Explosion

Lasers make the tricorder dream possible

The laser is on its way to realising the tricorder.

It's so easy for the crew of the USS Enterprise. If they want to know what something is made of, all they have to do is aim a tricorder at it and all is revealed. Back in the real world, working out whether you are faced with a bomb or a bag of cement is a lot harder and more dangerous.

The conflicts in Iraq and Afghanistan have shown how effective the so-called improvised explosive device (IED) can be. It's easy to make and hard to detect. So, the military and security services are looking to sensing technology to give them tricorders for finding telltale chemicals that indicate whether the barrel on the road is just a barrel or something far more deadly. The problem is being able to detect without getting into range of a bomb and the laser, which turns 50 this month, is fast becoming the technology of choice for what the military call 'standoff sensors'.

The laser has one prime advantage: you can bounce the light off things - and you need not be restricted to visible light. A number of R&D teams are focusing on the mid-infrared region of the electromagnetic spectrum because it can provide a wealth of information about chemical composition.

Organic chemists have used the mid-infrared region for decades to identify molecules. Many of the common bonds found in organic chemicals vibrate in the mid-infrared region, absorbing the radiation strongly at specific frequencies. The 'fingerprint region' - covering wavelengths from 5m to 20m - usefully pinpoints specific chemicals because each one has a specific absorption profile. It's why IR spectroscopy is one of the uncredited stars of programmes like CSI.

Lasers and spectroscopy are not good companions. Many spectroscopic techniques sweep through the spectrum. But most laser technologies use combinations of materials to generate very narrowly grouped wavelengths of light.

Tuning is possible. Chemists have been able to use tunable lasers, albeit room-sized, for techniques such as Raman spectroscopy - a complementary technique to infrared spectroscopy - since the early 1980s.

Tunable lasers use gratings or mirrors to select wavelengths that the core laser can generate. Sweeping through a frequency range is more tricky but is possible with new generations of semiconductor lasers such as vertical cavity surface emitting lasers (VCSELs) and quantum-cascade lasers.

In quantum-cascade laser, molecular beams are used to put down alloys in very thin layers to create a series of quantum wells, each one slightly lower in energy than the one before. The thickness of the well controls the frequency of the light from the electrons as they lose energy. As they descend the staircase of energy states, one electron can produce many photons before it moves out of the laser. Longer wavelengths are trickier because they demand very thin layers but the quantum-cascade laser has turned out to be a good candidate for work in the mid-infrared and even down into the terahertz region.

Heat pulse

Scotland-based Cascade Technologies uses a pulse of heat to control how their quantum-cascade laser sweeps through a range of wavelengths. Although the range a single sweep can cover is smaller than that of a traditional chemical spectrometer, it is possible to cover regions of the mid-infrared fingerprint zone to identify specific molecules.

Erwan Normand, chief scientific officer, explained at a recent Nanotechnology Knowledge Transfer Network seminar: 'If you apply a current pulse to the quantum-cascade laser for about 1s, the heat rise from the current causes a shift in the wavelength of the laser. You can scan two, three, even ten wavenumbers in one shift.'

In the mid-infrared region, a wavenumber corresponds to a wavelength of about 0.01m. But the shift is enough to let an image sensor capture a fingerprint to help identify gases in the atmosphere that the emitted light passes through.

'It's very rapid,' says Normand. 'You can do 100,000 scans in one second, so you get a very good signal-to-noise ratio.'

To identify chemicals, software fits the data from a sensor that receives the reflected or transmitted light to those molecules' predicted absorption curves, so that it won't trigger if a similar gas is picked up. The system has picked up applications in industry, looking for carbon dioxide in glass production or leaks from aerosol cans.

Cascade is looking beyond these controlled environments. 'We can do true standoff detection,' says Normand, pointing to the detection of IEDs as one possible use for the laser-based system. Soldiers and police could, potentially, be tens of metres away from a suspect package while they scan it.

Working with Sagem, the Cascade system has picked up the signature of hydrogen peroxide gas, which is emitted by some homemade explosives. 'It can pick up parts per billion of peroxide,' says Normand. In trials, 'the target lit up like a Christmas tree', he adds. Trials continue and the companies are looking at doing a trial of a gas detector in an airport next year.

Some mid-infrared laser-based systems are close to deployment. Northrop-Grumman started test flights for its ASTAMIDS mine detector in the summer of 2008, ten years after almost killing the project off for lack of funding. The system scans for landmines, using the difference in reflections from manmade objects compared to the ground around them.

With laser-illuminated standoff sensing, results can be picked up as images or video if an imaging sensor is tuned to sense the right wavelength range. It can pick the absorption up as 'smoke', says Graeme Malcolm, CEO of M-Squared as he plays a demo recorded at St Andrews University that shows a researcher armed with a length of pipe that is spewing out small amounts of methane. When tuned to one of methane's strong absorption frequencies - around 3.35m in this case - the researcher's dark shirt turns white, with the methane billowing around as smoke. E

F M-Squared is using research originally conducted at the University of St Andrews to massively increase the spectral range of individual lasers, even into the terahertz range, which lies in between microwaves and the infrared region. Researchers have called it the terahertz gap because there are no efficient, simple emitters of these waves.

However, the terahertz region is potentially very useful for both diagnostics and detection as it can peer through coverings in ways that infrared waves cannot. On top of that, it is possible to probe chemical composition. But a lot of the promise of terahertz lies in its relative novelty - undiscovered problems may still be lurking. So far only astronomers have exploited terahertz to a large extent, using the 300GHz to 1THz region where water does not absorb the radiation heavily.

Several years ago, the Home Office provided a group working at the University of Leeds with samples of explosives to see if a terahertz detector could pick them out. Professor John Cunningham says the experimental scanners could see peaks from individual chemicals, such as ROX and PETN, commonly used in plastic explosive.

Earlier this year, UK-based Teraview claimed its tests could identify PETN underneath clothing - arguing that its system could have identified the Detroit bomber at the airport had the system been deployed.

Professor Sir Michael Pepper, chief scientific officer at Teraview, says: 'What is required is an additional level of development to create automated and field-deployable systems that can address some of the challenges illustrated by this recent incident.'

Explosives

Since its initial work, the Leeds team has been funded by the UK Home Office to investigate how well terahertz radiation can pinpoint chemicals such as explosives. There is a potential catch: terahertz radiation probes the weak bonds between individual molecules, not the strong, higher-energy bonds that hold them together. Different mixtures of chemicals could throw off a scanner.

But Cunningham says more recent work with different mixtures of chemicals have shown comparatively small changes in the fingerprint spectra. 'There are usually other things mixed in such as plasticisers and dyes but they tend to have weak absorption spectra. The explosives generate the spectra rather than the additives. You can identify the constituents from the overall spectra,' he explains. 'It tends to be that the explosive retains a more crystalline structure within the overall mixture, so that the explosive part is still crystalline. As long as the crystallinity is maintained, you can identify it.'

Amorphous phases tend to have much weaker absorption spectra, Cunningham claims, but the team is working to uncover what controls terahertz absorption. 'We are trying to understand the spectra by modelling the intermolecular vibrational modes within explosives,' he says.

Impurities affecting a signature provide an opportunity for this kind of scanner. Teraview has promoted its scanners to pharmaceutical companies keen to identify counterfeit drugs. As they often have different levels of impurity, if they even contain the target drug at all, counterfeits should show up quickly.

In most cases, the laser plays an indirect role in generating terahertz radiation in practical instruments. Many involve firing a laser at a crystal, such as gallium selenide or lithium niobate. Some techniques involve mixing the output of two lasers, again through a crystal.

The approach employed by M-Squared uses the crystal to split a single laser signal into two lower-frequency waves, one of which is the terahertz wave output. The other is the 'idler'. By altering the angle of the light from the pump laser and the idler, it's possible to generate terahertz radiation over a wide range. The tuning process can be quite slow, so more recent work at St Andrews has focused on using inferometers to select narrow frequency bands from the broader output generated by the main terahertz generator.

M-Squared has applied a similar approach to generating a wider range of mid-infrared wavelengths to improve the overall frequency range of the laser-based system.

The team at Leeds is currently performing experiments with quantum-cascade lasers generating terahertz light. As with mid-infrared lasers, tunability is limited. Cunningham says the team is trying to assess whether this more limited range than is currently possible with indirect generation can still yield effective chemical detectors. 'We are working with the Home Office to identify how many modes you need before you can identify it as an explosive,' he says.

One advantage of the quantum-cascade laser over indirect generation, says Professor Edmund Linfield of the University of Leeds, is that the narrow linewidth of light it produces lends itself to heterodyne detection, a technique commonly used to identify gases in astronomy and other fields.

The first deployable standoff sensors are unlikely to fit easily into a hand but with new generations of solid-state lasers, a shoebox-sized instrument seems possible in the short term.

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