Reverse Symmetry Waveguide for Optical Biosensing

Abstract

The present chapter deals with a novel design of planar optical waveguide biosensors. The principle of reverse symmetry is based on making the refractive index (RI) of the waveguide substrate less than the RI of the medium covering the waveguiding film, which is usually an aqueous solution (RI ∼ 1.33). This is opposed to the conventional sensor geometry, where the substrate is glass or polymers with RIs of approximately 1.5. The reverse configuration can be used to tune the penetration depth of the evanescent electromagnetic field into the cover medium up to infinity; thus the waveguide can be tailor-made so that biological objects with any size can be probed by the evanescent field. This is an important improvement compared with, for example, surface plasmon resonance sensors, where the penetration depth is fixed by the choice of metal.