Synthesis

While many applications exploit the spatial confinement and field enhancement in the vicinity of plasmonic metal nanostructures, much less attention has been given to the field enhancement that occurs inside the metal. One phenomenon that hinges on this effect is the photoemission of electrons from nanostructured metals after excitation by ultrashort laser pulses. Several studies have investigated such ultrafast photoemission from planar films,(4) islandised metal films(5) and metallic gratings.(6, 7) In each of these studies, plasmon modes associated with the metal surfaces have been suggested as being involved in the photoemission process. It is thought that the role of the plasmon is 2-fold: high electric fields inside the metal can be used to explain observed enhancements in photoemission after excitation on resonance with plasmon modes, while high field gradients are thought to induce charge acceleration, through the ponderomotive force,(8) giving rise to high energy photoelectrons.(9)he semicontinuous films are fabricated by evaporation of metal under vacuum onto glass substrates at a deposition rate of ∼0.1 nm s–1, at a pressure of <10–5 mbar. For low (<∼20 nm) film thicknesses the metal does not properly wet the substrate (glass) surface and so a continuous film does not form, instead surface tension leads to the formation of metal islands.(16) Sample morphology is characterized by scanning electron microscopy (SEM), and the linear optical response assessed by acquiring extinction spectra. Examples of extinction spectra and of SEM images are shown in Figure 1. A range of samples were made, using both silver and gold, and altering the mass of metal deposited. We