Optical near-field induced current microscopy

A scanning near-field optical microscope operating in the visible in illumination mode and using 80 nm tapered aluminum-coated optical fiber tips is used in order to investigate spatially resolved photogalvanic effects in semiconductors. The measured signal, referred to as Optical Near-Field Induced photocurrent (ONIC), is mapped as a function of the tip position on the sample surface. In this mode of operation, the sample is also the photodetector. ONIC microscopy is tested on artificially fabricated subwavelength-sized nanostructures with known photoconducting properties. Using ONIC microscopy, we have accurately and directly measured the diffusion lengths of minority carriers photogenerated in heavily n-doped GaAs crystals.