Accurate detection of resonance by monitoring wavepacket width

Surface Plasmon Resonances (SPR) are detected through the dip they provoke in the reflection coefficient of a light beam send in a prism coupler. The reflected beam has two components (i) the part that is reflected by the... [ view full abstract ]

Surface Plasmon Resonances (SPR) are detected through the dip they provoke in the reflection coefficient of a light beam send in a prism coupler. The reflected beam has two components (i) the part that is reflected by the prism-metal interface and (ii) the part coming from the surface plasmon itself, leaking back trough the prism. One would thus expect a distortion of the reflected beam, characterized at resonance by a lateral shift and a change in the beam width.

Studying this phenomenon, we came to the surprising conclusion that while the beam should indeed be larger off resonance, it is theoretically narrower exactly at resonance. At resonance, the beam reflected by the bottom of the prism out of phase with the leakage coming from the surface plasmon. When the incident beam is narrow, the two components do not overlap, which actually results in a lateral shift of the beam and in a larger reflected beam. But when the incident beam is large, the two components overlap and interfere destructively. This counter intuitive phenomenon can be difficult to detect when the beam is very large, but we have shown that in the right conditions the reflected beam can be 10% narrower at resonance, a feature easy to measure.

It is particularly interesting to notice that, even slightly off resonance, the beam is larger, so that the change in the beam width occurs on a very narrow angular range. Monitoring the change is the width of the reflected beam can thus allow to measure more precisely the position of the resonance, than monitoring the reflection coefficient - thus improving the resolution of the SPR device. This is all the more interesting as SPR apparatus are reaching their theoretical limits in terms of resolution. Using this technique could allow to overcome these problem.

We finally underline that this phenomenon occurs not only in the case of SPR. Our conclusions can be extended to wave packets in general, even temporal ones, and are not limited to plasmonics.