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Terahertz (T-ray) technologies offer the promise to transform how we interact with the world around us, from delivering lightning-fast data transfer beyond 6G to providing X-ray-like imaging without any harmful radiation. Yet effectively and efficiently harnessing T-rays for mainstream commercial applications has continued to elude scientists and engineers, relegating T-ray technologies to the realm of science fiction.
However, recent breakthroughs in how terahertz waves are generated and detected have now brought the technology to a tipping point for mainstream adoption. T-ray scanners can now be small enough to fit on a desktop, and they have enabled widespread terahertz use in medical, corporate, manufacturing and security settings across the world, with more developments to come. Here’s how T-ray technology has evolved over the past few decades and what we can expect in the years ahead.
The origins of T-Ray
The electromagnetic spectrum has fascinated mankind for centuries, and in recent times, that spectrum has brought us radios, X-rays, cellphones, microwaves, lasers and more. But until the last two decades, there was one final, largely unexploited region of the electromagnetic spectrum, and that’s in the THz frequency range. In fact, ever since the ’60s, when scientists first began to experiment with T-ray technologies, this region right in the middle of the electromagnetic spectrum has been known as the Terahertz gap.
The reason for the gap is simple: THz signals are hard to generate and detect. You’ve experienced the ease with which electronics generate low-frequency signals whenever you turn on your car radio, use your microwave oven or access social media on your cell phone. At the opposite end of the spectrum, optical technologies are great at generating and detecting high-frequency signals like X-rays, infrared, and even visible light energy.
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In contrast, the THz range lies right at the intersection of what was historically possible with electronics and optics — slightly too high in frequency for conventional electronics to reach, and too low in frequency for optical approaches to be effective. Both optical and electronic systems strain to generate and detect signals in the THz range. Historically a THz source or detector required intricate infrastructure like cryogenic cooling systems using liquid helium or liquid nitrogen to keep THz sources from overheating and maintain THz detectors at stable reference temperatures. The cost and complexity of these systems limited THz technologies to the lab and to specialized applications including space and defense.
That is no longer the case. Over the past few decades scientists and engineers have slowly been chipping away at the boundaries, gradually narrowing the THz gap and laying the groundwork to at long last open up this final frontier in the electromagnetic spectrum.
Pushing the boundaries
Right below the THz frequency range lie millimeter waves (mmWaves), a close cousin to THz. mmWaves and THz signals share similar specialized characteristics when it comes to their applications for electronics and optics.
Technologies exploiting mmWave frequencies have grown exponentially over the past few years and provide a glimpse into the future of THz. Since millimeter waves are a key enabler for 5G communications, most of us now benefit from the use of mmWaves in our everyday lives without even realizing it. Additionally, advances in semiconductor technologies to generate and detect ever higher frequency signals on conventional circuit chips are now expanding to applications beyond 5G communications. These include automotive radar for autonomous vehicle navigation and counting the number of people inside a building and remotely monitoring their health.
Most people have also experienced various forms of mmWave imaging at airports and other security checkpoints. Since both mmWaves and THz energy do not generate ionizing radiation, they are safe to use for screening people, unlike X-rays. While your luggage is scanned by conventional X-ray scanners, which are lined with lead to contain the X-ray radiation, most airport checkpoints use mmWave scanners — the large circular scanners where you are asked to hold still for a few seconds — to screen people for concealed threats.
These growing mainstream applications for mmWave technology, for everything from high-speed wireless communication to security imaging, will continue to push frequencies even higher and pave the way for next-generation technologies based on THz.
The transition point
While the rapidly expanding applications of mmWave technologies continued to incrementally narrow the THz gap, major changes in technological capabilities were required for THz devices to operate at much higher frequencies and fully realize all that THz has to offer.
One of these major changes was the development of efficient sources to generate THz energy at levels high enough to be meaningful. Typically, pushing electronics to higher frequencies or pushing optical systems to generate lower-frequency signals results in a significant power loss, but work in the early 2000s on new laser technologies, called quantum cascade lasers (QCL), as well as diode-based multipliers promised to deliver higher power in smaller packages. These developments opened the door to a number of potential THz applications summarized in a 2002 paper published by the Institute of Electrical and Electronics Engineers (IEEE).
The ability to generate higher-power THz signals paved the way for three key areas of application: spectroscopy, imaging and high-speed data transmission. THz spectroscopy exploits the unique signatures or “fingerprints” of different materials in the THz range to identify them based on how the material interacts with the THz signal passing through or reflecting off it. Examples of THz spectroscopy include sorting plastic waste to categorize different types of clear plastic that look the same and detecting chemicals and explosives for security applications.
When it comes to imaging, THz promises an X-ray-like ability to “see inside” of items without the ionizing radiation generated by X-rays. However, unlike X-rays, T-rays are not able to penetrate metal and other conducting materials. Early work on T-ray imaging spanned applications from manufacturing quality control for pharmaceuticals to work at MIT demonstrating the ability to read text through multiple pages of a closed book.
On the communication and data transfer side, T-ray experiments in 2017 showed the potential for internet signals carried via T-rays to be considerably faster than current Wi-Fi technologies.
Unfortunately, the majority of these applications remained firmly in the R&D stage and didn’t immediately lead to commercialization of T-ray technologies. Researchers had yet to solve the cost and size issues with terahertz emitters and detectors that stemmed from the need to keep them at cryogenic temperatures, and development for both was still painfully slow.
That is, until breakthroughs over the past decade in uncooled THz technologies — both THz sources and detectors. Developments in uncooled quantum cascade laser emitters capable of operating at room temperature in compact sizes, along with the development of uncooled detector technologies using conventional electrical cooling systems, now provide much more practical components to support real-world applications. Suddenly, a whole new host of applications for T-ray became possible. And today, science has reached deep into those possibilities to develop remarkable technologies that have or likely soon will receive widespread adoption.
T-ray today: Passing phase or widespread adoption?
T-ray imaging is one of the first areas seeing an influx of commercial applications for these breakthroughs in the underlying technology. T-ray scanners today can operate with compact and more efficient power sources, solving the size problem, and benefit from detector technologies that don’t need cryogenic cooling. Although not yet low-cost, when compared with other commercial imaging systems such as X-ray, T-ray technologies have become quite competitive and are gaining widespread adoption.
We’ve already seen some commercial applications of THz imaging in the manufacturing and security fields. Commercially-available THz imaging devices, while still fairly expensive, are useful for quality control in manufacturing. For example, THz imaging can measure small changes of thickness in real time (like layers of paint) without damaging or corrupting the substance being scanned. Minor variations in thickness or pockets of air just under the surface of various materials are undetectable by most other imaging methods, but not by T-ray.
In security applications, THz scanners can provide a real-time 3D video view of weapons or substances someone might be hiding under a coat or in a bag, and the scanners don’t generate ionizing radiation like X-ray systems. In fact, passive THz systems exploit the fact that people naturally emit low levels of THz energy to detect concealed weapons or contraband and are increasingly being used in high-throughput applications to screen people.
Aside from screening people, T-ray scanners the size of a desktop printer or copier are better suited to certain screening use cases than X-ray for detecting concealed items and potential threats. Terahertz frequencies are particularly good at imaging soft items like powders and liquids, at up to 300 times higher sensitivity than X-rays, greatly enhancing the detection of drugs and chemical and biological threats. This includes fentanyl, ricin and anthrax along with other powders and substances that X-ray can’t detect in small quantities.
The increasing adoption of T-ray imaging in both manufacturing and security has led to a surge of R&D for terahertz technology in general, paving the way for even more innovation in mainstream T-ray technologies to come.
Future T-ray innovations
THz has already broken into mainstream use, thanks to the imaging capabilities of T-ray scanning technology. But scientists are exploring a wide variety of innovative uses spanning from long-range data communication through space to early cancer detection.
Although well-suited for ultra-fast data transfer rates to support 6G communications over short distances, long-range communications using THz are challenging. Currently, THz signals degrade over long distances, particularly through poor atmospheric conditions, so they’re not the most practical choice for extremely long-distance data transfer over land. On the other hand, outer space is pretty much free from atmospheric hindrances. So NASA has long been testing THz technologies for possible high-speed data transmission through space.
Finally, there’s medical diagnostics and imaging. Since terahertz imaging can differentiate between healthy and unhealthy tissue, THz imaging could help in cancer detection. Experts believe T-ray could be a game changer when it comes to detecting the exact boundaries of tumors and cancers in order to aid in early treatment and perhaps even enhance precision in robotic surgeries. For example, the Biomedical Terahertz Technology Center at the University of Massachusetts, Lowell is currently developing THz imaging technologies to aid in colon cancer screening and detecting cancer margins to aid in skin cancer surgery. Medical imaging with THz is already undergoing clinical trials throughout the world.
T-ray has made considerable progress, but while there are many opportunities now and in the future for experts to make use of THz technology, there are also issues yet to be solved. Signal degradation over long distances is still a problem. And deep scanning with T-rays is challenging, since THz signals are attenuated by a range of materials. Still, the technology has overcome several hurdles. With continued commitment to research and development, this is a technology to keep a close eye on. There are surely more breakthroughs to come.
Alexander Sappok, PhD is CEO at RaySecur.
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