Optical sensors ellipsometry

A chemical sensor is a device that transforms chemical information into an analytically useful signal. Chemical sensors contain two basic functional units a receptor part and a transducer part. The receptor part is usually a sensitive layer, therefore a well founded knowledge about the mechanism of interaction of the analytes of interest and the selected sensitive layer has to be achieved. Various optical methods have been exploited in chemical sensors to transform the spectral information into useful signals which can be interpreted as chemical information about the analytes [1]. These are either reflectometric or refractometric methods. Optical sensors based on reflectometry are reflectometric interference spectroscopy (RIfS) [2] and ellipsometry [3,4], Evanescent field techniques, which are sensitive to changes in the refractive index, open a wide variety of optical detection principles [5] such as surface plasmon resonance spectroscopy (SPR) [6—8], Mach-Zehnder interferometer [9], Young interferometer [10], grating coupler [11] or resonant mirror [12] devices. All these optical... 

Unlike the dependence of Aeff on film thickness alone (dNc /dd) that is sometimes used as a figure of merit for guided mode molecular sensors, 5m0d captures both the index and thickness dependence of the sensor response in a single parameter. While Dopt does not uniquely determine the film response for other optical techniques such as ellipsometry and reflectance difference, once d and n of the film are known, the optical thickness can be evaluated and comparisons are made between guided mode sensors and other techniques. 

An optical transduction method that is often used with ultrathin hlms, such as LB hlms, is that of surface plasmon resonance [30, 31]. Surface plasma waves are collective oscillations of the free electrons at the boundary of a metal and a dielectric. These can be excited by means of evanescent electromagnetic waves. This excitation is associated with a minimum in the intensity of the radiation reflected from the thin him system, called surface plasmon resonance (SPR). The sensitivity of SPR is noteworthy, and changes in refractive index of 10 may be monitored thus the technique compares favorably with ellipsometry. The method has been used with LB hlms to provide both gas detectors [29] and sensors for metal ions in solution [32]. 

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