Characterization of supported cylinder-planar germanium waveguide sensors with synchrotron infrared radiation.
Cylinder-planar Ge waveguides are being developed as evanescent-wave sensors for chemical microanalysis. The only non-planar surface is a cylinder section having a 300-mm radius of curvature. This confers a symmetric taper, allowing for direct coupling into and out of the waveguide's 1-mm(2) end faces while obtaining multiple reflections at the central <30-microm-thick sensing region. Ray-optic calculations indicate that the propagation angle at the central minimum has a strong nonlinear 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 absorption spectroscopy. Mode-specific performance of the cylinder-planar waveguides has also been investigated experimentally. As compared to a blackbody source, the much greater brightness of synchrotron-generated infrared (IR) radiation allows a similar total energy 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. These spectral oscillations are the principal technical limitation on using synchrotron radiation to measure evanescent-wave absorption spectra with the thin waveguides.
Vongsvivut
Jitraporn
J
Department of Chemistry, Faculty of Science, Chulalongkorn University, Bangkok 10330 Thailand.
Fernandez
Jason
J
Ekgasit
Sanong
S
Braiman
Mark S
MS
D015374:Biosensing Techniques; D004867:Equipment Design; D005857:Germanium; D007259:Infrared Rays; D008832:Microchemistry / Q000379:methods; D017550:Spectroscopy, Fourier Transform Infrared / Q000295:instrumentation* / Q000379:methods; D017356:Synchrotrons
D005857:Germanium