|An infrared pump laser scans a region of interest of the sample and an accompanying probe laser reads out the absorption spectrum of the constituent molecules. An image can thus be formed that represents the molecules making up the sample surface. This figure was selected for the cover of Journal of Physics D: Applied Physics.|
Oak Ridge, TN--A new dual-laser standoff-detection technique allows for the rapid identification of chemicals and biological agents from a distance, say the engineers at the Department of Energy's Oak Ridge National Laboratory (ORNL) who invented the system.1
Ali Passian of ORNL and his colleagues present a technique that uses a quantum-cascade laser to pump a target and a helium-neon laser to monitor the material's response as a result of photothermal changes. "The novel aspect to our approach is that the second laser extracts information and allows us to do this without resorting to a weak return signal," says Passian. "The use of a second laser provides a robust and stable readout approach independent of the pump-laser settings."
While this approach is similar to radar and lidar sensing techniques in that it uses a return signal to carry information of the molecules to be detected, it differs in a number of ways.
"First is the use of a photothermal spectroscopy configuration where the pump and probe beams are nearly parallel," Passian says. "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."
Could lead to hyperspectral imaging
This work represents a proof of principle success that Passian and co-author Rubye Farahi said could lead to advances in standoff detectors with potential applications in quality control, forensics, airport security, medicine and the military. In their paper, the researchers also noted that measurements obtained using their technique may set the stage for hyperspectral imaging.
"This would allow us to effectively take slices of chemical images and gain resolution down to individual pixels," said Passian, who added that this observation is based on cell-by-cell measurements obtained with their variation of photothermal spectroscopy.
R. H. Farahi et al., J. Phys. D: Appl. Phys. 45, p. 125101 (2012).