Gems in nanoscience

Abstract

In 1902 R M Wood published the paper 'On a remarkable case of uneven distribution of light in a diffraction grating spectrum' [1]. As was true of so much of his work, interest in his observations took flight, inspiring extensive research into associated new optical phenomena. What is now known as Wood's anomaly has been described as the result of excitations of oscillations in the conducting electron plasma, or 'plasmons'. These quasiparticles have become increasingly attractive to researchers, perhaps less for the dazzling colours they impart to glitzy gemstones over their potential to facilitate medical imaging, as well as integrated optics [2] and the transfer of information and energy at dimensions below the diffraction limit.Excitation of surface plasmons provides a means of enhancing optical near fields, thus empowering a range of signal detecting applications. A range of innovative techniques have been implemented to probe surface plasmon resonances. The evolution of plasmon resonance energy and mean free path as particle dimensions increase from nanostructures to bulk matter has been monitored through ellipsometry, thus providing an insight into the plasmon polariton coupling [3]. Electron energy loss experiments have also proved a valuable tool for mapping surface plasmons with a spatial resolution an order of magnitude better than can be achieved with scanning near-field optical microscopes [4].The exploitation of surface plasmons is primed to aid advances in medical imaging, diagnosis and therapy. Researchers in the US have developed a protein-enabled strategy to fabricate quantum dot nanoarrays where an increase in surface-plasmon-enhanced fluorescence of up to a factor of 15 has been achieved [5]. Understanding and dexterity in manipulating these enhancements has reached a high level of sophistication, and researchers in London have demonstrated the ability to increase the fluorescence enhancement by a factor of 4 and the decay rate by a factor of almost 30 by tuning the localised surface plasmon resonance of silver particle arrays to the emission wavelength of a locally situated fluorophore [6]. In the US and Belarus researchers have collaborated to investigate the potential of plasmonic nanobubbles, generated by laser activated nanoparticles, for combined diagnostics, therapy, and therapy guidance. Such nanobubbles can be optically tracked in the body, and their rapid expansion and collapse provides a localised mechanical impact on cells that can disrupt the cell membrane [7].Surface plasmon polaritons at nanostructures allow highly local control of light, which has a range of uses in electronic devices. Photovoltaics researchers in the US and the Netherlands have demonstrated enhanced short circuit current densities compared to cells having flat or randomly textured back contacts using nanostructured plasmonic back contacts, which maximise the interaction with the light [8].Progress in optoelectronics is another area where surface plasmon polaritons are playing an increasingly important role [9]. In this issue, researchers at the Institut des Sciences Molé culaire d'Orsay demonstrate that it is possible to excite propagating surface plasmon polaritons with a scanning tunnelling microscope, and detect them [2]. Their work also investigates the nature of the excited plasmons and how the intensities of the propagating surface plasmon polaritons and the localized plasmon emission can be enhanced by factors of 2 and 20 respectively by using a silver tip instead of a tungsten one. Wisdom may be worth more than silver and gold, but a little silver and gold can contribute a lot to unearthing new wisdom at the nanoscale.