Mid-IR evanescent-wave absorption spectra of thin films and coatings measured with a ~50-um-thick planar Ge waveguide sensor

摘要

Abstract: We have fabricated IR-transmitting Ge waveguides as supported single crystals 30 - 100 micrometers in thickness. These waveguides are useful as internal reflection elements for evanescent-wave absorption sensing and spectroscopy, when used in conjunction with a Fourier transform infrared microscope. This combination affords great sensitivity to small numbers of IR-absorbing molecules near the waveguide surface, and is especially useful for analyzing thin coatings on small-area substrates. For example, we have selectively observed absorption bands, as high as 20 times the noise level, from the adhesive layer of a 0.07 mm$+2$/ piece of Scotch tape without observing absorption from the backing. By comparing spectra taken using 70- and 30-micrometer thick waveguides, we demonstrate a clear increase in sensitivity to small samples with decreasing waveguide thickness. !10