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Spectroscopy Detects, Counts Toxic Molecules

Photonics Spectra
Apr 2004
Daniel S. Burgess

A team at the National Institute of Standards and Technology in Gaithersburg, Md., Eindhoven University of Technology in the Netherlands and the University of Maryland in College Park has demonstrated that a variant of cavity ringdown spectroscopy is suitable for detecting and quantifying toxic substances. Unlike many other techniques that offer such measurements of absolute surface coverage, this approach does not require ultrahigh-vacuum experimental conditions. The scientists suggest that it will have further applications in surface science and chemical sensing.

Spectroscopy Detects, Counts Toxic Molecules
Evanescent-wave cavity ringdown spectroscopy employs a monolithic optical element with a total internal reflection surface. Where the IR laser pulse experiences total internal reflection, it generates an evanescent wave that extends beyond the surface to form a sensing region. The presence of an absorbing sample in the sensing region frustrates the total internal reflection, increasing the per-pass loss and shortening the ringdown time. Comparing the decay rates with and without the analyte thus reveals the presence and concentration of the sample. Courtesy of Andrew C.R. Pipino, National Institute of Standards and Technology.

In cavity ringdown spectroscopy, the photon decay rate in a high-finesse linear resonator is used to determine the presence of particular substances. Without an analyte, the intensity of an injected laser pulse decreases over a characteristic time -- the ringdown time -- as some radiation is lost with each pass of the pulse through the resonator. If an absorbing gas-phase sample is present, the per-pass loss will be greater, shortening the ringdown time. Comparing the decay rates with and without the analyte thus reveals the presence and the concentration of the absorber, and because many passes are made in the resonator, the technique displays high sensitivity.

Evanescent sensing

Its variant, evanescent-wave cavity ringdown spectroscopy, relies on the same measurement principle, but the linear resonator is replaced by a monolithic optical element with a total internal reflection surface. Where the laser pulse experiences total internal reflection, it generates an evanescent wave that extends beyond the surface to form a sensing region. The presence of an absorbing sample -- of any phase -- in the sensing region frustrates the total internal reflection, thereby increasing the per-pass loss and shortening the ringdown time.

In the experiments, the scientists employed a monolithic folded resonator made of fused silica with a finesse of 28,500 at 1650 nm. An Nd:YAG-pumped optical parametric oscillator/optical parametric amplifier served as a source of tunable near-IR pulses, which passed through a pair of Glan-laser polarizing beamsplitters so that the researchers could choose to inject transverse electric or transverse magnetic modes. An InGaAs detector collected the output for measurement of the ringdown times. The environmental contaminant trichloroethylene and two other chloroethylenes served as the analytes.

The ability to probe with both transverse electric and transverse magnetic modes enabled the researchers to determine the absolute surface coverage. The degree of absorption by the analyte adsorbed to the surface of the resonator depends on its orientation to the surface, so the polarization anisotropy could be translated into the average molecular tilt angles of the sample and thereby into a measurement of coverage by invoking conservation of the integrated absolute intensity.

To assess the suitability of the technique for use in materials science and chemical sensing, the researchers compared their findings with measurements of trichloroethylene collected using a mass spectrometer and an optical waveguide technique. They noted that the former is an ex situ approach and is restricted to ultrahigh-vacuum conditions. At 1 × 10–7, the detection limit of evanescent-wave cavity ringdown spectroscopy was similar to that of the latter, but the optical waveguide had been optimized in order to enrich the local concentration of the substance in the evanescent wave.

The addition of an enrichment layer, such as a deuterated polysiloxane film, to the monolithic folded resonator thus should enable evanescent-wave cavity ringdown spectroscopy to display a lower detection limit.


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