Scanning near-field optical microscopy imaging

Characterization of surfaces and thin films has been revolutionized by the invention of scanning probe microscopes, i,e, scanning force microscopy, scanning tunnelling microscopy, and scanning near field optical microscopy [262-264], These methods not only allow imaging of molecular and supramolecular details, but can also be employed to probe and to manipulate chemical properties on a nanoscopic or molecular scale, e,g., mechanical SFM [265], chemical SFM [266], electrochemical STM [267,268],... 

Within the last years, single-molecule fluorescence imaging became possible at room temperature and ambient conditions, first with the use of scanning near-field optical microscopy and ultra-high spatial optical resolution (about 100 nm), and later with the use of diffraction limited microscopy techniques (resolution about 300 nm) such as the epi-fluorescence microscopy, internal reflection microscopy, and scanning confocal microscopy. 

When the diffusion is very slow and high spatial resolution is more important than high frame rates, such as in a polymer film, illumination mode scanning near-field optical microscopy can be used to image the diffusion of individual fluorophores. This application will be discussed in the Section V.B. 

Figure 21. Six sequential images of the same sample area of individual carbocyanine dye molecules spread over a polymethylmethacrylate (PMMA) film as recorded by scanning near field optical microscopy. The image dimensions are 2.3 x 2.7-/nn cut out from 4 x 4-fim records of 256 x 256 data points. The excitation polarization was random in (A-D) and linear along y and x, respectively, in ( ) and (F). The emission polarization was measured along y and x in (B) and (C) and not otherwise. Some fluorescence spots are labeled for discussion in the text. The various shapes of the fluorescence peaks (circular spots, rings, arcs, and double arcs) are striking. These shapes can be explained by molecular dipoles being excited by the inhomogeneous electric field at the aperture. (Adopted from [83].)...

Scanning near field optical microscopy (SNOM) opens the perspective to apply optical imaging and spectroscopy techniques to soft matter far below the classical diffraction limit. A use of the novel SNOM technique [6,7] based on an aperture less probe provides a lateral optical resolution in the range of 1-10 nm. 

Scanning near-field optical microscopy is a technique that provides a way of circumventing the optical diffraction limit SNOM images with a spatial resolution of A/20 have been reported [27-33]. Because SNOM is an optical technique, it may be used to perform Raman spectroscopy with sub-A/2 spatial resolution [34-47]. 

The evolution of Raman instrumentation has been recently reviewed (Lewis, 2001a), from the first measurements by C.V. Raman, when the spectra were excited with a mercury lamp and recorded with a small prism spectroscope equipped with a photographic plate (Raman, 1928), to the new high resolution microscopes, in Raman scanning near-field optical microscopy (Adar, 2001). This review includes Raman microscopy (Baldwin, 2001), Raman imaging (Treado, 2001), the adaptation of Raman spectrometry to industrial environment (Slater, 2001), Raman spectroscopy of catalysts (Wachs, 2001) and process Raman spectroscopy (Lewis, 2001b). 

It is instructive to compare the SH microscopy results with the near-field images of Fig. 9.6 which show the same kind of light-emitting nanofibers. The spatial resolution of SH microscopy is lower ( 700 nm) compared with scanning near-field optical microscopy (SNOM) the contrast ratio, however, is much better. Nevertheless, it recently also became possible to determine molecular orientations via SH-SNOM (Fig. 9.14). 

Scanning near-field optical microscopy can be used to obtain detailed images of localized surface plasmons, e.g., around lithographically fabricated gold dots (Hecht et al. 1996 Kim et al. 2003). The resolution, however, is limited by the small detection sensitivity, which limits the minimum aperture size of the scanning optical fiber. Usually one obtains a resolution of the order of 100 nm. 

For nearly two hundred year s microscopy as a science depended on the use of visible and, rarely, near UV electromagnetic radiation. In the early part of this century developments in theoretical Physics opened other avenues of seeing objects. The following is not an exhaustive list but does illustrate the expansion in the science of microscopy which began earlier this century and which continues today. First came the use of electrons in the forms of transmission and scanning electron microscopies [TEM and SEM 3,4]. Then, relatively recently, came the use of sound as an imaging medium in the development of acoustic microscopy [5,6]. Most recently, near-field optical microscopy [7] and the family of scanning probe microscopies have been developed [8]. 

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