Time-frequency analysis: a tool to discriminate artefacts from near-field optical data

Near-field optical data are non-stationary, which means that their spectral content caries with the position of the tip, due to both the scanning-prob e recording process and the variations of the optical signal. Therefore tim e-frequency representations are potentially powerful tools for local charac terization as they distribute the energy of the analysed signal over the ti me and frequency variables, and faithfully depict the signal local behaviou r. In this paper, the time-frequency distributions are shown to be appropri ate tools to analyse near-field optical data by using it first on simulated data, and second on experimental nearfield optical images. Within this con text, we observe that time-frequency analysis allows a possible separation of relevant optical signals from artefacts, especially in the usual case wh ere the near-field optical signal is lower band than the feedback data.