Near field optical behaviour of C supertips

Carbon supertips are classically grown at the end of silicon AFM probes by electron beam induced deposition in an SEM. These tips are used as PSTM converters of an evanescent wave at 1.06 mu m on a semiconductor surface. It is shown that the classical conversion model is obeyed. The coupling effective length "a" of these tips is shown to lie in the range of 100-200 nm and the variation of a is studied as a function of the conical angle of the supertip. Experimental data conveniently agree with the model of the "virtual" photons developed by Hori. From these results it can be concluded that to be resolutive supertips require a very sharp end (small radius) but essentially also a large conical angle, in the first 200 nm of the tip. This latter conclusion could be surprising but it can be justified by the shorter interaction length for the largest conical angles. (C) 1999 Elsevier Science B.V. All rights reserved.