Plasmonics is the study of the coupling between photons and electron oscillations in conducting materials, such as metals, and has seen tremendous advancement in the last 30 years due to improvements in nanofabrication techniques. Structures supporting plasmons can give strong localised fields, guiding of light and a means to control light-matter interactions and thus ‘plasmonics’ research can span a broad range of topics. Our research considers a variety of aspects of nanoplasmonics as outlined below.

1.    Mesoscopic plasmon speckle

Surface plasmons polaritons (SPPs) are electromagnetic excitations, coupled to coherent oscillations of electrons, propagating along the interface between, typically, a metal and a dielectric. Perpendicular to the interface SPPs experience an exponential decay in their amplitude and are hence quasi-2D bound modes. Evanescent wave contributions can dominate in the near field and can give rise to large local intensities and endow SPPs with a high sensitivity to perturbations in the vicinity of the surface. Consequently SPPs have found myriad uses, such as bulk sensing, high resolution spectroscopy (e.g. SERS) and microscopy. As with all wave phenomena, propagating SPPs can undergo scattering by surface defects or adsorbed particles, a phenomenon frequently employed in design of nanoplasmonic devices. By virtue of their coherent nature, SPPs can also exhibit interesting interference effects, such as focusing, standing wave patterns and, in the case of many interfering SPPs, a near field plasmonic speckle pattern. Study of the properties and uses of conventional optical speckle, arising from interference of many randomly interfering optical waves, has burgeoned into a vast scientific field, with numerous applications, however its plasmonic analogue has hitherto been neglected. We hope to develop a fuller understanding of both the topological, polarisation and statistical properties of near field plasmonic speckle patterns. Beyond the fundamental insights this offers, mesoscopic systems operating in the multiple scattering regime have been shown to be extraordinarily sensitive to configurational changes, down to sub-wavelength displacements of a single scatterer. This is especially true in two dimensions, where coherent multiple scattering effects can emerge for even weak disorder. Mesoscopic plasmon speckle thus affords an interesting route to high sensitivity plasmonic sensors.

2.    Dynamic plasmon scattering

Fluctuations in plasmonic speckle patterns can arise when the scattering configuration undergoes some form of dynamic change. If plasmon scattering arises from proteins bound to a metal surface, fluctuations can originate, for instance, from further molecular binding, conformational changes, protein motion (molecular motors) and/or transient interactions. The nature of the resulting temporal variations is, however, governed by the size, shape and kinetics of molecules near the sensor surface. This project thus seeks to develop methods analogous to those employed in correlation spectroscopy, dynamic light scattering and diffusive wave spectroscopy, to extract key parameters of the underlying processes and constituent molecules.

3.    Localisation microscopy of plasmonics structures

Small metallic nanostructures can support localised plasmon oscillations. The properties of such plasmonic structures can be tailored through changing the shape of the system. Because of this, plasmonic structure design is a key technique in the manipulation of light at sub-wavelength scales, consequently it has much to offer related areas of physics and technology.  Whenever complex design is required quality characterisation techniques are needed to verify ideas and simulations with experimental reality. The very nature of the length scales involved make conventional, diffraction limited, microscopy unsuited for the study of the electric field distributions generated by plasmonic structures, which can exhibit features <10 nm in size.  To this end we must employ a super-resolution approach.  Localisation microscopy uses our ability to return the near-field position of a point source emitter to nanometre accuracy from knowledge of the point spread function generated in the far-field. Using fluorescent molecules as near-field probes and localising their position from the far-field, the structure of these plasmonic near-fields can be resolved. Applying this technique to nano-antenna systems we hope to gain a greater understanding of antenna design leading to the further advancement of plasmonics.

Contact for more information

Prof Peter Török

Dr Matthew Foreman