Action at a distance: Standoff spectroscopy

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  • Published: Apr 1, 2012
  • Author: David Bradley
  • Channels: Infrared Spectroscopy
thumbnail image: Action at a distance: Standoff spectroscopy

Remote sense

Identifying chemicals from a distance could take a step forward with the introduction of a two-laser visible-infrared system being developed at Oak Ridge National Laboratory.

Writing in the Journal of Physics D: Applied Physics, Ali Passian and colleagues explain how they have used quantum cascade lasers to "pump" their sample target and a second laser to probe the material's response to the temperature-induced changes that take place during the pumping process. The researchers suggest that the information they obtain using this hands-off approach could allow them to quickly identify contaminants of chemical and biological agents on the sample.

Pump-probe systems are not new, but, explains Passian, a member of the laboratory's Measurement Science and Systems Engineering Division, "The novel aspect to our approach is that the second laser extracts high-resolution spectroscopic information and allows us to do this without resorting to a weak return signal." He points out that by using the second laser they can obtain a robust and stable reading that is essentially independent of the settings used to fire the pump laser.

A laser standoff

The approach is different in that, "First is the use of a photothermal spectroscopy configuration where the pump and probe beams are nearly parallel," Passian adds. "We use probe beam reflectometry as the return signal in standoff applications, thereby minimizing the need for wavelength-dependent expensive infrared components such as cameras, telescopes and detectors."

Passian and co-author Rubye Farahi explain that this is a proof of principle which could lead to significant advances in standoff detectors for quality control applications, in forensics, airport security, medical diagnostics and, inevitably, the military given that the technique can be applied at a much greater distance than with conventional spectroscopic techniques used in a laboratory. The technique may also facilitate the development of hyperspectral imaging where bands of the electromagnetic spectrum in conjunction with spatial information are being collected and processed by the same instrument.

"This would allow us to effectively take slices of chemical images and gain resolution down to individual pixels," Passian says. Cell-by-cell measurements obtained with their variation of photothermal spectroscopy would be extended in hyperspectral imaging to provide not only high-resolution chemical information about a sample but also topographical information.

Common touch

The team has tested their approach on common substances such as organophosphate pesticides, paint, cosmetics, cellulose and polystyrene. Comparisons with reference spectra demonstrate its validity. "The results are applicable to the standoff detection of a larger category of chemicals, as well as applications in other fields of research," the team says.

ORNL's Passian and Farahi worked on the project with colleague Laurene Tetard and Thomas Thundat of the University of Alberta with support from ORNL's Laboratory Directed Research and Development program.

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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