Surface Plasmon Resonance

Spatial interference of the specular (left) and conically scattered (right) light for a thin silver film on high index glass in the Kretschmann ATR configuration.

Since their introduction nearly 30 years ago, biosensors based on surface plasmon resonance (SPR) have become one of the most popular tools used interrogate bimolecular interactions. Despite this, and somewhat surprisingly, there are still many phenomena in this system which are not well understood.

In our research, we look at the influence of focussed beams on plasmon excitation using the Kreschmann attenuated total reflection (ATR) configuration. The effect of such a focussed beam is many-fold.

First, if one uses (for example) an incident Gaussian beam at the SPR resonance angle, the far field pattern is a familiar "notched Gaussian" -- where the notch is coincident with surface plasmon polarition excitation on the metal film. Curiously, this pattern is not static but evolves spatially with propagation into the far field. This effect is a spatial manifestation of the metal's casual response to incident light and can be understood directly with Fourier optics principles.

Portion of the SPR cone ring, showing both interference and speckle.

Second, focussed beams cause an increase in the amount light directionally emitted from the system. This directional emission is due to surface rougness modifiying the in-plane momentum of SPPs and the result is a hollow cone of light about (nearly) the SPR resonance angle. Embedded in this cone is information related to the fundamental mesoscopic transport mechanisms of surface plasmon polaritons themselves.

Here we study such phenomena in the context of highly sensitive SPP biosensing.