IMAGING OF SURFACE-PLASMON LAUNCH AND PROPAGATION USING A PHOTON SCANNING TUNNELING MICROSCOPE

The spectroscopic capability of the photon scanning tunneling microsco pe is exploited to study directly the launch and propagation of surfac e plasmons on thin silver films. Two input beams, of different wavelen gth, are incident through the prism in a prism-Ag film-air-fibre tip s ystem. Both excite surface plasmons at the Ag-air interface and light of both wavelengths is coupled into the fibre probe via the respective surface plasmon evanescent fields. One laser beam is used for instrum ent control. The second, or probe beam is tightly focused on the sampl e, within the area of the unfocused or control beam, giving a well-def ined and symmetrical, confined surface plasmon launch site. However, t he image at the probe wavelength is highly asymmetrical in section wit h an exponential tail extending beyond one side of the launch site. Th is demonstrates in a very direct fashion;the propagation of surface pl asmons; a propagation length of similar to 11.7 mu m is measured at a probe wavelength of 543.5 nm. On rough Ag films the excitation of loca lised scattering centres is also observed in addition to the launch of delocalised surface plasmons.