Near-field beam displacement at surface plasmon resonance
Résumé
A finite-size beam exciting a surface plasmon polariton (SPP) in a prism coupling configuration experiences an in-plane displacement that can be used for the characterization of plasmonic components by means of near-field optical microscopy. We first demonstrate experimentally the existence of this displacement by taking near-field images of finite-width metal strips. Next, the properties of this shift are analyzed in detail. We investigate the dynamic of the near-field shift for an incident Gaussian beam as a function of illumination conditions. For beams with a narrow spectrum, we propose a straightforward derivation showing that the displacement depends on the average angle of incidence according to a Lorentzian law characteristic of the SPP resonance. For smaller beams with typical sizes of an order or smaller than the plasmon damping distance, we give a heuristic expression relating the beam displacement to the amount of incident energy that can couple to the SPP mode. By using an analogy with tunneling experiments through a dielectric barrier, we demonstrate a direct-space analog of the Hartman effect. Finally we show that the beam displacement is a convenient parameter that can be used to optimize SPP mode excitation in plasmonic waveguiding geometries.