Detection of an evanescent field scattered by silicon tips in an apertureless scanning near-field optical microscope

We describe how a commercial silicon cantilever can be used both as a force probe and a local scatterer of light. This tip can thus be used as the key element of an apertureless Scanning Near-field Optical Microscope (SNOM) combined with an Atomic Force Microscope (AFM). We use a Total Internal Reflection (TIR) illumination to provide a Fresnel evanescent wave with well-defined characteristics. We show how the weak signal issued from the near-field scattered by the tip can be extracted from the background noise. Finally we present how a signal related to the near field can also be obtained when the tip is oscillated at a frequency close to the resonance frequency of the cantilever. These results are compared with a simulation of the lock-in detection. Finally we describe the role of the vibration amplitude on the signal.