Subsurface Scatterers

It is possible to increase the optical path through a photodetector by incorporating different stmctures within the active region itself so that they act as scatterers of light. In this way, a beam that would else leave the detector remains within the absorptive domain. Stiucturally, one could use inclusions, fillers, pores, voids, etc. 

A method of the use of scatterers incorporated into the absorptive region was described by Vynck et al. who produced a random array of nanoholes in absorptive film [204]. Multiple scattering here is two-dimensional and leads to an appearance of disordered quasi-guided modes. Their structure is schematically presented in Fig. 2.29. 

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Hanrahan, P., and W. Krueger. 1993. Reflection from layered surfaces due to subsurface scattering. Pp. 165-174 of the Proceedings of the Association for Computing Machinery Special Interest Group on Computer Graphics and Interactive Techniques 1993 International Conference. New York ACM Press/Addison-Wesley. 

We can also visualise the subsurface as being made up of an enormous number of point scatterers or diffractors. (Fig b). Each contributes a diffraction curve (hyperbola) to the reflection section. Migration focuses the energy in these curves to a single point. 

The major role of TOF-SARS and SARIS is as surface structure analysis teclmiques which are capable of probing the positions of all elements with an accuracy of <0.1 A. They are sensitive to short-range order, i.e. individual interatomic spacings that are <10 A. They provide a direct measure of the interatomic distances in the first and subsurface layers and a measure of surface periodicity in real space. One of its most important applications is the direct determination of hydrogen adsorption sites by recoiling spectrometry [12, 4T ]. Most other surface structure teclmiques do not detect hydrogen, with the possible exception of He atom scattering and vibrational spectroscopy. 

Another very important technique for fundamental consideration of multicomponent systems is low energy ion scattering (LEIS) [Taglauer and Heiland, 1980 Brongersma et al., 2007]. This is a unique tool in surface analysis, since it provides the ability to define the atomic composition of the topmost surface layer under UHV conditions. The signal does not interfere with the subsurface atomic layers, and therefore the results of LEIS analysis represent exclusively the response from the outer surface. In LEIS, a surface is used as a target that scatters a noble gas ion beam (He, Ne, ... 

A series of LEED intensity studies, together with ion-scattering spectroscopy, established that a missing row structure was the correct model for the (1 x 2) phase,14 with some small subsurface relaxation and reconstruction.10... 

S. C. Park, M. Kim, J. Noh, H. Chung, Y. Woo, J. Lee and M.S. Kemper, Reliable and fast quantitative analysis of active ingredient in pharmaceutical suspension using Raman spechoscopy, Aruil. Chim. Acta, 593,46-53 (2007). P. Matousek, I.P. Clark, E.R.C. Draper, et al.. Subsurface probing in diffusely scattering media using spatially offset Raman spechoscopy, Appl. Spectrosc., 59, 393 00 (2005). 

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