Influence of tip modulation on image formation in scanning near-field optical microscopy

Modulation of the probe height in a scanning near-field optical microscope (SNOM) is a technique that is commonly used for both distance control and s eparation of the near-field signal from a background. Detection of higher h armonic modulated signals has also been used to obtain an improvement in re solution, the elimination of background, or artifacts in the signal. This a rticle presents a theoretical model for the effects induced in SNOM images by modulation of the probe. It is shown that probe modulation introduces a spatial filter into the image, generally suppressing propagating field comp onents and enhancing the strength of evanescent field components. A simple example of detection of a single evanescent field above a prism is studied in some detail, and a complicated dependence on modulation parameters and w aveform is shown. Some aspects of the application of this theory in a gener al experimental situation are discussed. Simulated images are displayed to explicitly show the effects of varying modulation amplitude with first and second harmonic detection. Finally, we discuss the suppression of backgroun d artifacts due to propagating fields through the use of higher harmonic de tection. (C) 2001 American Institute of Physics.