Optical near-field harmonic demodulation in apertureless microscopy

Spatial derivatives of the optical fields scattered by a surface can be inv estigated by apertureless near-field optical microscopy by modulating sinus oidally the probe to sample distance and detecting the optical signal at th e first and higher harmonics. Demodulation up to the fifth harmonic order h as been accomplished on a sample of close-packed latex spheres by means of the silicon tip of a scanning interference apertureless microscope. The wor king principles of such microscope are reviewed. The experimental configura tion used comprises a tuning-fork-based tapping-mode atomic force microscop e for the distance stabilization, and a double-modulation technique for com plete separation of the topography tracking from the optical detection. Sim ple modelling provides first indications for the interpretation of experime ntal data. The technique described here provides either artefact-free near- field optical imaging, or detailed information on the structure of the near fields scattered by a surface.