A thin gold film (~50 nm) sits between a high-index prism and an analyte medium. At the plasmon resonance angle, light tunnels through the metal and excites a surface plasmon wave — dramatically reducing reflectance.
The resonance angle shifts when the analyte index changes — the basis of SPR biosensors, which can detect sub-nanomolar binding events without labels.
Drag the angle slider to scan through resonance. Notice the sharp dip in reflectance at θ_sp.