Surface plasmon random scattering and related phenomena
Schumann, Robert Paul
Surface plasmon polaritons (SPPs) are collective electron excitations with attendant electromagnetic fields which propagate on a metal-dielectric interface. They behave, in many ways, as model two-dimensional electromagnetic waves. However, because the evanescent field of the SPPs extends a short distance outside the interface, a near-field probe can modify the wave propagation. We use this behavior to study both SPP scattering within the plane of the interface and also the transition to free-space propagation out of the plane. We have, in particular, studied the multiple scattering of SPPs excited on rough silver films. Our laboratory possesses apertureless near-field scanning optical microscopes (A-NSOMs), the probes of which can act as an in-plane scatterer of SPPs. Subsequent momentum-conserving decays of the SPPs generate an expanding hollow cone of light to which information about the direction and phase of the SPPs on the surface is transferred. A focus of our studies has been SPP multiple scattering when one of the scatterers (the tip) can move. This problem is very closely related to a similar problem in mesoscopic electronic transport, involving "universal conductance fluctuations". It is also related to various radar-detection, microwave communications and medical imaging problems. In parallel with actual experimental measurements, we have also conducted extensive Monte Carlo simulations of the scattering. Multiple scattering leads to the appearance and detection of "speckle" in the far field. A speckle field, however, is more properly considered in terms of its embedded optical vortices and so we have used holographic techniques to study these. We have demonstrated that vortices can be manipulated, created and destroyed by movement of the STM probe tip. Optical vortices are an example of the effect of "geometric" or "topological" phase in physics and as such link the trajectory of a parameter in one space to the phase observed in another. In our case, the trajectory of the A-NSOM tip parallel to the sample surface plane generates topological phase in the far field, manifestations of which are vortices.