Refractometry using beam profile reshaping with surface waves and its application to bioassay.

摘要

In this dissertation, we develop the theory underlying the application of surface waves to detection of small changes in the index of refraction of a substrate in a multi-layered structure. The analysis can be used to improve the sensitivity of bioassay devices in which the presence of a contaminant in a blood sample is registered as a change in the effective index of refraction of the substrate in the structure used to excite the surface waves. For this purpose, we compare the sensitivities of devices based on both single and double boundary surface plasmons excited by either prism or grating coupling. We show that the sensitivity of present bioassay devices based on single boundary surface waves can be improved by use of double boundary surface waves, and by more detailed observation of changes in the profile of the reflected light beam near the resonance minimum in the attenuated total reflection (ATR). Our analysis is the first to apply the theory of reflection from a double boundary grating to describe the excitation, by a finite width beam, of a double boundary surface wave. To avoid the necessity of angle tuning in a bioassay device based on surface waves, we incorporate into the theory the effect of a lens, designed to spread the incident light beam over a range of angles including the plasmon angle. We show, for the first time, that the inclusion of the lens in the double boundary geometry makes it possible to observe oscillations in the profile of the reflected beam. These oscillations arise from the interference between a specularly reflected field at the first interface of the geometry and the re-radiated field of the surface plasmon. We derive the conditions under which the oscillations in the profile are observable, and discuss the potential of the oscillations for increasing the sensitivity of the surface-wave bioassay.