Refractive indices waveguide evanescent waves

In many practical situations, the waveguiding layer consists of an optically dense media formed as a thin film with a thickness dp and refractive index nP. This film is usually applied on a solid substrate (typically glass) with refractive index ns less than the RI of the film. The macroscopically thick substrate creates a mechanically stable foundation for the above film, which has a thickness typically in the submicron range. In this case, the light can travel by TIR inside the film, in a zigzag manner (see Fig. 15.2a), creating the evanescent wave both in the aqueous cover solution with refractive index nc and in the substrate10. 

The second (real) term accounts for the exponential decay of the electric field intensity in the direction normal to the interface. The reflected beam combines with the incident beam, forming a standing electromagnetic wave at the interface (Fig. 9.9). The electric field that penetrates to the optically rarer medium of refractive index n, the evanescent field, plays a critical role in many optical sensors based on the waveguiding principle. Its depth of penetration dv is defined as the distance at which the initial intensity Eq decays to 1/e of its value. Thus from (9.18), dv is... 

Fig. 5.2. (a) Evanescent field of the fundamental propagation mode in an optical waveguide, (b) Interaction of the evanescent wave with a biomolecular reaction for sensing purposes. The adsorption of the receptor layer and the recognition process produces a change of the effective refractive index of the waveguide inside the evanescent field and this change is quantitatively related with the concentration of the analyte to be measured. 

The curing of an epoxy resin adj acent to an embedded silica optical fibre was monitored in situ by evanescent wave spectroscopy. The epoxy resin was partially fluorinated and had a lower refractive index than the silica fibre. The lower refractive index of the epoxy resin allowed the silica optical fibre to be used as a waveguide for the internal reflection of the near-IR light. The epoxy resin curing was determined as a function of time and temp, by analysis of the near-IR spectmm from the epoxy adj acent to the fibre obtained by the interaction of the evanescent wave that occurred at each internal reflection with the low refractive index epoxy resin. The results obtained showed that epoxy ring-opening and... 

MEMS-Based Biosensor, Fig. 7 (a) Schematic of an evanescent wave traveling in an optical waveguide. The traveling light in the waveguide is influenced by analyte-induced changes of the refraction index in the adjacent... 

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