OBSERVATION OF OPTICAL WAVE-GUIDES BY USING A PHOTON SCANNING TUNNELING MICROSCOPE

The Photon Scanning Tunneling Microscope (PSTM) is based on the frustration of total internal reflection by the apex of an optical fiber placed near the surface of reflection. It was so far been used to obtain topographic information. This article shows that it also allows one to reach local variations of the refraction index of the sample. The PSTM, like many local probes, is operated on a constant intensity level. In that cases a theoretical analysis based on a simple model shows that a change of the refraction index can actually be seen with the PSTM. We find thc corrugation height depends on the distance between the tip and the sample surface. The local refraction index of microguides diffused in glass has been studied experimentally with the PSTM. Relative variations in the order of 10(-3) have been observed. If the sample surface is not perfectly flat, images simultaneously display the topography and the local index changes of the sample. Spectroscopic images can be performed in order to separate the two contributions. This can be achieved by modulating the fiber to sample distance at high frequencies and by reading the corresponding intensity variations. We show that under specific conditions the intensity variations mainly depend on the refraction index.