Resonance Enhanced Dynamic Light Scattering (REDLS) at interfaces

Bernhard Menges, Werner Steffen, Georg Fytas

Keywords: field enhancement, evanescent wave optics, surface plasmon polaritons, dynamics at interfaces, transport phenomena

The ability to study motions on the nanometer scale close to a surface is of paramount importance for the understanding of dynamic behaviour, e.g. particles or polymers at interfaces, transport phenomena to and from surfaces, thin films or membranes. Local structure and properties of material of and in the immediate vicinity of a solid surface often differ from those in the bulk state due to the physical and chemical influence of the interface. These effects extend up to fractions of a micrometer only. Observation of the near-interface region requires very sophisticated techniques to obtain information within such a thin layer.

Fig. 1: Scheme of SPR-PCS set-up for field enhanced dynamic light scattering

The combination of Surface Plasmon Resonance and Photon Correlation Spectroscopy (SPR-PCS) is a means to achieve this goal. With an adapted detection system and a modified SPR setup we accomplished for the first time to perform such an experiment.

While in the last years the evanescent wave generated by total internal reflection has attracted a great deal of interest as a probe with high spatial resolution, our approach is to utilize a surface plasmon as the incident electromagnetic field instead of the evanescent field generated from total internal reflection. The surface plasmon polariton delivers a much greater enhancement of the evanescent field. It thus opens the possibility to study the diffusion on surfaces at very low levels of laser power and e.g. low concentration of particles in solution.

Here we describe the first experimental realization of a dynamic light scattering experiment close to an interface with surface plasmon polaritons as source of the scattered light. As a proof of concept the diffusion of polystyrene-latex particles close to the metal surface was studied. The particle radii can be extracted from relaxation function g(q,t) within certain limits

Fig. 2: SPR-PCS data and fit of a 60 nm PS-Latex at a scattering vector q =0.019nm_-1. The inset shows the surface Plasmon reflectivity scan of this system. The PCS data was obtained at the minimum.

So far we achieved to probe particles with radius as small as 10 nm. The scattering volume in this experiment is limited to the space between the solid surface and 200 nm above.

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References:

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