Photo-acoustics to detect explosives

After having just reported on a new and novel use of the photo-acoustic effect for use in the printing industry (Photo-acoustics measure ink thickness), here's another story that's (cliché alert!) hot of the press: Researchers at Oak Ridge National Laboratory and the University of Tennessee have used the effect to demonstrate a new way of remotely detecting explosives¹.

The novel approach they took was to illuminate a remote target with pulsed laser light and then expose a quartz crystal tuning fork (QCTF) to the light reflected from it. The reflected light absorbed by the QCTF generates pressure waves at the air/surface interface and thereby causes it to resonate. These devices are highly sensitive and do not require microphones to detect the sound produced by the photo-acoustic effect, they instead use the piezoelectric voltage produced by the quartz when it is driven into oscillation. As the intensity of light absorbed at its surface increases, the amplitude of oscillation and the voltage across it also increases.

More conventional uses of the photo-acoustic effect are concerned with the absorption characteristics of the target and require highly sensitive microphones to be situated close to it. This new approach, however, allows both the light source and the detection equipment to be situated away from the target.

A quantum cascade laser with tuneable wavelength emission was used and, to maximise the light collection efficiency, a 20 cm diameter spherical mirror was used to capture the light scattered from the target and to reflect it onto the QCTF. If a substance in the target region is strongly absorbing at a given wavelength, the light scatter from the target will decrease at that wavelength, resulting in a drop in voltage. By scanning across the operational wavelength range of the laser, which was approximately 9.3 to 9.8 μm, a spectral signature of the target sample could be recorded from the photo-acoustic response of the QCTF.

Stand-off distances from the target to the laser and the detection apparatus, ranged from 0.5 to 20 m. The target itself was a stainless steel plate on which samples were placed for testing. A spectrum of the clean plate was recorded to provide a background measure, against which spectra of the target containing sample substances could be compared. Samples of explosive materials were detected by their unique reflection characteristics and the researchers reported a detection limit of around 100 ng/cm².

This research demonstrates an effective form of remote sensing which does not require the illumination source or the detector to be near to the target. The operating distance could be increased by using a higher power laser source (the wavelength emissions from these lasers are considered eye safe) or by employing a larger collecting mirror.

In a security application, this technique might enable the equipment to be situated out of sight and yet operate over a wide area. Used in airport security, remote optical detectors could do away with invasive searches and the need to swab or vacuum laptop computers for evidence of trace explosives. Other promising applications for technology that can remotely sense explosive materials include the detection of roadside bombs and the identification of potential suicide bombers.

1) Van Neste, C.W., Senesac, L.R., Thundat, T. (2008). Standoff photoacoustic spectroscopy. Applied Physics Letters, 92(23), 234102. DOI: 10.1063/1.2945288