Abstract-Toward Standoff Sensing of CBRN with THz Waves

• Kang Liu, 
• Xi-Cheng Zhang
• • 
    

https://link.springer.com/chapter/10.1007/978-94-024-1093-8_1

With the soaring demands for remote spectroscopy in homeland security and environmental monitoring, terahertz (THz) wave sensing has drawn a significant amount of attention because of its capability to acquire chemical spectral signatures non-invasively. THz-Radiation-Enhanced-Emission-of-Fluorescence (THz-REEF) makes broadband stand-off remote spectroscopy possible, due to its omni-directional emission pattern of the fluorescence and minimal ambient water vapor absorption of THz wave near the target. Two years ago, we have demonstrated coherent THz wave detection at a distance of 30 m. The development of intense broadband table-top THz source is imperative to all the extreme THz science research, including THz remote sensing. Recently, we have also demonstrated an enhanced THz radiation using dual-color ring-Airy beam induced air-plasma compared to a Gaussian beam induced plasma. It is believed that the great tunability of ring-Airy beam, its wavepacket stability at nonlinear focus regime, and the strong resilience to turbulent medium of the Airy beam family, make the plasma induced by this novel wave a promising broadband THz source for THz remote sensing and spectroscopy.

Can higher powers open windows for terahertz security?

Andy Extance investigates the new sources that could broaden adoption of terahertz imaging

http://www.electrooptics.com/features/feature.php?feature_id=388

Electro Optics: April 2016

As of today, terahertz imaging and sensing is yet to fully live up to the hopes that potential users in security applications might have had for it. The term terahertz became popular among spectroscopists referring to the electromagnetic spectrum between the infrared and microwave in the 1970s. Interest from researchers in using technology producing and detecting such light for identifying hidden threats grew through the 2000s, as better terahertz sources became available. The promise they saw is today being exploited in a few airport scanners – but that’s only a very narrow niche. However, higher-power sources have entered the market; could they drive broader adoption?  

Terahertz’s appeal comes because it interacts with matter differently to other types of light. Whereas infrared, for example, induces molecules to perform bending and stretching motions, terahertz light causes collective motions of groups of polar molecules like water. Consequently, it’s potentially great for detecting what others want to keep from you. Because it’s not absorbed by non-polar materials like cardboard and clothes, terahertz offers similar capabilities for imaging what’s within things to X-rays, but without the health risks. Explosives, chemical or biological weapons do absorb terahertz light, giving unique spectroscopic ‘fingerprints’.  

Yet to be practical, security systems have to be fast, reliable, robust, and reasonably inexpensive. Combining all these requirements has proven ‘difficult to achieve’, explained Anselm Deninger, director for terahertz technologies at Toptica Photonics in Munich, Germany. ‘With most commercial terahertz systems it takes at least a minute to record a high-quality spectrum,’ Deninger said. ‘If you want to scan hundreds of envelopes per hour, this is clearly too slow.’ Cost is also not amenable to wide deployment, he added.

Consequently, the significance of the non-destructive testing market for Toptica’s terahertz systems is greater than the market for security applications. ‘Via time-of-flight techniques, pulsed terahertz systems can quantify the thickness of paint layers, or wall thickness of plastic pipes or bottles,’ Deninger said. ‘This is a much more dynamic field right now. I do believe that the terahertz market will grow. One might debate whether or not the growth will be driven by defence and security – in my view, likely not.’

Toptica produces two complementary terahertz system brands: TeraFlash and TeraScan, with maximum output power of 65µW. TeraFlash is a pulsed system, based on the company’s FemtoFErb, a 1,560 femtosecond pulsed erbium fibre laser. The laser pulse is split in two; one part travels to a semiconductor-based terahertz emitter, creating a terahertz beam that then interacts with a sample before travelling to the detector. The other part serves as a ’readout’ pulse at the detector where it samples the incident terahertz field, much like a sampling oscilloscope does. Deninger noted that Teraflash can offer either very broad bandwidth, covering frequencies as high as 6THz, or speed, recording a complete spectrum in 20ms. ‘Of course, we trade measurement speed for spectral bandwidth – this is true for any terahertz system, but even at maximum speed, we still obtain an impressive signal,’ he said. 

TeraScan combines beams from two distributed feedback (DFB) semiconductor lasers, obtaining continuous wave terahertz light whose frequency is the difference between the two input lasers. Its main advantage is the spectral resolution, distinguishing absorption lines down to single megahertz amid the terahertz range. ‘In both systems, the dynamic range of the terahertz power is very high,’ Deninger said. ‘Thus, you will still see a signal in the case of highly absorbing samples, within physical limits, of course.’

Toptica’s DFB lasers were used in a terahertz system designed to identify toxic chemicals by Goodrich ISR Systems in Danbury, Connecticut. ‘This worked well, but for reasons not disclosed to us, the project was discontinued,’ Deninger commented. Now, his company is taking part in a German consortium – including the city of Mannheim’s fire brigade – looking at trace gas detection. ‘They are keen to identify gases released in an industrial disaster, such as a fire in a factory. This has a direct impact on the protective gear firefighters need. We still need to work out how we best bring gas samples to the spectrometer, but the new TeraScan seems to be a great instrument, owing to its signal quality, spectral resolution, and frequency repeatability. The project has already detected gas on the parts-per-million level.’

The need to bring samples to the spectrometer highlights an inherent challenge facing this technology. ‘Terahertz light is strongly attenuated by water vapour – omnipresent in air – so remote sensing simply does not work: after a few metres, only selected “window frequencies” survive,’ Deninger explained. This is not sufficient for ‘stand-off spectroscopy’ applications detecting threats at a distance, he added. ‘I have talked to people who wanted to detect buried land mines with the help of terahertz light, and I had to tell them terahertz rays will not pass through soil, so this does not work either. Applications that I consider realistic include detecting trace amounts of toxic gases in public places, buildings, or subway stations. Also, the spectroscopic analysis of mail envelopes seems feasible. This does not require extended path lengths, and paper is reasonably transparent, so one might be able to check envelopes for explosives, or illicit drugs inside.’

More power to them

Alan Lee, who co-founded Mountain View, California’s Longwave Photonics in 2010, agreed that security and defence applications using terahertz spectroscopy technology, although interesting and very promising, ‘are still quite limited’. ‘For now, the need for high resolution terahertz spectroscopy is still mainly driven by laboratory and industrial internal R&D,’ he said. ‘However, with the maturation of terahertz technology and with more researchers adopting our high-quality Easy-QCL source, I can see a strong need might emerge in this area.’

Longwave’s semiconductor quantum cascade laser (QCL) is currently a test platform for research and development, enabling applications in the 2 to 5THz frequency range to be explored. These include homing in on the narrow frequency windows where terahertz light does propagate through atmospheric moisture. ‘One large challenge is producing lasers that operate in these windows, which we can do by creating DFB lasers for specific frequencies,’ Lee explained. ‘We’ve been able to develop DFB devices that have milliwatt average power levels with nice beam patterns and single frequency operation. Milliwatt power is also sufficient to use room temperature direct detectors like pyroelectric detectors and microbolometer focal plane arrays for real-time imaging. Otherwise, terahertz sources that produce microwatts of average power must typically use either a more complicated heterodyne detection technique or a high-sensitivity liquid helium cooled detector.’   

However, confining the electrons responsible for emitting the terahertz light in semiconductor QCL structures also requires very low temperatures. ‘To date, the maximum operating temperature of a terahertz QCL is 200K, but they really work best below liquid nitrogen temperatures of 77K,’ Lee explained. Yet Longwave packs its lasers in compact pulse-tube cryocoolers that provide closed-cycle refrigeration, without needing cryogens like liquid nitrogen or even – they claim – maintenance. ‘We’ve made these systems to be flexible so that researchers can exchange laser modules to access different parts of the 2 to 5THz frequency range, or take advantage of DFB or even higher power Fabry-Perot devices,’ Lee added.  

This technology allows spectroscopy at a distance, such as in remote sensing of atmospheric gases and observing emission from molecular gases in astronomy. ‘We recently prepared a device that was made at MIT for use on the STO-2 NASA balloon mission in Antarctica,’ Lee said. The final instrument, assembled by the Netherlands Institute for Space Research (SRON), will use a 4.74THz QCL to provide some of the first observations of emission from neutral oxygen. 

Lee is optimistic that similar capabilities can help in security applications. ‘I would argue that the ultra-high frequency resolution and high power of our terahertz source could lead to an advanced buried explosive detection system,’ he said. ‘Not only could it generate a binary, true or false result, but the ability to do fine spectrum analysis would reveal a chemical fingerprint which is crucial for explosive identification.’

Working on the terahertz image

Higher-power QCLs could also enhance the prospects of replacing X-ray security imaging, according to Pierre Gellie co-founder of Paris, France’s Lytid. Lytid, which was spun out of Paris Diderot University in 2015, also offers a milliwatt-output ‘TeraCascade’ QCL. It emits specifically at 2.5THz, but provides both continuous wave and pulsed operation from the same system. Higher power sources can illuminate more pixels on a detector, Gellie emphasised. ‘That’s very interesting in imaging – you could perform several million measurements per second,’ he said. ‘That could enable terahertz cameras producing several hundred thousand signals at one time and several tens of images per second. Eventually scanners could work quickly.’

Closed-cycle, ‘maintenance-free’ refrigeration is also included in Lytid’s TeraCascade QCL systems. ‘Our market study shows people want a terahertz source with higher power that is very easy,’ Gellie said. ‘We’ve worked on integration and user-friendliness a lot – it’s easier to use than a smartphone. It’s one press on the touch screen, you wait 20 minutes for it to cool down and then you’re ready to go. You have all you need in a single box – a cooler, all the electronics to drive the cooler and the quantum cascade laser. That’s why it’s bulkier than just a diode laser – and also makes the system more expensive. Obviously it has to be as transparent as possible for the end user in any real-world application. This is what we’re aiming for, and what’s missing in other sources on the market.’

Having been established so recently, Lytid is initially targeting the better-developed industrial non-destructive testing market, where it provides real-time terahertz imaging. Currently that’s the greatest interest in TeraCascade from the defence industry, Gellie explained. ‘It’s a tool for thickness measurements, finding faults in very high grade designs in aerospace engines, and also on armoured vehicles. Of course, going towards industrial applications, having a fully integrated system, the most reliable components are a must-have. But they don’t need to worry about the source any more – they can focus on their application.’ TeraCascade won a Prism Award in the Scientific Lasers category at Photonics West 2016, Gellie added.

TeraCascade’s reliability would be well suited to use airport scanners, Gellie suggested – although even at the milliwatt level it doesn’t yet have the necessary power. ‘Airport scanners now use millimetre wave technology that provides low-resolution image patches and can only detect small areas,’ he explained. Consequently, security applications either require manual scanning or systems using emitter and receiver arrays. ‘With terahertz you could have much higher resolution – you could actually see the proper shape of the object,’ Gellie explained. ‘But nowadays it’s difficult to implement; there are few sources, they are not powerful enough and also detectors are not quite sensitive enough. You’re talking about needing 1W of power with actual receiver technology today. If they improve too you might be able to do something with a few tens of milliwatts of power.’

Lytid is working on power improvements, but the need for better detectors highlights the key barrier to broader uptake. ‘For terahertz technology to go mainstream you have to do more on components and the whole system,’ Gellie said. ‘There are no terahertz optical fibres right now, there probably won’t be any time soon. We hope that there are some advances still to be made. ’ Similarly to how Lytid came to commercialise the TeraCascade, he feels that those advances are most likely to involve technology transfer from academic labs. 

Producing systems that operate at other frequencies is another area that Lytid would like to explore. Gellie believes this would take terahertz imaging in a highly desirable direction for security use.  ‘Using different frequencies you can go towards spectroscopic imaging,’ he explained. ‘That would be the holy grail for this application, not just being able to detect what kind of object is hidden, but also the chemical composition, for example finding explosives and drugs. This has been shown in academic publications – it’s still in an infant stage, but it’s very promising. It will come.’ 

_{Andy Extance is a freelance science writer based in Exeter, UK}

Could screening deter future train terror attacks?

http://www.securitynewsdesk.com/could-screening-deter-future-train-terror-attacks/

In the aftermath of the August 2015 incident in France, Tim Compston, features editor at SecurityNewsDesk, asks if there is anything that can be done to deter future attacks?

The ‘lone wolf’ attack on a high speed train travelling from Amsterdam to Paris in August 2015 – which was stopped in its tracks by the brave actions of those on board – underlines just how vulnerable rail passengers are whilst on the move. Worryingly, it appears that the suspect was able to join the service in Brussels without raising any suspicions and, allegedly, was carrying a concealed hand gun, assault rifle with 270 rounds of ammunition, a box-cutter and petrol.

So what can the authorities and rail operators do to respond to the heightened threat level? Well, for his part, the French interior minister, Bernard Cazeneuve – following discussions with European ministers – has already announced a series of measures, including more identity and baggage inspections and police patrols. Also under consideration is name-based ticketing on international trains.

ThruVision, Digital Barriers

Logistical challenges
On whether mass passenger screening, like that at airports, could be viable, Mike O’Neill, managing director, of UK-based Optimal Risk Management Limited, feels that the logistics involved simply make it a non-starter. This echoes the sentiments of Guillaume Pepy, president of SNCF, who pointed out that in France there are 20 times as many rail passengers as air passengers.

O’Neill goes on to say that more limited action may be possible: “You could have spot checks and even passenger profiling in terms of looking at behavior and the bags they are carrying.” He adds that the fact most weapons break-down to fairly small pieces makes life even more difficult. For O’Neill it is the work ‘behind-the-scenes’ by the security services to identify individuals ahead of time which is the game changer: “It is has to be intelligence-led.”

Stand-off screening
Turning to Mark Marriage, transport solutions specialist at Digital Barriers, he suggests that, for concealed object detection, compact ‘stand-off’ people screening could offer a solution: “We use a non-invasive screening method which you just walk past.” Marriage explains that it employs not only CCTV but also a terahertz camera to receive and interpret the natural terahertz energy emitted by individuals and the environment.

ThruVision, Digital Barriers

Real-time video
Another challenge for rail operators who have cameras on their trains, and staff being kitted out with body worn video, is how to transmit footage back in a consistent way to an external control room. Some security experts like Optimal Risk’s Mike O’Neill argue that as events unfold so quickly there is little that such video footage can add to how an incident plays out: “It is for evidential use, not really for preventative use,” says O’Neill.

In the view of Simon Jenkins, product manager for TransVu at AD Group, while it is theoretically possible to take footage from a fast moving train, in practice it is probably of fairly low quality and not necessarily in real-time because existing connections aren’t reliable enough and don’t provide sufficient bandwidth: “No one is taking video off moving trains at the moment,” concludes Jenkins.

Offering his thoughts on this subject, Mark Marriage at Digital Barriers believes that, despite the pitfalls highlighted, technology with its roots in military operations could make real-time streaming from trains a practical proposition. He explains that a compression algorithm, developed by Digital Barriers, allows the optimising of video to the available bandwidth so avoiding latency and break-up.

Adding analytics
Considering other innovations in the context of rail security, Dr Rustom Kanga, ceo at Omniscient, is an enthusiastic advocate of the intelligent use of video surveillance: “You can do face recognition of people of interest in a crowd.” Dr Kanga says that for railway stations it is feasible to recognise people on a suspect list at say 20 metres distance.

Added to this, Dr Kanga reckons that flagging-up an abandoned bag in a crowded scene is much easier now thanks to ‘non-motion detection’: “In a railway station where everything is moving it makes more sense to apply ‘non-motion detection’ which focuses on the things that have actually stopped moving.”

So to conclude, we are likely to witness an even greater emphasis on intelligence-led efforts, the extension of spot checks of individuals and their bags across Europe, and new and enhanced technologies – like stand-off screening – coming down the track.

Electronic terahertz imaging for security applications

Janez Trontelj and Aleksander Sešek

A compact system for 2D and 3D sensing has been built using semiconductor and microelectromechanical systems components.

http://spie.org/newsroom/technical-articles/6300-electronic-terahertz-imaging-for-security-applications?highlight=x2410&ArticleID=x116952

15 January 2016, SPIE Newsroom. DOI: 10.1117/2.1201601.006300

The terahertz (THz) frequency band of the electromagnetic spectrum (i.e., below 1THz) is used in security applications mainly for detecting hidden hazardous objects. Barrier materials that hide dangerous items carried by potential terrorists, however, are normally clothes and are semi-transparent to THz waves. There is also a need to investigate the contents of different types of packages, such as those that are shipped by mail or in other cargo transports. In addition, frequencies at the lower end of the THz region (or even lower, at the high end of the microwave range) can be used to detect hidden objects underground, or those that are hidden behind brick or concrete walls.

As well as security applications, THz systems are also of interest for industrial and health purposes.1 Such THz systems—constructed with solid-state, rather than optical, components—have three main parts. First, a THz source for illumination of the scene is required. Second, a THz detector for measurement of the received THz signal must be included. Finally, a signal processing unit is needed to amplify and analyze the input THz signal. There are many fully integrated microwave sources available that provide a suitable drive to a frequency multiplier for these purposes. Moreover, advances in semiconductor technology have led to the promise of integrated devices that operate at frequencies close to 1THz.2, 3

In this work, we present a THz system that is based completely on solid-state electronic components that we have fabricated with a mature, well-proven, stable, and low-cost mass production technology.4 We built our THz source from a solid-state frequency multiplier5, 6 and an integrated microwave source, which exhibits a high frequency stability and low phase noise. The most important parameters that need to be considered for the THz sources are the output power, illumination beam size, and directivity. The latter depends on the chosen THz antennas and lenses, frequency modulation range, and low phase noise. The optimization of these parameters was thus an important part of our THz system development.

The THz system that we have developed operates with a frequency-modulated continuous-wave THz illumination source. The detection principle for our system is based on THz mixing of the illumination beam and the reflected beam from the target. Since there is a difference in the travel length of the two beams, the result of the mixing is a low-frequency beam that is linearly related to the modulation rate. Our system has a modulation rate of 148GHz/s, which produces a low-frequency mixing product of 1kHz per 1m difference in the traveling path of the illumination and reflected beams. We produce the local oscillator (LO) in our THz system with the use of a beam splitter that directs 40% of the transmitted power to the sensor. Our system performance depends critically on this LO generation and its connection to the mixer.

The single most important element of our system is the THz sensor. For this, we use an antenna-coupled nano-bolometer microelectromechanical systems (MEMS) device, which is fabricated in a linear array of eight pixels. We use the sensor to perform signal detection, as well as simultaneous mixing of the LO signal and received signal from the target. To achieve excellent sensitivity and signal-to-noise (S/N) ratios, we suspend the THz sensors in a vacuum. We have thus measured a sensor sensitivity of 1000V/W, with a noise-equivalent power of about 5pW/p Hz. We are able to develop and fabricate narrow-band and wide-band antennas for specific applications. For our sensors, we most often use nano-bolometers with narrow-band antennas (central frequency and bandwidth of 300 and 40GHz, respectively) and wide-band antennas in the 0.1–1THz frequency range. Two 16-pixel THz sensor arrays that operate at 300 and 600GHz, as well as a four-pixel array with an application-specific integrated circuit for front-end signal processing, are shown in Figure 1. Both of these sensor arrays were fabricated using silicon MEMS and a CMOS process. The excellent performance and S/N ratio of our sensors are reflected in the dynamic range we can achieve with our THz system. We obtain a dynamic range of up to 90dB for standoff targets at distances of about 3m, in open fields and harsh atmospheric conditions (e.g., rain, fog, snow).

 

Figure 1. Photographs of the dual-frequency terahertz (THz) sensor system. Left: Two arrays, each of 16 sensors, which operate at central frequencies of 0.3 and 0.6THz (with bandwidths of 10 and 60GHz, respectively). Right: An array of four wide-band sensors (0.2–1THz), with a dedicated four-channel application-specific integrated circuit for front-end signal processing.

We have used our THz system to acquire the images shown in Figure 2. The 2D image includes information about the intensity of the beam reflected from the object, whereas the 3D image also provides information regarding the object's position in space. From both of these images, much can be learned about the physical and material properties of the target. We use the nano-bolometric quadratic characteristic as a mixer of the transmitted and received THz beams. The resultant signal provides the data required for 2D imaging (i.e., amplitude, and x, y position), as well as the distance of the reflected signal (needed to image the third dimension).

 

Figure 2. Images obtained with the THz system. Left: A 2D image of a coil wound on a plastic reel and packaged in a box. Right: A 3D THz image of a cigarette lighter that is half-filled with gas (inset shows a photograph of object).

With the high sensitivity of our THz sensor, we can obtain depth resolution down to a few micrometers. This capability therefore opens up a number of applications, e.g., positioning systems and tomography imaging of materials that are transparent or semi-transparent at THz frequencies. Such tomography imaging of objects is illustrated in Figure 3. The objects shown are packaged within visually opaque materials, such as plastic wraps, paperboards, and cartons. Furthermore, we have developed a miniature version of our THz sensor so that it can be easily embedded into larger systems or at a high level of integration within a computer system. Our mini-THz sensor array (see Figure 4) can be used to detect a THz beam or to capture reflection/transmission THz images in real time. It incorporates all the necessary electronics, a data processing module, and a USB-to-serial driver that facilitates the information stream of the captured THz beam.

 

Figure 3. THz tomography imaging result for several objects that are transparent or semi-transparent at THz frequencies. The objects shown in the upper panel include (top left and bottom right) parts of thin silicon wafers (600μm thick), (bottom left) part of a printed circuit board, and (top and middle right) 1 and 2 Euro cent coins. The green multilayer material—Styrofoam—is transparent to THz radiation. The bottom panel shows the THz tomography image of the various objects packaged within visually opaque materials (i.e., expanded polystyrene, plastic wrap, and carton).

 

Figure 4. The miniature THz sensor system. Shown on the right with its USB connection.

We have developed a new THz imaging system that is built entirely from solid-state electronic components. Our system includes a frequency-modulated continuous-wave THz illumination source and antenna-coupled nano-bolometer MEMS sensors. We can use our system to obtain 2D and 3D images, with excellent sensitivity, S/N ratios, and depth resolution. In addition, we have produced a miniature version of our system, which can be embedded within larger infrastructures. Our THz imaging system is thus suitable for several security applications. Our most important future challenge is to develop a miniature, integrated, and efficient THz source to replace the inefficient frequency multipliers. Our preliminary design for such a source shows promising results.

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Janez Trontelj, Aleksander Sešek

Faculty of Electrical Engineering
University of Ljubljana

Ljubljana, Slovenia

Janez Trontelj is a professor and chair of the Microelectronic Technologies and Laboratory for Microelectronics. He is active in the design of magnetic and mechanical microsystems, mainly for the automotive industry. For the last few years, he has led the THz imaging project.

Aleksander Sešek is an assistant in the Laboratory for Microelectronics. He is mainly involved in the design of integrated sensor systems and in the development of THz sensor systems for security, medical, and industrial applications.

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References:

1. P. H. Siegel, Terahertz technology, IEEE Trans. Microwave Theory Techn. 50, p. 910-928, 2002.

2. A. Tessmann, A. Leuther, H. Massler, V. Hurm, M. Kuri, M. Zink, M. Riessle, H. P. Stulz, M. Schlechtweg, O. Ambacher, A 600 GHz low-noise amplifier module, IEEE MTT-S Int'l Microwave Symp., 2014. doi:10.1109/MWSYM.2014.6848456

3. J.-M. Rollin, D. Miller, M. Urteaga, Z. M. Griffith, H. Kazemi, A polystrata®820 mW G-band solid state power amplifier, IEEE Compound Semiconductor Integrated Circuit Symp., 2015. doi:10.1109/CSICS.2015.7314481

4. J. Trontelj, A. Sešek, Electronic FM CW THz system for security applications. Presented at SPIE Photonics West 2016.

5. http://vadiodes.com/index.php/en/products/integrated-mixer-amplifier-multiplier-chainIntegrated mixer, amplifier, multiplier chain from VDI. Accessed 24 December 2015.

6. M. Vigilante, P. Reynaert, An E-band low-noise transformer-coupled quadrature VCO in 40 nm CMOS, Euro. Solid State Circuits Conf. 40, p. 423-426, 2014.doi:10.1109/ESSCIRC.2014.6942112

This Tool Could Sniff Out a Paris Bomb More Than a Football Field Away

National Journal Short

FRIDAY, NOVEMBER 20, 2015

Patrick Tucker, Defense One

http://www.nationaljournal.com/s/102523/this-tool-could-sniff-out-paris-bomb-more-than-football-field-away?mref=scroll

Parisi­an po­lice moved in to ap­pre­hend people sus­pec­ted of launch­ing last week’s ter­ror at­tacks, a wo­man det­on­ated a sui­cide-bomb vest. The re­cent ap­pear­ance of such weapons in West­ern cit­ies has alarmed many in law en­force­ment. One pos­it­ive out­come of the mil­it­ary’s ex­tens­ive ex­per­i­ence with such devices is a new sys­tem that can help guards de­tect a bomb vest from up to 100 meters away.

The new ex­per­i­ment­al set of sensors is dubbed the Stan­doff Sui­cide Bomber De­tec­tion Sys­tem, or SS­BDS. De­veloped by the Joint Im­pro­vised-Threat De­feat Agency, or JIDA, it has already seen ac­tion; in 2012, the De­fense De­part­ment took an early ver­sion to Afgh­anistan.

The SS­BDS is not a single ma­gic lens, but an en­semble of sensors that meas­ure ra­di­ation at the mid­wave and long­wave in­frared as well as the tera­hertz wavelengths. There’s also a vis­ible-light cam­era. Tera­hertz ra­di­ation may sound like something to avoid—but be­cause it uses low en­ergy and isn’t ion­iz­ing, it’s less dan­ger­ous to hu­man tis­sue than x-rays. It has been used as part of se­cur­ity ap­plic­a­tions since 2004.

On Tues­day, JIDA of­fi­cials demon­strated the sys­tem to re­port­ers at Vir­gin­ia’s Fort Bel­voir. When dir­ec­ted at a per­son, the SS­BDS of­fers three views: a grainy black-and-white, which is the in­frared; a bright or­ange glow, which is the tera­hertz; and a reg­u­lar pic­ture from the cam­era.

The pres­ence of a sui­cide belt won’t set off alarms but a trained—or simply at­tent­ive—eye can use the mul­tiple sensors to see dark shapes or spots—areas of neg­at­ive space that in­dic­ate an ab­nor­mal­ity, which should prompt a closer in­vest­ig­a­tion. In es­sence, you’re look­ing not for ex­plos­ives but for holes in the pic­ture where there should be sol­id white or or­ange.

“It’s a per­fect sys­tem for dual use with De­part­ment of Home­land Se­cur­ity. Just think of Par­is,” one JI­DA scient­ist said.

The SS­BDS is not a small sys­tem. Each bank of sensors stands at per­haps three feet tall, and a hand­held ver­sion would be prac­tic­ally im­possible. “You’re lim­ited by phys­ics,” said an­oth­er one of the De­fense De­part­ment sci­ent­ists on the team. But it could be in­cor­por­ated in­to the ar­chi­tec­ture of a train sta­tion, foot­ball sta­di­um, or con­cert ven­ue.

In JIDA’s demon­stra­tion, which was modeled on a real-world test­ing scen­ario, a base in Afgh­anistan, a sub­ject, dressed in a thawb, or white dress­ing gown, stood in a booth. The booth shiel­ded him from sun­light, which can in­ter­fere with the tera­hertz sensor—which points out just one of the dif­fi­culties in turn­ing the SS­BDS in­to a use­ful, de­ploy­able sys­tem.

The next step for SS­BDS, if its re­ques­ted re­search-and-de­vel­op­ment money comes through, is the in­teg­ra­tion of a new, cut­ting-edge sensor for hy­per­spec­tral ima­ging. Take a thermal im­age of the sort you would see with a heat cam­era and then slice it up in­to dif­fer­ent wavelengths, al­low­ing op­er­at­ors to identi­fy not just dark spots but also, po­ten­tially, the pres­ence of ex­plos­ives. That would cut down on false pos­it­ives and in­crease the range of the device.

“We can take dif­fer­ent ma­ter­i­als, such as your jack­et, my jack­et, her shirt. Al­though they’re very sim­il­ar in ma­ter­i­al makeup, they ac­tu­ally have their own unique sig­na­tures, so every piece of ma­ter­i­al has its own sig­na­ture, just like a fin­ger­print,” said a third sci­ent­ist.

The cost of a pro­duc­tion sys­tem could run past a mil­lion dol­lars, but those num­bers could come down as man­u­fac­tur­ing comes up since all of the sensors are com­mer­cial off-the-shelf.

SS­BDS wasn’t in­ten­ded for use in a ci­vil­ian set­ting or a West­ern city but to pro­tect troops on for­ward op­er­at­ing bases. But the need for sys­tems like it has evolved tre­mend­ously—and rap­idly.

On Fri­day even­ing, just 15 minutes in­to a soc­cer match between France and Ger­many, a man wear­ing a vest laced with ex­plos­ives ap­proached one of the en­trances to the St­ade de France, a sta­di­um filled with 80,000 spec­tat­ors in­clud­ing French Pres­id­ent François Hol­lande. When guards frisked him, he backed away and det­on­ated the vest.

New York City Po­lice Com­mis­sion­er Wil­li­am Brat­ton seized on this as an omin­ous sign of the fu­ture: “All of them, were equipped with these sui­cide vests, which are a great con­cern if you’re ask­ing your of­ficers to rush in, which is the tac­tic here in Amer­ica now, [when] re­spond­ing to the act­ive-shoot­er scen­ario,” he said on the pop­u­lar Morn­ing Joe news pro­gram.

No sensor sys­tem can re­place the judg­ment of an in­tel­li­gent and ob­ser­v­ant hu­man be­ing, a be­ing, hope­fully, armed with the best equip­ment. The events last week and this week in Par­is are test­a­ment to that.

Abstract-Discrimination and identification of RDX/PETN explosives by chemometrics applied to terahertz time-domain spectral imaging

J. Bou-Sleiman, J.-B. Perraud, J.-P. Guillet, P. Mounaix

IMS, CNRS, Bordeaux Univ. (France)

B. Bousquet

CELIA, CNRS, Bordeaux Univ. (France)

N. Palka

Military Univ. of Technology (Poland)

Proc. SPIE 9651, Millimetre Wave and Terahertz Sensors and Technology VIII, 965109 (October 21, 2015); doi:10.1117/12.2197442

http://proceedings.spiedigitallibrary.org/proceeding.aspx?articleid=2466618

Detection of explosives has always been a priority for homeland security. Jointly, terahertz spectroscopy and imaging are emerging and promising candidates as contactless and safe systems. In this work, we treated data resulting from hyperspectral imaging obtained by THz-time domain spectroscopy, with chemometric tools. We found efficient identification and sorting of targeted explosives in the case of pure and mixture samples. In this aim, we applied to images Principal Component Analysis (PCA) to discriminate between RDX, PETN and mixtures of the two materials, using the absorbance as the key-parameter. Then we applied Partial Least Squares-Discriminant Analysis (PLS-DA) to each pixel of the hyperspectral images to sort the explosives into different classes. The results clearly show successful identification and categorization of the explosives under study.
 © (2015) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.

Abstract-Passive imaging of concealed objects in terahertz and long-wavelength infrared

Passive imaging of concealed objects in terahertz and long-wavelength infrared Marcin Kowalski, Mariusz Kastek, Michal Walczakowski, Norbert Palka, and Mieczyslaw Szustakowski  »View Author Affiliations

http://www.opticsinfobase.org/ao/abstract.cfm?uri=ao-54-13-3826

Applied Optics, Vol. 54, Issue 13, pp. 3826-3833 (2015)
http://dx.doi.org/10.1364/AO.54.003826

View Full Text Article

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Terahertz and infrared radiation have unique properties applicable to the field of surveillance and security systems. We investigated the possibility of detecting potentially dangerous objects covered by various types of clothing using passive imagers operating at 1.2 mm (250 GHz) and long-wavelength infrared at 6–15 μm (20–50 THz). We developed a measurement methodology that assumes to investigate theoretical limitations, performance of imagers, and physical properties of fabrics. To evaluate stability of the detection capabilities of imagers, we performed measurement sessions each lasting 30 min. We present a theoretical comparison of the two spectra and results of experiments using state-of-the-art equipment.

© 2015 Optical Society of America

Course-Terahertz Technology in Homeland Security and Defense (SC718)

Course Level: Introductory
Instructor: Ruth M. Woodward, HT Consultants Ltd. (United Kingdom)
http://spie.org/x1145.xml?course_id=M0000782&topic_id=5

Course Details

This course will introduce the basic techniques in terahertz technology for imaging and spectroscopy in homeland security and defense. The course reviews the progress of the technology in the security and defense arena, including recent developments in sources and detectors. The focus will be specifically on the application of the technology for improvised explosive device (IED) detection and chemical and biological sensing. The methods required for the successful imaging and spectroscopy of target materials will be discussed, in addition to the appropriate implementation of standards for identifying the viability of terahertz technology in this arena. The student will receive an understanding of the challenges and potential of terahertz technology in homeland security and defense.

Learning Outcomes

This course will enable you to:

• describe the current progress of terahertz technology in homeland security and defense
• explain the signatures observed in the terahertz region and associated merits for time or frequency domain analysis
• understand the limitations of the terahertz technology for stand-off detection
• identify problems associated with image generation and spectroscopy
• understand the appropriate experiments required for identify the viability of terahertz technology in security and defense
• conclude the areas where terahertz technology has potential

Intended Audience

This material is directed at scientists, researchers, managers and government interested in the application of terahertz technology for homeland security and defense initiatives. Those currently in the field of terahertz technology or wishing to enter and explore this field will find this course valuable.

Instructor

Ruth M. Woodward is director and founder of HT Consultants Ltd., a specialist consultancy firm in terahertz technology. She has been involved in the application of terahertz pulsed imaging and spectroscopy to the medical industry since its foundation in 1999. During her PhD research on ‘Terahertz pulsed imaging and spectroscopy applied to dermatology’, she identified a terahertz contrast between diseased and normal tissue in basal cell carcinoma, the most common form of skin cancer, which led to an explosion of interest in the application of terahertz technology to the medical industry, and the subsequent commercialization of terahertz imaging systems in this area. As Chair of SPIE Global Homeland Security Technical Group Standards Subcommittee, she also takes an active interest in the implementation of product and safety standards in security and defense, in addition to alternate methods for improving homeland security.

New terahertz device could strengthen security

                                                                                                    Manijeh Razeghi

http://phys.org/news/2014-11-terahertz-device.html

We are all familiar with the hassles that accompany air travel. We shuffle through long lines, remove our shoes, and carry liquids in regulation-sized tubes. And even after all the effort, we still wonder if these procedures are making us any safer. Now a new type of security detection that uses terahertz radiation is looking to prove its promise. Able to detect explosives, chemical agents, and dangerous biological substances from safe distances, devices using terahertz waves could make public spaces more secure than ever.

But current terahertz sources are large, multi-component systems that sometimes require complex vacuum systems, external pump lasers, and even cryogenic cooling. The unwieldy devices are heavy, expensive, and hard to transport, operate, and maintain.

"A single-component solution capable of room temperature and widely tunable operation is highly desirable to enable next generation terahertz systems," said Manijeh Razeghi, Walter P. Murphy Professor of Electrical Engineering and Computer Science at Northwestern University's McCormick School of Engineering and Applied Science.

Director of Northwestern's Center for Quantum Devices, Razeghi and her team have been working to develop such a device. In a recent paper in Applied Physics Letters, they demonstrate a room temperature, highly tunable, high power terahertz source. Based on nonlinear mixing in quantum cascade lasers, the source can emit up to 1.9 milliwatts of power and has a wide frequency coverage of 1 to 4.6 terahertz. By designing a multi-section, sampled-grating distribution feedback and distributed Bragg reflector waveguide, Razeghi and her team were also able to give the device a tuning range of 2.6 to 4.2 terahertz at room temperature.

The device has applications in medical and deep space imaging as well as security screening.

"I am very excited about these results," Razeghi said. "No one would believe any of this was possible, even a couple years ago."

 Explore further: Continuous terahertz sources demonstrated at room temperature

Read more at: http://phys.org/news/2014-11-terahertz-device.html#jCp

China debuts first terahertz scanner for security checks

http://www.china.org.cn/china/Off_the_Wire/2014-05/08/content_32331709.htm

China has developed its first terahertz (THz) security scanner, which is expected to have broad applications, according to an announcement at a defense industry exhibition on Thursday.

China's first terahertz security scanner debuted at the ninth China International Defense Electronics Exhibition. It was independently developed and manufactured by the China Electronics Technology Group Corporation, one of China's defense industry giants.

Security scanners of this kind can work without interruption, scanning 500 people per hour, or about one person per second, five times the efficiency of manual checks.

The scanners are expected to be put into mass use in 2015 in public places such as airports, train stations, stadiums and foreign embassies.

The development will chip away at the United States' monopoly and dominance in the terahertz application market, according to Chinese experts.

Terahertz radiation is a kind of electromagnetic radiation with wavelengths between those of infrared rays and microwaves, boasting a strong penetration force without harmful effects.

The human body naturally transmits terahertz waves, which can be received by the terahertz body scanner.

By contrasting the image projected on the scanner screen with the background image of the human body, security staff can determine if a person is hiding something in their clothes as the object would deflect or hinder the terahertz transmission path.

Compared with traditional security check devices, terahertz scanners are much safer and more reliable, said one of the developers.

Terahertz scanners can not only detect metal objects but also non-metal materials such as ceramics, powders and liquids.

Moreover, terahertz scanners do not require bodily contact with those being examined.

The image projected on the screen does not show details of the person being scanned, allowing for protection of privacy. 

Terahertz Bi-Directional Reflection Function Measurements for Homeland Security

http://nist.gov/pml/div682/grp00/terahertz-bi-directional.cfm
My Note: I can't determine the date this published, but it's new to me, and I wanted to share it.

Summary:

Far-infrared (or THz, 25 to 300 micron wavelength) femtosecond laser methods are employed to generate high power broadband pulses for far-field imaging applications. We are developing real-time pulsed imaging of objects as well as exploring THz hyperspectral imaging as a means to discover concealed threat materials relevant to Homeland Security. Measurement of the bi-directional reflection function (BRDF) at THz frequencies is being conducted to better understand pulse propagation and distortion effects after reflection from common concealment materials.

Description:

We employ novel ultrafast laser terahertz techniques to generate high power picosecond pulses for reflection imaging applications. A large area (2 cm x 2 cm) biased GaAs photoconductive switch produces THz pulses with sufficient bandwidth (0.3 to 2 THz) for far-field reflection hyperspectral imaging. Reflected pulses are collected by a large area reflection telescope and the THz image is relayed onto a (1x1) cm² ZnTe electro-optic image up-convering crystal. Weak 800 nm gating pulses from the Ti:Sapphire laser system (seeTerahertz Apparatus/Facilities page) are used to extract the image imposed on the detection crystal and relayed onto a cooled CCD detector for imaging.  

In parallel to our THz spectroscopy efforts, we are also developing novel methods for real-time pulsed THz imaging of far-field objects and to perform hyperspectral imaging of inhomogeneous materials, threat agents and other species relevant to Homeland Security needs. This wavelength range is particularly attractive because THz radiation readily transmits through most plastics, cloth, papers and other non-metallic, low-density matierals. Detailed analysis of high power source and THz pulse propagation is required as well as specialized collection optics to optimize image resolution. Spectral characterization of materials in an image scene at several meter reflection distances is being explored.    

BRDF measurements with (top) and without (below) the index card sample taped to the sample mount front surface. The without sample case shows strong evidence of THz pulse reflection from the sample mount. These measurements were taken without the input iris that is normally set at diameter of 1.5 cm (to restrict the beam size) in front of the sample. Evidence for stray optical reflections is also found from responses before zero delay time.

Angular pulsed Terahertz reflection amplitude map for a sample of corduroy cloth with 3.2 mm rib spacing. (a) Power dependent response showing temporal oscillations resulting from individual rib scatter radiating from the cloth front and rear surfaces; (b) Superimposed Fourier Transform amplitude and power reflection map of the raw scattering data exhibiting the spatial equivalent of diffraction and fit to the data exhibiting several orders of diffraction.

In earlier imaging experiments (circa 2003-2008), it was found that near-field imaging (<1 meter) of reflective objects retained expected resolution at THz frequencies. However, objects placed at ca. 3 meters from the generator produced distorted images, probably arising from pulse interference or depolarization effects. To explore these possibilities, DHS is currently funding a collaborative effort to directly measure and model GHz through THz BRDF of a variety of concealment materials. Since the BRDF has not previously been measured in this wavelength region, our new results will be compared to conventional BRDF models to test their applicability and accuracy for predicting image quality for hyperspectral and related applications. 

Bi-directional Reflection Function apparatus CAD design (top) and its implementation attached to pulsed THz imaging apparatus (bottom). The stacked rotation stages permit manual selection of the incident angle of the pulsed THz output beam and angularly synchronized collection mirrors allow propagation of horizontal or vertically polarized radiation to be collected and processed by the imager.

These investigations use a state-of-the-art, KHz rep-rate amplified 45 femtosecond pulsed laser for broadband (0.2-2.5) THz GaAs antenna generation and detection using a ZnTe electro-optic crystal. See the Terahertz Apparatus/Facilities page.

Selected Publications:  

• M. B. Campbell and E. J. Heilweil, “Non-invasive detection of weapons of mass destruction using THz radiation,” in Proceedings of SPIE Vol. 5070 Terahertz for Military and Security Applications, edited by R. Jennifer Hwu, Dwight L. Woolard, (SPIE, Bellingham, WA, July, 2003), page 38.
• Shu-Zee A. Lo, D. Novotny, E. N. Grossman, and E. J. Heilweil, “Pulsed terahertz bi-directional reflection distribution function (BRDF) measurements of materials and obscurants,” in S.P.I.E Proceedings for Session #8022 entitled "Passive Millimeter-Wave Imaging Technology XIV," Orlando FL, April 25-29, 2011.