Time-frequency analysis of near-field optical data for extracting local attributes

Near-field microscopy has been developed to characterize optical properties of materials below the diffraction limit. It consists of scanning a probe, which can be of atomic dimensions, a few nanometers above a material surface, and detecting electromagnetic interaction. The resulting near-field optical images are conventionally analyzed by means of Fourier-based methods although these data are nonstationary. This observation suggests that time frequency analysis is potentially a powerful tool for extracting attributes such as local resolution of near-field optical microscopes. We use bilinear time-frequency distributions and their optimized version by the AOK procedure to analyze experimental near-field optical and magneto-optical raw images. We show that this approach allows local characterization of optical resolution and separation of relevant optical information from artifacts caused by the scanning probe recording process.