  We propose and demonstrate, through simulation and experiment, how the
interaction of an optical field within a waveguide designed for chemical
sensing and, more generally, evanescent field spectroscopy can be enhanced
substantially by strategic deposition of high index surface layers. These
layers draw out the optical field in the vicinity of probing and take advantage
of field localisation through optical impedance matching. Localisation of the
evanescent field to the inner layer in turn is accompanied by whispering
gallery modes within the channels of a structured cylindrical waveguide,
further enhancing sensitivity. A novel demonstration based on self-assembled
layers made up of TiO2 within a structured optical fibre is demonstrated, using
a simple porphyrin as the spectroscopic probe. This technique offers
optimisation of the limitations imposed on practical waveguide sensors that are
highly sensitive but nearly always at the expense of low loss. The principles
have potential ramifications for nanophotonics more generally and these are
discussed.
