Terahertz (THz) radiation is a type of electromagnetic radiation which falls in between infrared and microwave radiation in the electromagnetic spectrum in the range of one to 30-70 THz. The cosmic microwave background (CMB) radiation is a natural source of terahertz radiation, and thermal radiation emitted by the human body and by various objects at room temperature falls within the high-frequency range of THz waves.
Since the 2000s, a number of promising applications for THz radiation have been explored, such as navigation in low visibility conditions, environmental monitoring, volcanology, medicine, astronomy and security, i.e. detecting items hidden under clothes. Indeed, security services at airports, stadiums, sensitive facilities and some private companies would like to know the content of their visitors' pockets and bags. Metal detectors and X-ray machines have been used for security purposes for many years. However, the former cannot detect plastic or ceramic weapons, drugs and explosives – a major limitation nowadays, when a lethal gun can be produced, except for a couple of parts, using plastic and a 3D printer.
As to X-ray scanners, while they can show in detail the contents of suitcases and bags, using them for security purposes on people is hazardous and thus prohibited.
Technology using THz radiation is the next logical step in the development of security systems. "Terahertz radiation combines high penetrating power with good spatial resolution," Smirnov explains. "In fact, it can fully penetrate fabric and can penetrate many non-metallic objects reasonably well." Such scanners are already installed at many airports: a person enters a booth where they are exposed to radiation, and a detector captures either the waves passing through them (producing a contrasted image based on different wave transmission ratios) or the reflected light (producing an image based on different light reflection ratios); the monitor then displays the contours of items, e.g. hidden under clothes or in shoe soles.
A downside of these scanners is that they cannot be used remotely and discreetly, but require people to pass through a metal detector or place their hands and feet on yellow circles inside a cabin with a terahertz emitter and stand still while the scan takes place. "Someone who is up to no good will avoid passing through detector frames," suggests Smirnov. "In addition, active screening can be sensed by a radiation-detecting device."
An ideal security system today should:
According to Smirnov, only passive THz scanners which detect waves emitted by an object can fully meet these requirements today. A team of MIEM scientists, jointly with the RAS Institute of Radio Engineering and Electronics (RAS IREE), have developed a prototype of such a device. "Rather than expose someone to radiation, you can simply observe the THz waves emitted by his or her body; these waves can easily penetrate fabric such as sweaters, shirts and jackets, but once they hit any other object, whether metal or not, the latter will partially absorb the radiation and become visible as a contrasting image against the background radiation coming from the body itself."
Both direct detectors and more complex superheterodyne receivers can be used as sensors in passive screening systems; the former are simpler and cheaper, while their sensitivity even today is only limited by the background radiation. The latter’s main advantage is that superheterodyne receivers enable spectral analysis, i.e. can be used to identify what substance something is made of, and thus detect explosives or drugs.
As a prototype, the MIEM team produced a scanner using a one-pixel direct detection receiver. "It was a trial version just to demonstrate the system's capabilities; the frame was small, approximately one metre by one metre," describes Smirnov. "We examined the object from a distance of about three to five metres and could detect a phone or a pack of cigarettes in the breast pocket of a jacket."
According to the developers, scanners using a passive detector can only show the outlines of an item against the backdrop of the human body, but work faster than most active screening systems used today, thanks to a new type of detector designed by the MIEM and RAS IREE scientists. Their next aim is to further improve scanning speed, so that ideally an image may be transmitted in real time without delay.
Such systems can be used during any public gathering at risk of a terrorist attack. "Scanners can be installed in stadiums and concert halls, and people will not even know that they are being scanned," the expert explains. "In the future, it may be possible to add spectral analysis to enable recognition of various substances."
According to the scientist, less than a thousand companies in the world are now capable of producing such detectors, and then only for use as expensive research instruments, such as those employed by SOFIA (the Stratospheric Observatory for Infrared Astronomy), a joint project between NASA and the German Aerospace Centre.
The solution proposed by Russian scientists has passed its R&D phase with good prospects for commercialization – a number of companies providing security services have expressed an interest. If successful, this will be the first industrially produced passive terahertz scanning system.