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D Barchiesi

Publications and source records attributed to D Barchiesi.

6 recordsLinked to original sources
PubMed2001

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 varies with the position of the tip, due to both the scanning-probe recording process and the variations of the optical signal. Therefore time-frequency representations are potentially powerful tools for local characterization as they distribute the energy of the analysed signal over the time and frequency variables, and faithfully depict the signal local behaviour. In this paper, the time-frequency distributions are shown to be appropriate tools to analyse near-field optical data by using it first on simulated data, and second on experimental near-field optical images. Within this context, we observe that time-frequency analysis allows a possible separation of relevant optical signals from artefacts, especially in the usual case where the near-field optical signal is lower band than the feedback data.

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PubMed2000

Optical near-field data analysis through time-frequency distributions: application to the characterization and separation of the image spectral content by reassignment.

The near-field optical images have been traditionally analyzed by Fourier analysis and, recently, by wavelet analysis. Those data are nonstationary, which means that their spectral content varies with time, owing to the scanning-probe recording process; therefore time-frequency representations are, potentially, powerful tools for local characteristics extraction or shape separation, since they distribute the energy of the analyzed signal over the time and frequency variables and faithfully depict the signal local behavior. In this study we show that Cohen's class time-frequency distributions and their modified version by the reassignment method are appropriate tools for the analysis of near-field optical data. We demonstrate this by using these tools first on simulated data and second on experimental near-field optical images. Within this context we observe that time-frequency analysis allows one to easily characterize local frequencies, which involves a possible separation of relevant optical signal from artifacts.

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PubMedDate not supplied

Spectroscopic study of the image formation in near-field microscopy, near an evanescent-homogeneous switching.

Near-field optical microscopes provide highly resolved images of various samples. However, images are difficult to interpret owing to their sensitivity to illumination conditions. Moreover, by contrast with classical microscopy, the near-field signal combines the contributions of evanescent and propagative modes. In this study, we present results of a spectroscopic study in near-field. Our purpose is to explain how a switching of one diffracted mode from homogeneous to evanescent can modify image formation. The main point is to establish a relation between the evanescence of one diffracted mode and the fringes that are often observed in near-field experimental images. Moreover, on a metallic sample, the possible occurrence of plasmon resonance contributes to image distortion in a mainly different way. We use a Fourier series Rayleigh 3D method to explain image formation.

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PubMedDate not supplied

Surface imaging in near-field optical microscopy by using the fluorescence decay rate: a theoretical study.

In this paper, we study the fluorescence decay rate of a molecule above a corrugated interface, and particularly the variations of the decay rate as a function of the lateral position of the molecule. As a first step, one has to determine the field diffracted by a corrugated interface when the incident field is the field emitted by a dipole. For this purpose, we have used a perturbative Rayleigh method, and we show that the decay rate variations can be connected to the surface profile via a transfer function. Some numerical calculations of this transfer function and of decay rate variation images are presented for dielectric and metallic samples. The visibility of the theoretical images is up to 20% and, moreover, resolution of the images is good enough to use the fluorescence lifetime of molecules as signal in a life-time scanning near-field optical microscope. The technical problems are discussed briefly.

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PubMedDate not supplied

Characterization of reflection scanning near-field optical microscopy and scanning tunnelling optical microscopy/photon scanning tunnelling microscopy working in preliminary approach constant height scanning mode.

The resolution in near-field images is currently determined by the visual inspection of recorded images. One of the major questions in near-field optical microscopy is 'what resolution can be reached, the tip-to-sample distance being known?' This knowledge is critical when choosing the scanning step and the distance between the tip and the sample, in a preliminary scan. This preliminary scan is often the only way to detect the interesting parts of the sample, with limited risk of tip crash and topographical artefacts. The method proposed here needs two scans of the same area, of the same sample, in constant height mode, recorded at two tip-to-sample distances. The pseudotransfer function is the ratio of the Fourier transform of these two data maps. This function enables the evaluation of the limit of resolution. Theoretical considerations are introduced to assess the method.

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