Figure 2.

Schematic diagram of the optical waveguide lightmode spectroscopy (OWLS) setup. Light from a He-Ne laser is coupled into the waveguiding film via an optical grating. Only at a specific angle of incidence α, under constructive interference conditions, is a guided mode excited and a peak in light intensity detected. The evanescent field penetrates about 100–200 nm into the bulk solution and senses changes of the refractive index in this region. Thus, molecular adsorption can be precisely monitored by measuring the incoupling angle as a function of time.