Utilizing microwaves, with a wavelength of 3 centimeters, passing through a opticzl aperture of 1. The system illustrated in the figure includes an external laser to provide illumination, a photomultiplier detector for optical signal collection, and a computer and electronic control unit for management of specimen and probe positioning and image acquisition. Near-field scanning optical microscope — Wikipedia Though there are many issues associated with the apertured tips heating, artifacts, contrast, sensitivity, topology and interference amongst othersaperture mode remains more popular. The information generated as a result of nanoilthography the interaction between the probe and specimen is collected and recorded by the computer point-by-point during the raster movement. Contributing Authors Jeremy R. Although the scanning probe microscope family encompasses a vast array of specialized and highly varied instruments, their common operational motif is the employment of a local probe in close interaction with the specimen.
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NSOM makes use of evanescent or non propagating fields that exist only near the surface of the object. These fields carry the high frequency spatial information about the object and have intensities that drop off exponentially with distance from the object. Because of this, the detector must be placed very close to the sample in the near field zone, typically a few nanometers. As a result, near field microscopy remains primarily a surface inspection technique.
The detector is then rastered across the sample using a piezoelectric stage. The scanning can either be done at a constant height or with regulated height by using a feedback mechanism. As illustrated, the tips used in the apertureless mode are very sharp and do not have a metal coating. Though there are many issues associated with the apertured tips heating, artifacts, contrast, sensitivity, topology and interference amongst others , aperture mode remains more popular.
This is primarily because apertureless mode is even more complex to set up and operate, and is not understood as well.
The major ones are illustrated in the next figure. Apertured modes of operation: a illumination, b collection, c illumination collection, d reflection and e reflection collection.
This hybrid probe can deliver the excitation light through the fiber to realize Tip-enhanced Raman spectroscopy TERS at tip apex, and collect the Raman signals through the same fiber. Some of these mechanisms are constant force feedback and shear force feedback Constant force feedback mode is similar to the feedback mechanism used in atomic force microscopy AFM.
Experiments can be performed in contact, intermittent contact, and non-contact modes. In shear force feedback mode, a tuning fork is mounted alongside the tip and made to oscillate at its resonance frequency. The amplitude is closely related to the tip-surface distance, and thus used as a feedback mechanism. By using the change in the polarization of light or the intensity of the light as a function of the incident wavelength, it is possible to make use of contrast enhancing techniques such as staining , fluorescence , phase contrast and differential interference contrast.
It is also possible to provide contrast using the change in refractive index, reflectivity, local stress and magnetic properties amongst others. The light source is usually a laser focused into an optical fiber through a polarizer , a beam splitter and a coupler.
The polarizer and the beam splitter would serve to remove stray light from the returning reflected light. The scanning tip, depending upon the operation mode, is usually a pulled or stretched optical fiber coated with metal except at the tip or just a standard AFM cantilever with a hole in the center of the pyramidal tip. Some of the common near-field spectroscopic techniques are below.
However, apertureless NSOM can be used to achieve high Raman scattering efficiency factors around Topological artifacts make it hard to implement this technique for rough surfaces. The Raman signal is found to be significantly enhanced under the AFM tip. This technique has been used to give local variations in the Raman spectra under a single-walled nanotube. A highly sensitive optoacoustic spectrometer must be used for the detection of the Raman signal. Fluorescence NSOM is a highly popular and sensitive technique makes use of the fluorescence for near field imaging, and is especially suited for biological applications.
The technique of choice here is the apertureless back to the fiber emission in constant shear force mode. This technique uses merocyanine based dyes embedded in an appropriate resin. Edge filters are used for removal of all primary laser light. Resolution as low as Near field infrared spectrometry and near field dielectric microscopy  use near-field probes to combine sub-micron microscopy with localized IR spectroscopy.
The most common root for artifacts in NSOM are tip breakage during scanning, striped contrast, displaced optical contrast, local far field light concentration, and topographic artifacts. It is normally limited to surface studies; however, it can be applied for subsurface investigations within the corresponding depth of field. In shear force mode and other contact operation it is not conducive for studying soft materials.
It has long scan times for large sample areas for high resolution imaging.
Near-field scanning optical microscope