Digital Detection of Optical Signals in a Near-Optical-Field Microscope

— A digital electronic system for recovering an optical signal in an apertureless scanning near-optical-field microscope (ASNOM), which operates in the elastic scattering mode, is presented. A photodetector signal results from optical homodyning of light, for which a Michelson interferometer scheme is used. Oscillations of the probe tip at its resonance frequency (30–300 kHz), which initiate a periodic change in the tip–surface distance, are used as the modulating effect in the ASNOM. The amplitude of one or several higher harmonics of the tip oscillation frequency is used as the useful signal in the photocurrent spectrum. The additional phase modulation of homodyning light is provided by a periodic change in the length of the reference arm in the Michelson scheme, which allows one to reliably measure not only the amplitude of a near-field optical signal but also its phase. Digitization of the detector photocurrent signal directly at the input of the detecting system made it possible to perform complex algorithms for extracting useful signals. The modularity of the data-processing algorithms provides flexible signal-detection variants, as well as the reliable modification and adjustment of individual parts of an algorithm. The features of the instrument that generate measurement errors are considered.