Nonlinear microscopy advantages

A third advantage of nonlinear interferometry is that the mixing term scales linearly with the sample concentration through E. In the case of CARS microscopy, the anti-Stokes field can be expressed as... 

According to the selection rules, the HRS spectroscopy can in principle be used as an alternative for IR spectroscopy [19]. Indeed, this nonlinear spectroscopy has several advantages IR-mode detection is possible even in IR-opaque media and its spatial resolution is much better than IR microscopy. Moreover, HRS signals appear in the doubled frequency region, which is far from the intense excitation laser line, and hence, low-energy vibration modes are easily observable. 

The small cross sections in multiphoton processes are of course a weakness of nonlinear spectroscopy. Especially in microscopy, this problem becomes serious because of the small volume of a sample. By the use of the signal enhancement techniques, however, the disadvantage can be turned into an advantage of back-ground-free selective measurements. For example, the combined use of HRS with the plasmonic enhancement provides us a chemical imaging with nanoscale spatial resolution when a laser-illuminated metal tip is located adjacent to a sample surface, signal enhancement is locally induced near the tip. This spatial resolution is expected to overcome optical diffraction limit. Such tip-enhanced spectroscopy has already been reported in conventional CARS [15]. 

In this section we briefly discuss some special techniques of linear and nonlinear Raman spectroscopy that have particular advantages for different applications. These are the resonance Raman effect, surface-enhanced Raman signals, Raman microscopy, and time-resolved Raman spectroscopy. 

The chapter is organized as follows. Section 8.2 provides a short reminder of what acoustic shear waves can and cannot do. Shear waves have distinct advantages (compared to other surface anal3Ttical techniques like optical reflectometry or atomic force microscopy [AFM]), but there are also some caveats to be kept in mind. Section 8.3 briefly summarizes some predictions from simple planar models of slip. An experimental result, which stands as an example for an experience in the authors laboratory, is presented in section 8.4. Section 8.5 provides the results from FEM calculations. Section 8.6 discusses nonlinear phenomena and acoustic streaming, in particular. 

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