Until the Rensselaer/Rochester work, standoff THz sensing of explosives was difficult due to fundamental limitations in the technology caused by ambient water vapor in the air, which disturbs THz radiation. Clough, Liu and Zhang added a new twist: their technology uses fluorescence and sound waves from a plasma laser to boost the effective distance of THz spectroscopy from a few feet to many meters. Clough says what’s exciting is that the laser plasma is now being used both as a THz emitter and detector. “This means the device uses sound waves to remotely “see” or “hear” the THz signals,” he says.
It works this way. By focusing two laser beams into the air, small bursts of plasma are created, which in turn creates THz pulses. Another pair of lasers is aimed near the target to create a second plasma for detecting THz pulses after they have interacted with the suspect material. The detection plasma produces acoustic and fluorescence waves as it ionizes the air.
Clough discovered that by using a sensitive microphone to “listen” to the plasma, he could detect THz wave information embedded in these sound waves. This audio information can then be converted into digital data and instantly checked against a library of known THz “fingerprints” to determine the chemical composition of the unknown material.
Clough said that although the technology is extremely new and remains relatively unexplored, “with the fast-paced evolution of laser technology, and the mature acoustic and optical detection technology already available, these sensing methods have potential for commercialization.”
Some recent explosive detection techniques focus on chemical identification, but require close proximity to the object in question.
Desorption electrospray ionization, for instance, is an ambient ionization technique that can be used in mass spectrometry for chemical analysis. DESI, developed a few years ago by Purdue chemistry professor Graham Cooks, and now available commercially from Prosolia in Indianapolis, is used mostly for close-quarter, direct contact detection of trace levels of explosives from ranges on the order of 10 mm, with no surface preparation necessary. DESI works by directing charged droplets produced from a pneumatically-assisted electrospray onto a surface to be analyzed at atmospheric conditions. Aside from explosive agents, DESI is also effective at rapidly detecting chemical warfare simulants from a variety of insulating or conducting surfaces, from human skin to leather car seats.
Other detection methods use imaging technologies to screen for explosive threats. German researchers at the Fraunhofer Institute for Applied Solid State Physics are currently developing an optical standoff explosive detection technique capable of revealing surface trace contaminations. This method uses a compact mid-infrared laser that can be tuned rapidly over the wavelength range of interest. The German scheme, called IRLDEX, or Infrared Laser-based Imaging Detection of Explosives, is capable of detecting at a distance of 5 to 25 meters the slightest trace of explosives on suspect surfaces, such as might be left behind by fingerprints.
Scientists at NASA’s Jet Propulsion Laboratory reported in the IEEE Transactions on Terahertz Science and Technology (Sept. 2011) they have a standoff device for screening humans based on THz imaging radar. The device is capable of rapid through-clothing imaging of concealed objects, including explosives, weapons, and contraband. In 2010, the team successfully extended the imaging radar’s standoff capabilities from four meters to 25 meters, while reducing imaging times from two minutes to less than one second.
Princeton Lightwave of Cranbury, New Jersey was recently awarded a patent on a novel shortwave infrared imaging technology that achieves standoff explosive detection distances of 25 to 50 meters, and possibly more. This method (called the Solid-State Hyperspectral Imager for Realtime Standoff Explosives Detection using Shortwave Infrared Imaging) is based on measuring the reflected short-wave infrared radiation on an imaged scene and comparing it to the reflected response of known explosive materials.
Princeton Lightwave technical program manager Bora Onat says testing demonstrates detection probabilities of 90 percent, with a corresponding false alarm rate of 10 percent. The technology is compact, lightweight, low-power, and eye-safe since it does not require high energy laser beams to ionize target particles. It is designed to provide inconspicuous operation at checkpoints or for mounted area or crowd surveillance.