Planar-surface-waveguide evanescent-wave chemical sensors

It is shown that planar-surface waveguides have several attractive features for evanescent-wave sensing. We present expressions, in terms of normalized waveguide parameters, for the penetration depth of the evanescent wave in the sensing medium, the absorption coefficient due to the latter, and the fluorescence collection efficiency from the sources present in this medium, which acts as the cover of the multimode waveguide. A comparison with optical-fibre sensors shows that planar guides not only offer reasonable performance in terms of absorption and fluorescence collection efficiency, but their asymmetry allows control of the penetration depth, which can be made relatively independent of the guided mode. Finally, we demonstrate that single-mode planar guides can also have comparable performance in situations where evanescent-wave chemical sensing using single-mode fibres can be quite challenging.