The optimization of azimuth angle settings of a polarizer-compensator-sample-analyzer off-null ellipsometric sensor system to obtain maximum intensity changes with respect to changes in the properties of a sensing layer, with and without considering changes in a reflectance, is studied. Optimal conditions in the two cases are derived analytically under the assumption that linear relationships exist among the changes in the parameters of the sensing layer. The validity of these optimal conditions is verified by numerical examples. The advantage of using ellipsometry compared with reflectometry to readout sensing information for some sensing samples is also discussed. © 2004 Optical Society of America.

A gas sensor system based on ellipsometric readout is presented. It includes a gas chamber and a compact null ellipsometer operated in off-null mode. Small, low-cost optical components are used to demonstrate that this advanced methodology can be implemented in simplified instrumentation. The off-null ellipsometric sensing principle and transducer mechanisms of the sensing layers, as well as the instrumentation, are described. The application of the sensor system is exampled with experimental results on low-concentration alcoholic gases (methanol, ethanol, and 2-propanol) using porous silicon as a sensing layer. Optimization of the optics of the sensor system, improvement of sensitivity or alteration of selectivity by modification of sensing layers, and multisensing by using several ellipsometric units in parallel are discussed.

Ellipsometry has sufficient sensitivity for sensor applications and is here used as an optical readout method in a gas sensing system. Porous silicon is used as sensing layers in which vapors of solvents can adsorb and condensate due to capillary effects. A miniaturized multi-beam ellipsometer system is proposed and the concept is demonstrated by measurements on alcohol vapors. Optimization of the sensor system is discussed and improvement of sensitivity and alteration of selectivity by metal deposition in porous silicon layers are presented.

The concept of an ellipsometric sensor array system - "an optical nose" - is presented. The system is used to monitor and discriminate gases. The optical readout is based on polarizer-compensator-sample-analyzer (PCSA) off-null ellipsometry. Thin porous silicon sensing layers with and without surface chemical modifications are used to measure low concentration methanol, ethanol and 2-propanol gases. The main contents of the thesis are:

- Principle and construction of a prototype PCSA ellipsometric gas sensor system.

- Optimization of the azimuth angle settings of the optical components in PCSA off-null ellipsometry to obtain the maximum readout sensitivity.

- Application of return-path off-null ellipsometry in gas sensing as an alternative way to construct ellipsometric gas sensors.

- Modification of porous silicon sensing layers by copper deposition to obtain different gas sensitivity for the enhancement of selectivity.

- Multi-sensing concept of the optical nose to detect and discriminate alcohols using porous silicon sensing layers and pattern recognition techniques.

So far, a prototype PCSA ellipsometric gas sensor system has been constructed and tested. Low concentration methanol, ethanol and 2-propanol vapors are detected and discriminated based on the PCSA off-null ellipsometric gas sensor system and porous silicon sensing layers.

Gas sensing was performed using spectroscopic ellipsometry and porous silicon films. Modification of the porous layer by polymer deposition showed an increase in sensitivity to organic solvent vapor of up to 135%. The increase in sensitivity is strongly dependent on polymer concentration. At high concentrations, too much polymer is deposited, presumably blocking the pores, causing a decrease in sensitivity. At sufficiently low concentrations, the polymer causes a strong increase in sensitivity. This is assumed to be caused by the polymer being deposited inside the pores, where its interaction with the vapor influences the sensitivity. At very low concentration, the sensitivity approaches values obtained without polymer modification. The sensitivity increase is different for different vapors, pointing to possible selectivity enhancement.

The dependence of the azimuth angle settings on the change in off-null intensity of a polarizer-.compensator-sample-analyzer ellipsometer owing to changes in sample properties is studied. First, a closed-form expression for the relationship between azimuth angles that fulfill the null condition is presented. An approximation for the off-null light intensity near null that is valid for small changes of the p- and s-reflection coefficients of an isotropic sample is then derived. This approximation shows that the intensity change near the null can be described by changes in the ellipsometric parameters tan q, and Delta only. Expressions for finding the azimuth angle that gives the maximum possible intensity change for a given change in the sample parameters are also derived. The importance of optimization of azimuth angle settings for different samples is investigated and found to depend on tan psi. Numerical and experimental results chosen from the investigation of gas sensors based on porous silicon are included to verify the approximations as well as the optimization. (C) 2003 Optical Society of America.

The sensitivities of porous silicon layers modified by copper deposition to low vapor concentrations of methanol, ethanol and 2-propanol are studied with spectroscopic ellipsometry. The porous silicon layers are prepared with electrochemical etching in hydrofluoric acid, and copper deposition is done in aqueous CuSO4. The ellipsometric vapor sensitivities, in terms of the ellipsometric spectral shifts due to gas exposures, of these samples and their oxides are studied and compared. It is shown that ellipsometric vapor sensitivities of porous silicon layers to alcohols are improved by copper deposition. It is also found that the oxidation of copper deposited in porous silicon layers shows improved selectivity to methanol. © 2002 Elsevier Science B.V. All rights reserved.