Supported planar germanium waveguides for infrared evanescent-wave sensing

We have fabricated miniature planar IR waveguides with thicknesses of 30-50 mm, consisting of 12-mm long, 2-mm wide strips of Ge supported on ZnS substrates. Evidence for efficient propagation of broadband IR light through these waveguides is provided by the presence of characteristic high- and lowfrequency optical cutoffs of Ge; by the observation of an oscillatory interference pattern in the transmittance spectrum, which exhibits a dependence on waveguide thickness and propagation angle that closely matches waveguide theory; and by the detection of strong evanescent-wave absorption from small (2 mm2) droplets of liquid, e.g., water, on the waveguide surface. As also predicted by theory, the surface sensitivity (detected light absorbance per unit area of sample-waveguide contact) is shown to increase as a function of incidence or bevel angle.