Reflection geometry Refractive index

Figure 4. Reflectivity in the Kretschmarm geometry of ATR consisting of an SF14 glass prism (refractive index - 1.65), a gold layer (thickness - 50 nm), and a low refractive index dielectric medium (refractive index - 1.32), and wavelength - 800 nm.

Within standard BPM-schemes almost any geometry n(x,y,z) can be considered except for abrupt z-dependent changes of the refractive index. Fortunately, lO-devices usually are intended to have low loss, good transmission and low back-reflection, which is congruent with the above limitation. Thus, BPM got such popular over the years... 

Fig. 16.1. Typical geometry for common-path epi-scattering Raman measurements of biofluids, in vitro. The common path ensures that the excitation and collection volumes overlap (aside from slight changes in refractive index between the laser and Stokes-shifted wavelengths). A dichroic beamsplitter is required as drawn here, it is reflective at the Stokes-shifted wavelengths, but more commonly it is reflective at the laser wavelength...

Figure 27 Schematic representation of the all-optical parallel processing in guided mode geometry and the calculated reflectance for a polymer film (1600 nm) on a silver layer (50 nm). The complex refractive index of a polymer layer is (a) 1.60, (b) 1.58, and (c) 1.60 + 0.02i.

Fig. 1 Waveguide geometry, (a) Cylindrical waveguide with light propagation direction 2, (b) circular cross section with refractive indices rii and /t2. (c) corresponding refractive index profile for total internal reflection guiding...

Using a reflection geometry, one can employ the technique known as REFLEXAFS, which consists of measuring the ratio of the reflected and incident intensities as a function of energy. Although an EXAFS spectrum can be obtained from such a measurement, the process is somewhat involved since the reflectivity is a complex function of the angle of incidence, the refractive index, and energy. 

IR absorption in the presence of composite films In Section 6.4, we noticed that the presence of a thin film over the semiconductor surface may lead to a decreased IR absorption from the electrolyte in ATR geometry. A quantitative analysis of this effect requires going beyond the treatment leading to Eqs. (l)-(2), which did not take absorption of medium 2 into account. The result actually depends on the refractive index of the film. If it is lower than that of the semiconducting substrate Ui, a loss in electrolyte absorption is indeed to be expected (most frequent case for an oxide film). If it is equal or close to Uj, total reflection takes place at the outer edge of the film, and no change in electrolyte absorption is to be expected. 

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