Enhancing Sensitivity of Mid-infrared Waveguide Spectroscopy with a High-Index Thin Film

Waveguides are useful in mid-infrared spectroscopy for achieving high sensitivity on a small surface area. Here we demonstrate that by adding a thin high-index film, a highly asymmetric sensitivity enhancement can be achieved between the top and bottom of the waveguide. By enhancing the sensitivity only on the top side of the waveguide, we produced a 15 μm thick diamond waveguide on an absorbing silicon dioxide cladding. This waveguide is as sensitive as a much thinner waveguide while simplifying edge coupling of broadband light. Losses were reasonable between 5.5 and 9 μm wavelength, demonstrating that the asymmetric enhancement allows otherwise poor cladding materials to be used.