Scanning optics limitations

Perhaps the most significant complication in the interpretation of nanoscale adhesion and mechanical properties measurements is the fact that the contact sizes are below the optical limit ( 1 t,im). Macroscopic adhesion studies and mechanical property measurements often rely on optical observations of the contact, and many of the contact mechanics models are formulated around direct measurement of the contact area or radius as a function of experimentally controlled parameters, such as load or displacement. In studies of colloids, scanning electron microscopy (SEM) has been used to view particle/surface contact sizes from the side to measure contact radius [3]. However, such a configuration is not easily employed in AFM and nanoindentation studies, and undesirable surface interactions from charging or contamination may arise. For adhesion studies (e.g. Johnson-Kendall-Roberts (JKR) [4] and probe-tack tests [5,6]), the probe/sample contact area is monitored as a function of load or displacement. This allows evaluation of load/area or even stress/strain response [7] as well as comparison to and development of contact mechanics theories. Area measurements are also important in traditional indentation experiments, where hardness is determined by measuring the residual contact area of the deformation optically [8J. For micro- and nanoscale studies, the dimensions of both the contact and residual deformation (if any) are below the optical limit. 

For samples thicker than the depth of field, the images are blurred by out-of-focus fluorescence. Corrections using a computer are possible, but other techniques are generally preferred such as confocal microscopy and two-photon excitation microscopy. It is possible to overcome the optical diffraction limit in near-field scanning optical microscopy (NSOM). 

The resolution of a conventional microcope is limited by the classical phenomena of interference and diffraction. The limit is approximately X/2, X being the wavelength. This limit can be overcome by using a sub-wavelength light source and by placing the sample very close to this source (i.e. in the near field). The relevant domain is near-field optics (as opposed to far-field conventional optics), which has been applied to microscopy, spectroscopy and optical sensors. In particular, nearfield scanning optical microscopy (N SOM) has proved to be a powerful tool in physical, chemical and life sciences (Dunn, 1999). 

A series of solution-processible and tractable polymetallaynes of Pt and their mixed-metal analogues were demonstrated to be excellent OPL materials to nanosecond laser pulses at 532 nm, with optimized optical transparency/ nonlinearity trade-offs. The optical-limiting behavior of selected platinum(II) polyynes was investigated by the Z-scan technique. Polyynes 19, 21, 22, 27, 29,... 

Near-Field Scanning Optical Microscopy (NSOM) is a technique which enables users to work with standard optical tools integrated with scanning probe microscopy (SPM). The integration of SPM and certain optical methods allows for the collection of optical information at resolutions well beyond the diffraction limit. 

The most widely employed material characterization techniques in third-order nonlinear optics are third-harmonic generation (THG) [21], degenerate four wave-mixing (DFWM) [22], Z-scan [6], and optical limiting by direct two-photon absorption (TPA) and fluorescence spectroscopy induced by TPA [23]. All of them will be discussed in the following. Further measurement techniques such as electric-field induced second-harmonic generation (EFISH) [24], optical Kerr... 

Near-field scanning optical microscopy (NSOM) has been developed as a combination of scanning probe microscope and optical microscope in which the spatial resolution is determined by scanning probe microscope resolution while the signals detected are coming from several optical interactions. As a result, NSOM has achieved a higher spatial resolution than that of the classical optical microscopy that uses a conventional lens, which is strictly limited by the diffraction... 

Keywords. Two-photon absorption. Non-linear transmission, Z-scan, Optical power limiting. Up-converted lasing... 

Tlie usual experimental techniques developed to study the optical Kerr effect in materials have already been described in a preceding chapter of this book. We only mention here the methods which have especially been used for nanocomposite materials as colloidal solutions or thin films Degenerate four-wave mixing (DFWM) and optical phase conjugation, which provide the modulus of x only and may be completed by Interferometry techniques to get its phase as well, optical limiting, optical Kerr shutter, and z-scan, which is probably the most common technique used in recent years due to its ability to provide simultaneously the nonlinear refraction and absorption coefficients of the same sample point [118],... 

Near-field Scanning Optical Alicroscopy (NSOM) overcomes these limitations by squeezing the fight through an aperture of approximately 50... 

Near-field scanning optical microscopes (NSOM or SNOM) are mainly used in fluorescence and VIS measurements. They provide optical images with spatial resolution less than the Abbe s limit of Ajl. The high lateral resolution is commonly achieved by using the optical near-field, e. g. in close vicinity of a very narrow fiber tip. Figure 5.16 illustrates the design of a near-field microscope. 

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