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Symmetrically tapered <30-micron-thick quasi-planar Ge waveguides as chemical sensors for microanalysis

journal contribution
posted on 2002-12-01, 00:00 authored by Jitraporn Vongsvivut, S Shilov, S Ekgasit, M Braiman
Symmetrically tapered planar IR waveguides have been fabricated by starting with a ZnS coated concave piece of single-crystal Ge, embedding it in an epoxide resin as a supporting substrate, and then grinding and polishing a planar surface until the thickness at the taper minimum is <30 μm. Such tapering is expected to enhance a waveguide's sensitivity as an evanescent wave sensor by maximizing the amount of evanescent wave energy present at the thinnest part of the waveguide. As predicted by theory, the surface sensitivity, i.e., the absorbance signal per molecule in contact with the sensing region, increases with decreasing thickness of the tapered region even while the total energy throughput decreases. The signal-to-noise ratio obtained depends very strongly on the quality of the polished surfaces of the waveguides. The surface sensitivity is superior to that obtained with a commercial Ge attenuated total reflection (ATR) accessory for several types of sample, including thin films (<10 ng) and small volumes (<1 μL) of volatile solvents. By using the waveguides, light-induced structural changes in the protein bacteriorhodopsin were observable using samples as small as ∼50 pmol (∼1 μg). In addition, the waveguide sensors can reveal the surface compositions on a single human hair, pointing to their promise as a tool for forensic fiber analysis.

History

Journal

Applied spectroscopy

Volume

56

Issue

12

Pagination

1552 - 1561

Publisher

Society for Applied Spectroscopy

Location

Frederick, Md.

ISSN

0003-7028

eISSN

1943-3530

Language

eng

Publication classification

C1.1 Refereed article in a scholarly journal