Evanescent wave sensor of sensitivity larger than a free space wave

A ray optic description of evanescent wave sensing allows a homogeneous treatment of the sensitivity of free space and guided wave slab sensors. It is shown in particular that the sensitivity of evanescent wave sensors is generally smaller or much smaller than that of a free space wave traversing the probed medium except in the remarkable case of large contrast slab waveguides propagating a restricted number of TM modes where the sensitivity can exceed that of a free space wave