Developments of symmetrically tapered germanium waveguides as chemical sensors for microanalysis

Abstract

Symmetrically tapered planar infrared (IR) waveguides have been fabricated from a ZnS-coated piece of single-crystalline germanium (Ge), embedded in an epoxide resin as a supporting substrate. The planar surface was subsequently ground to the thickness at the central minimum of < 30 micrometre. As predicted by theory, the surface sensitivity increases with decreasing thickness of the tapered region by maximizing the amount of evanescent wave energy present at the thinnest part of the waveguide. 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 microliter) of volatile solvents. By using the waveguides, light-induced structural changes in the protein bacteriorhodopsin (bR) were observable using samples as small as ~ 50 pmol (~ 1 microgram). Such waveguide sensors can also reveal the surface compositions on a single human hair, pointing to its promise as a tool for forensic fiber analysis. In addition, ray-optic calculations indicate that the propagation angle at the central minimum has a strong non-linear dependence on both angle and vertical position of the input ray. This results in rather inefficient coupling of input light into the off-axis modes that are most useful for evanescent-wave spectroscopy. As compared to a blackbody source, the much greater brightness of synchrotron IR radiation allows a similar total throughput, but restricted to a smaller fraction of the allowed waveguide modes. However, such angle-selective excitation results in a strong oscillatory interference pattern in the transmission spectra, which limits the use of synchrotron radiation with the thin waveguides.