VIRTUAL PHOTON SCATTERING AT SUBWAVELENGTH SIZED TIPS

Silicon AFM tips can profitably be used as optical sensors for near fi eld optical microscopy. In particular they are able to convert evanesc ent waves (also called virtual photons) into propagating ones which ar e conveniently guided in the tip and transferred to the air at the bac k of the cantilever. In this paper it is shown that virtual photons sc attered at the tip end are also converted into real ones which are obs erved in the far field. Contrary to the previous observations of simil ar conversion at STM metal tips the scattered emission at dielectric s ilicon tips does not have a dependence with distance of a full exponen tial decay but rather follows the more complex dependence of the ''die lectric capture''. This observation is consistent with Mie's theory of scattering of real photons by small particles. Experiments are perfor med with both a and p polarizations showing a regular dependence of th e scattered intensity with the incidence angle. The same experiment is also performed with metallized silicon tips showing a metal scatterin g behaviour. This results can help to achieve a better control of the tip position in the close range of distances in a Photon Scanning Tunn eling Microscopy (PSTM) experiment.