Observation of a nanometer size confined transient phenomenon at the gold STM tip interface under UV illumination

In the development of a hybrid STM-SNOM apparatus, we studied the influence of the illumination on the tip-sample system. Classically, the only effect measurable should be a thermal expansion and a nano-sized mechanical displacement due to a tiny radiation pressure on the tip. However, we observed different kind of perturbations that we had difficulties to explain. We demonstrated that the phenomenon appears to be strongly related to the surface properties of the sample, so we believe that it is not a direct effect of the thermal expansion, nor mechanical movement induced by the irradiation. We describe here the nature of the phenomenon examined, we give details on the experimental condition and setup parameters and we attempt to rigorously discuss the result obtained showing relation to an electro-capacitive coupling of the system at the surface interface. We believe this observation may shed light on the proximity processes that occurs under the STM probe in the presence of light and improve the fundamental understanding of nano-sized interfaces.