It is shown, that the sensitivity of the effective refractive index on the
cladding index in evanescent optical waveguide sensors, can be larger than
unity, This implies that the attenuation of a guided wave propagating in a
waveguide immersed in an absorptive medium can be made larger than that of
a free-space wave propagating through the same medium, The conditions and p
hysical explanation for this puzzling behavior are identified and as a prac
tical application, an absorption sensor, based on a suspended silicon slab
waveguide, is proposed where the sensitivity is enhanced by a factor of 1.3
5.