Optical near-field harmonic demodulation in apertureless microscopy

Spatial derivatives of the optical fields scattered by a surface can be investigated by apertureless near-field optical microscopy by modulating sinusoidally the probe to sample distance and detecting the optical signal at the first and higher harmonics. Demodulation up to the fifth harmonic order has been accomplished on a sample of close-packed latex spheres by means of the silicon tip of a scanning interference apertureless microscope. The working principles of such microscope are reviewed. The experimental configuration used comprises a tuning-fork-based tapping-mode atomic force microscope for the distance stabilization, and a double-modulation technique for complete separation of the topography tracking from the optical detection. Simple modelling provides first indications for the interpretation of experimental 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.