Linear Optical Techniques at Surfaces and Interfaces

In this chapter we shall consider linear optical techniques which are used for the investigation of surfaces and interfaces. They involve a linear susceptibility of the sample, Nonlinear optical techniques will be discussed in Chapter 6. 

In Section 3.1.1 we have discussed the phase changes on reflection. For the two-phase model of a crystal-vacuum interface which does not take into ac-coimt any transition layer, the complex bulk dielectric function of the crystal 

1) An exception is the case where the IR radiation is resonant to excitations in the overlayer. 

Optus and Spectroscopy at Surfaces and Interfaces. Vladimir G. Bordo and Horst-Giinter Rubahn Copyright 2005 WILEY-VCH Verlag GmbH Co. KGaA, Weinheim ISBN 3-527-40560-7 

Any real surface contains a layer whose optical properties differ from those in the bulk crystal. That may be a thin film on the surface, in particular an oxide film, contamination, relaxed or reconstructed layer, or surface roughness. Therefore with the help of Eq. (5.1) an effective dielectric function, (e), is determined, which corresponds to an average over the region penetrated by the incident light. In order to extract the optical properties of a transition layer, the substrate contribution to (e) must be evaluated. This is usually performed by applying a three-phase model (see Section 3.1.3). Then the ellipsometric ratio, p, can be written using Eq. (3.40). The complex dielectric function (its real and imaginary parts) and the thickness of the transition layer (phase 2) are considered as the three unknown parameters. However, the measurements of the complex quantity p provide only two equations for them. To obtain the third one, it is necessary to invoke additional, physically reasonable restrictions. 

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