June 26, 2008--Researchers at the Department of Energy's Oak Ridge National Laboratory (ORNL; Oak Ridge, TN) can detect explosives at distances exceeding 20 yards using a laser and a device that converts reflected light into sound.
The method is a variation of photoacoustic spectroscopy but overcomes several problems associated with this technique originally demonstrated by Alexander Graham Bell in the late 1880s. Most notably, ORNL researchers are able to probe and identify materials in open air instead of having to introduce a pressurized chamber, which renders photoacoustic spectroscopy virtually useless for security and military applications.
The technique detailed in Applied Physics Letters 92 by researcher Charles Van Neste and colleagues, involves illuminating the target sample with an eye-safe pulsed laser and detecting the scattered light with a quartz crystal tuning fork.1 The laser is an infrared quantum-cascade laser from Daylight Solutions (Poway, CA), tunable from 9257.98 to 9804.07 nm in 0.01 nm increments.
"We match the pulse frequency of the illuminating light with the mechanical resonant frequency of the quartz crystal tuning fork, generating acoustic waves at the tuning fork's air-surface interface," said Van Neste of ORNL's Biosciences Division. "This produces pressures that drive the tuning fork into resonance." The amplitude of this vibration is proportional to the intensity of the scattered light beam falling on the tuning fork, which because of the nature of quartz creates a piezoelectric voltage.
Van Neste and co-authors Larry Senesac and Thomas Thundat note that other advantages of quartz tuning fork resonators include compact size, low cost, commercial availability, and the ability to operate in the field.
For their experiments, researchers used tributyl phosphate and three explosives--cyclotrimethylenetrinitromine, trinitrotoluene, commonly known as TNT, and pentaerythritol tetranitrate. They were able to detect trace residues with lasers 100 times less powerful than those of competing technologies.
While the researchers have been able to detect explosives at 20 m using larger collection mirrors and stronger illumination sources, they believe they can achieve detection at distances approaching 100 m.
This research was funded by DOE's Office of Nonproliferation Research and Development and the Office of Naval Research. UT-Battelle manages Oak Ridge National Laboratory for the Department of Energy.
1. C. W. Van Neste et al., Appl. Phys. Lett. 92, 234102, (2008).