift

“…In our previous work, in Ref. [ 15 ], we used a relatively rough Au film surface prepared by radio frequency (RF) sputtering with substrate heating. Rough Au film would increase the surface area, hence, increase the number of adsorbed molecules, which is preferred for sensing applications.…”

“…As such, Au film was prepared by EB evaporation without substrate heating giving a relatively smoother surface morphology, as shown in Figure 5 a. The root mean square (RMS) roughness of the EB evaporation fabricated film is 0.84 nm—much smoother than the RF sputtered film with RMS roughness 5.67 nm [ 15 ]. The surface morphology is slightly changed with the formation of BT-SAM, as shown in Figure 5 b, with an RMS roughness of 1.2 nm.…”

“…The basic system configuration can be found elsewhere [ 15 ]. In brief, we used a linearly polarized helium-neon (HeNe, = 632.8 nm) laser.…”

“…However, it breaks down for beams focused to a small beam waist since nonlinear phase terms can no longer be ignored for such cases [ 4 , 25 , 26 ]. A more accurate calculation of can be made by getting the location of the beam centroid at the focus plane where, for appreciably small beam waists, becomes negligible [ 4 , 15 , 25 ].…”

“…Previously, in Ref. [ 15 ], we demonstrated SPR-enhanced -dominated GH shift measurement as a potential RI sensor using gold (Au) substrate. However, the surface property of Au has been met with controversy as to whether Au surface is hydrophobic or hydrophilic.…”