Near-field scanning optical microscope

Summary

Near-field scanning optical microscopy (NSOM) or scanning near-field optical microscopy (SNOM) is a microscopy technique for nanostructure investigation that breaks the far field resolution limit by exploiting the properties of evanescent waves. In SNOM, the excitation laser light is focused through an aperture with a diameter smaller than the excitation wavelength, resulting in an evanescent field (or near-field) on the far side of the aperture. When the sample is scanned at a small distance below the aperture, the optical resolution of transmitted or reflected light is limited only by the diameter of the aperture. In particular, lateral resolution of 6 nm and vertical resolution of 2–5 nm have been demonstrated. As in optical microscopy, the contrast mechanism can be easily adapted to study different properties, such as refractive index, chemical structure and local stress. Dynamic properties can also be studied at a sub-wavelength scale using this technique. NSOM/SNOM

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https://en.wikipedia.org/wiki/Near-field_scanning_optical_microscope

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This thesis studies apertureless Scanning Near Field Optical Microscopy, a technique that uses the apex of a very sharp tip to obtain local optical information with lateral resolution much beyond the diffraction limit. Both theoretical and experimental results are discussed. The theoretical work is a significant advance towards the quantitative convergence of experiments and theoretical predictions, and should be useful in aiding the interpretation of measured images. Extended tips and substrates are used, and the detector is also carefully modeled. A static tip in vacuum serves to study the influence of the tip and illumination geometry on the far fields and on the near fields in the proximity of the tip apex, the volume used to probe the sample. Including a gold substrate and the commonly used demodulation scheme allows to study the discrimination of the components carrying the local information. A very good discrimination is verified for silicon tips and small oscillation amplitudes, as far as the tip interacts closely with the substrate and the oscillation remains highly sinusoidal. The imaging process is studied by including patterned substrates. The obtained signal is mostly sensitive to a few nanometers of depth into the sample, and the influence of the scanning conditions on the level of signal, background suppression and lateral resolution is characterized. Further, a closer look into the behavior of the extended physical detector reveals the influence of the spatial inhomogeneities of the scattered fields and, for interferometric measurements, the large significance of the optical phase. Experimentally, different techniques are first described that can facilitate images with clear local information. A cross polarization scheme is introduced which is very useful for non-perturbative measurements. It is applied to the mapping of the the field distribution surrounding plasmonic structures, for both the phase and the amplitude. Beyond dipolar resonances, I also study coupled dipoles and quadrupole field distributions. When imaging artifacts are avoided, the obtained images closely resemble theoretical expectations.

EPFL2007

Scanning Near-Field Optical Microscopy of Living Cells in Liquid, Elaboration of New SNOM Probes and Detection Methods

Kanat Dukenbayev

The aim of the present PhD thesis is the elaboration of new, home-made Scanning Near-field Optical Microscope (SNOM or NSOM), and the demonstration of its great potential for high-resolution topography and fluorescence investigations of soft and solid samples, as well as the possibility to work in air and liquid environments showing high-resolution topographical and fluorescence abilities to investigate soft and solid samples. The first chapter starts from with the description of the general idea of SNOM and with a brief introduction into to the history and theory of optical image formation, and leading to the resolution limits in conventional optical microscopy. The following section will discuss about the different state-of-the-art different SNOM configurations and theirs peculiarities. The second chapter discusses the SNOM instrumentation and its operation in liquid. The central idea of the proposed approach is the application of the "joint recipient principle" method. Then we will describe the double-resonance principle of the SNOM sensor, and the preparation of glass optical fibers by chemical etching. In addition, the description of the important technical components (lasers, filter and so on) for the SNOM experiment concludes this chapter. The third chapter focus is on the time-gated pulse excitation and optical signal detection of SNOM images. The technical configuration of the SNOM is presented in more details. The fourth chapter starts from with a broad overview of living bacteria Escherichia coli (E.coli) and of the Green Synechococcus – type of Picocyanobacteria cell lines, and presents the distinctive advantages for of the SNOM investigations working in native physiological conditions. The following sections emphasize our obtained high-resolution experimental SNOM result measurements, then followed a discussion of the results follows. The sample preparation and deposition procedures of the living cells for the scanning will are also discussed in this chapter. The chapter five introduces our recently developed SNOM sensor, based on polymethylmethacrylate (PMMA) plastic optical fiber. The chemical preparation of sharp (>100 nm) tips, and its significant advantages will be illustrated by experimental results.

EPFL2011

Near-field optical probes provide subdiffraction-limited excitation areas for fluorescence correlation spectroscopy on membranes

Dusan Vobornik

Near-field optical probes have been used to produce a subdiffraction-limited ob- servation area for fluorescence correlation spectroscopy (FCS) experiments on supported membranes. The design of a bent, etched fiber probe that is compatible with biological im- aging in an aqueous environment is described. This probe design is used for proof of princi- ple experiments to measure lipid diffusion in a fluid-supported bilayer. A reduction in exci- tation area of approximately one order of magnitude (relative to a confocal FCS experiment) is obtained with a probe aperture diameter of 140 nm. We also demonstrate a simple ap- proach for modeling the autocorrelation decay due to diffusion within the excitation profile at the near-field scanning optical microscopy (NSOM) probe aperture. The use of probes with smaller apertures is expected to provide an additional order of magnitude reduction in the observation area, thus enabling the study of cellular membranes with higher concentra- tions of fluorophores than is currently possible with diffraction-limited techniques.

2009

Scanning Near-Field Optical Microscopy of Living Cells in Liquid, Elaboration of New SNOM Probes and Detection Methods

Kanat Dukenbayev

EPFL2011

EPFL2007