Near-field optics on silicon-electrolyte junctions

A technique allowing near-field photocurrent (PC) mapping of silicon surfaces in contact with an electrolyte is presented. The illumination source is an optical fibre tip with a 100-nm aperture. A shear force detection system controls the tip-sample distance while scanning the tip across the silicon-electrolyte interface. Topographic and PC images on SiO2/Si mesas both show 300 nm resolution. It is shown that this PC contrast is induced by the tip-topography interaction and hence the PC resolution is limited by the resolution of the topography. Indeed, PC mapping on topography-less patterned porous-silicon/silicon samples shows that the lateral resolution is only limited by the aperture size which is of the order of 100 nm.