Surface-sensitive FTIR spectral measurements of nanogram samples using 30-100-μm-thick planar Ge waveguides

Details Surface-sensitive FTIR spectral measurements of nanogram samples using 30-100-μm-thick planar Ge waveguides Surface-sensitive FTIR spectral measurements of nanogram samples using 30-100-μm-thick planar Ge waveguides

Jun 1998

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998aipc..430..577b&link_type=abstract

The eleventh international conference on fourier transform spectroscopy. AIP Conference Proceedings, Volume 430, pp. 577-580 (1

Computer Science

Elemental Semiconductors And Insulators, Semiconductors

Scientific paper

We have developed techniques for fabricating supported planar Ge waveguides that are useful for measuring broadband mid-IR evanescent-wave absorption spectra from small numbers of molecules. The waveguides are ground and polished as thin as 30 μm, starting from larger pieces of commercially-available single-crystal Ge. Such waveguides are useful as extremely sensitive multiple-internal-reflection elements, when used in conjunction with an FTIR spectrometer with microscope accessory and a small-area detector. With careful polishing and alignment, optical losses can be reduced to those predicted as resulting from ray spreading within the waveguide and from reflection at the air-Ge interface. For example, by using a Spectra-Tech Research IR-Plan™ microscope with a (100 μm)2 detector element, we have been able to demonstrate broadband IR power transmission through a 30-μm-thick, 3-mm-wide waveguide that is 4% of the open-beam throughput focused through a 30 μm×3 mm slit at the sample focal plane. Supported planar Ge waveguides between 30 and 100 μm in thickness function as internal reflection elements with 10-20 reflections per mm of length. Using such waveguides, we have been able to make surface-sensitive spectral measurements of tiny samples. For example, we can easily observe the FTIR absorption spectrum selectively from the adhesive layer on 3M Scotch™ tape, without interfering absorption from the tape backing. A 0.07 mm2 piece of tape, carrying only several μg of adhesive, still produces distinct bands with absorbance values as large as 0.18. The great sensitivity to monolayer samples covering small contact areas has allowed us to measure useful FTIR spectra of the plasma membranes of intact cells, e.g. individual 1-mm-diameter frog oocytes, submerged in aqueous media. The protein and lipid components in a patch of such a single-bilayer membrane give absorbances on the order of 0.01.

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