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James J Stone

Susan E Plunkett

Mark S Braiman

Miniature planar IR waveguides of thickness 30&#x2013;50 &#x3bc;m, consisting of 12-mm long, 2-mm wide strips of Ge supported on ZnS substrates and tapered quasi-tapered waveguides, tapered from a thickness of 1 mm at the ends to a minimum of 1&#x2013;100 &#x3bc;m at the center, are disclosed. The surface sensitivity is increased as a function of incidence or bevel angle. The tapered waveguide improves the efficiency of the optical coupling both into the waveguide from an FTIR spectrometer, and out of the waveguide onto a small-area IR detector. The tapering makes it possible to dispense with using an IR microscope couple light through the waveguide, enabling efficient coupling with a detector directly coupled to an immersion lens. This optical arrangement makes such thin supported waveguides more useful as sensors, because they can be made quite long (e.g. 50 mm) and mounted horizontally. Furthermore, even with a 20-&#x3bc;m&#xd7;1-mm cross section, sufficient throughput is obtained to give signal/noise ratios in excess of 1000 over most of the 1000&#x2013;5000 cm-range, with just 2 min of scanning at 8 cm-resolution.

Support planar and tapered quasi-planar germanium waveguides for infrared evanescent-wave sensing

Miniature planar IR waveguides of thickness 30-50 &mgr;m, consisting of 12-mm long, 2-mm wide strips of Ge supported on ZnS substrates and tapered quasi-tapered waveguides, tapered from a thickness of 1 mm at the ends to a minimum of 1-100 &mgr;m at the center, are disclosed. The surface sensitivity is increased as a function of incidence or bevel angle. The tapered waveguide improves the efficiency of the optical coupling both into the waveguide from an FTIR spectrometer, and out of the waveguide onto a small-area IR detector. The tapering makes it possible to dispense with using an IR microscope couple light through the waveguide, enabling efficient coupling with a detector directly coupled to an immersion lens. This optical arrangement makes such thin supported waveguides more useful as sensors, because they can be made quite long (e.g. 50 mm) and mounted horizontally. Furthermore, even with a 20-&mgr;m&times;1-mm cross section, sufficient throughput is obtained to give signal/noise ratios in excess of 1000 over most of the 1000-5000 cm range, with just 2 min of scanning at 8 cm solution.

We have fabricated miniature planar IR waveguides of thickness 30-50 .mu.m, 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 low frequency optical cut-offs of Ge; by the observation of an oscillatory interference pattern in the transmittance spectrum, which exhibits a dependence on waveguide thickness and propagation angle closely matching waveguide theory; and by the detection of strong evanescent-wave absorption from small (2 mm.sup.2) 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) was shown to increase as a function of incidence or bevel angle.

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Charlottesville, VA, United States of America

James J Stone