Optical Detection Using Computer Screen Photo-assisted Techniques and Ellipsometry
Abstract: Two main subjects, ellipsometry and computer screen photo-assisted techniques (CSPT), form the main line in this thesis. Ellipsometry is an optical technique based on the detection of polarization changes of light upon interaction with a sample. As most optical detection techniques it is non-intrusive and an additional advantage is its high surface sensitivity: thickness resolution in the order of pm can in principle be achieved. Therefore, ellipsometry is widely used as a technique for determination of optical constants and layer thickness for thin-layer structures. Lately ellipsometry has also been proposed for sensing applications, utilizing the detection of changes in the properties of thin layers. One application is described in this thesis concerning the detection of volatile organic solvents in gas phase using modified porous silicon layers, fabricated by electrochemical etching of silicon to create nm-sized pores. This greatly increases the surface area, promoting gas detection because the number of adsorption sites increases. Other applications of ellipsometry discussed in this thesis are based on combination with CSPT.CSPT is a way to exploit existing optical techniques for use in low-cost applications. In CSPT the computer screen itself is used as a (programmable) light source for optical measurements. For detection a web camera can be used and the whole measurement platform is formed by the computer. Since computers are available almost everywhere, this is a promising way to create optical measurement techniques for widespread use, for example in home-diagnostics. Since the only thing that needs to be added is a sample holder governing the physical or chemical process and directing the light, the cost can be kept very low. First, the use of CSPT for the measurement of fluorescence is described. Fluorescence is used in many detection applications, usually by chemically attaching a fluorescent marker molecule to a suitable species in the process and monitoring the fluorescent emission. The detection of fluorescence is shown to be possible using CSPT, first in a cuvette-based setup, then using a custom designed micro array. In the latter, polarizers were used for contrast enhancement, which in turn led to the implementation of an existing idea to test CSPT for ellipsometry measurements. In a first demonstration, involving thickness measurement of silicon dioxide on silicon, a thickness resolution in the order of nm was already achieved. After improvement of the system, gradients in protein layers could be detected, opening the door toward biosensor applications. Some further development will be needed to make the CSPT applications described here ready for the market, but the results so far are certainly promising.
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