By Damien Johnson
Approximate Reading Time: 15 Minutes
The terahertz (THz) frequency band is a fascinating and relatively unexplored part of the larger electromagnetic spectrum (EMS). THz can enable users to ‘see-through’ materials and structures and is now widely used in laboratory quality control testing, for non-destructive testing and transmitting information between servers in close proximity. However, because the transmitted signal degrades rapidly in the atmosphere, its application is limited to controlled settings. Soon, technological leaps will enable development of power sources that will prevent THz degradation in the atmosphere and at distance. When this happens, THz will be an exquisite tool to help the United States find and target items of interest. It will allow intelligence professionals and field operators to quickly discriminate targets based on composition, counter adversary concealment or deception techniques, and identify items of interest based on chemical resonance that is visible only through the use of THz imaging.
Imagine a future where intelligence analysts will have the ability to process satellite images of weapons caches in underground bunkers. Security patrols will have the ability to ‘see-through’ suspected terrorist domiciles or into bomb-making facilities. Unmanned aerial vehicles will be able to positively identify maritime vessels carrying illicit drugs or victims of human trafficking inside shipping containers. Low observable technologies like those found on the F-35 fighter and B-2 bomber will be ineffective, as radar fidelity will outpace low-observable technologies. At the same time, big data updates to the war will be transmitted rapidly to artificial intelligence nodes to produce a common operating picture for military commanders. If correctly developed, terahertz (THz) technology is the key to unlocking this next evolution of warfighting—increasing our ability to find and target adversaries and their capabilities at lightning-fast speed.
The THz frequency segment is a fascinating and relatively unexplored part of the larger electromagnetic spectrum (EMS). On the low end of the spectrum are radio waves and microwaves, characterized by a relatively low frequency and a long wavelength. On the high end of the spectrum are X-rays and Gamma rays, having high frequencies and short wavelengths. Figure 1 is a graphical representation of the EMS and shows the approximate location of the THz band. The THz segment of the EMS occupies a large sector between the microwave and infrared bands (understood to be in the frequency range from roughly 0.1 THz to 10 THz (corresponding to wavelengths from 3 mm down to 30 μm).
Figure 1. Electromagnetic Spectrum and “THz Gap” Location
The EMS is critical to our daily activities and encompasses all known and measurable wavelengths and frequencies. Because of its inherent capability to link warfighting equipment, electronics, and subsystems together, the United States and its adversaries are beginning to understand the importance of exploiting EMS dependencies and are designing new applications to achieve strategic and operational advantages. If harnessed correctly, the THz band of the EMS has the potential to accelerate our military capabilities ahead of an adversary. The conceivable military applications of THz sensors are broad for intelligence, surveillance and reconnaissance (ISR), to include detection of isolated personnel behind enemy lines, fixing targets and terminal guidance of precision weapons. Since the 1950s, researchers and scientists have branded this part of the spectrum many things, some of which are still popular in modern research: the THz Gap, submillimeter, and extreme far infrared.
The following article explores the hype of this technology. It assumes that we can think ‘outside the box’ to combine techniques with upcoming technological evolutions to make THz a reality in a 2030 security environment. It explores current usage in laboratory environments and hypothesizes future military uses. In the next 10 years, miniaturization of THz technologies and hyper-efficient power sources will be realized. These breakthroughs will enable most of this discussion, turning theory into reality. Until then, it is in our national interest to brainstorm ways in which we can use the THz to our advantage.
Concepts, Limitations, and Current Applications
There is no doubt THz radiation is extremely useful. Most notably, because of its ability to penetrate deep into objects for imaging. THz radiation is also non-ionizing, making it safe for humans and organic matter (unlike X-ray radiation). Although microwaves have this ‘see-through’ capability, their comparatively large wavelengths do not provide the same fidelity and image resolution as THz images. Because of this, THz is ideal for security imaging, can be used in concert with radar systems, and can help wireless networks transmit data at incredible speed. Unfortunately, two limitations stifle THz growth.
First, atmospheric attenuation is a fundamental problem. THz signal strength diminishes at an extreme rate. A 1-watt transmission at a frequency of 1 THz diminishes to almost nothing (10-30 percent of original strength) after one kilometer. This degradation can be even more severe if water is present in the atmosphere. Second, the power requirement to overcome this attenuation is not presently realistic outside of a laboratory environment — the immense power requirement is a critical subcategory of common design considerations: Size, Weight, Power and Cooling (SWaP-C). Collectively, SWaP-C presents a barrier known as the ‘terahertz wall.’ Currently, SWaP technology for mobile transport of THz devices does not exist to enable stand-off capabilities.
Figure 2. “THz Wall” Demonstration of Communication Transmissions & “THz Fingerprints of Explosives and Fabrics”
So far, most THz research is in the proof-of-concept phase for use in a combat or security environment. This significant power limitation must be overcome before THz can exist in an operational setting. Presently, there is no clear solution on how to solve this problem; a new breakthrough in power supply and battery capacity in needed. However, laboratory THz usage is thriving. Current application (as an imaging tool) includes non-destructive testing and quality control (QC) of various materials, including plastics, concrete, and ceramics. Recently, THz imaging has been used in laboratory tests and security screening to examine the condition of space shuttle components, examine text in books that are too sensitive to examine physically, and to localize illicit drugs based on specific composition. Due to the power constraints, its use has been restricted to an approximate distance of 10 meters or less.
In addition to non-destructive testing and QC application, lab tests in the THz band (defined in IEEE Standard 802.15.3d, published October 2017) or “300 GHz band” demonstrate a high-speed communication channel capable of achieving speeds of 80 Gbit/s. For non-scientists, this immense speed can most appropriately be described as ‘6G’. This limitation is significant, but there is potential use in places where vast amounts of data need to be rapidly exchanged within the same room. Space and Air Operation Centers (AOCs) are prime examples. These centers are an operational decision-making environment and a hub for multiple information sources. The ability to sort, transfer, and fuse information nearly instantly between servers through the use of near field communications (NFC) is helpful in informing leaders and outpacing an enemy. For example, a data transfers of 25-50 Gigabits (GBs) could be transferred to a single machine in close proximity in one second.
Once the power limitation is solved, several military applications can be realized: Ground-based ISR, radar system integration, airborne and space-based ISR, and precision targeting and communications are capabilities that can offer a massive advantage. As such, THz technology should be considered part of the proverbial arms race that exists for artificial intelligence (AI), quantum computing, and machine learning (ML).
Regarding ground-based THz systems, security corridors or mobile patrols could soon ‘see-through’ structures, clothing, vessels or transport containers, and most other non-liquid material to probe for concealed materials. A future security environment where threats can be detected and countered could thwart criminals and belligerents manufacturing or smuggling explosives. It could also help locate bomb-making materials, weapons/mines, illegal drugs, and rare earth minerals.
In 2014, researchers from Sandia National Laboratories, Rice University, and the Tokyo Institute of Technology began working on a ground-based security screening system for THz technologies. THz machines have a distinct advantage over X-ray machines or millimeter waves (these tubular devices are often found in airport screening corridors immediately following the metal detectors). They are better suited for detecting explosives and illicit materials and more reliable in identifying almost any chemical composition under the right conditions. Operationalizing this type of screening system in a dense urban environment against enemy state or non-state actors would be especially useful. Much of this has already been demonstrated within controlled environments. In June of 2016, China’s People’s Daily Online reported that the State Administration of Science, Technology, and Industry for National Defense completed research and development (R&D) efforts of China’s first solid-state THz imaging system. The Chinese article also proposes applications for urban and anti-terrorist combat, such as scanning behind walls or inside compounds.
Potential Applications in a Future Security Environment
In a combat environment, THz machines such as these would be capable of scanning passageways or mounted on vehicles for IED detection. They would take ISR methodology efforts to a new level. In addition, displaying targets of interest in a two-dimensional rendering at a rate of approximately 16 frames per second (similar to current commercial THz security imagers) would allow the reconstruction of images as a video presentation. Perhaps the most practical application in an urban environment would exist in subterranean transportation networks such as subways, the proposed hyperlink network in California, or underground shopping centers and dwellings with high rates of violent crime and criminal activity.
In a non-combat environment, it could be used to help detect plastic or minimal metal land mines on current or former battlefields; most anti-personnel mines are a combination of metal and plastic (and manufactured to avoid detection by metal detectors). Current technology for land-mine detection requires analysis of the soil temperature and is measured in three dimensions, then injected into cumbersome software algorithms that make rough estimates with limited confidence. This detection technique uses Field Programmable Gate Array (FPGA) technology. THz spectroscopic imaging is a logical alternative to FPGA, as it can detect almost any material under the right conditions with relatively high confidence.
THz imaging in concert with air surveillance radar systems is another technological pairing that is beginning to be explored. In late 2017, The South China Morning Post released an article claiming China’s largest arms manufacturer, China North Industries Group Corporation successfully tested THz instruments capable of detecting stealth aircraft. The origin of this technology in China is unclear and understandably leads some readers to skepticism and doubts of legitimacy. China’s (questionable) past pursuits of foreign and dual-use technologies suggest that the country will continue to grow THz technology in competition with other international powers such as the United States. The radiation produced, or ‘T-rays’, can penetrate composite metals to examine underlying aircraft metals. These metals are specific to certain types of aircraft, making the process of finding and positively identifying low observable (LO) aircraft fast and easy.
According to the article, an executive with the manufacturer claims, “the radar-absorbent coatings on the F-35 will look as thin and transparent as stockings.” The existence of this capability would drive a fundamental change to nations employing stealth technologies. Given the current anti-access/area denial (A2AD) situation in eastern China and the South China Sea, this could be a great advantage for the Chinese air defense and directly affect the United States’ perceived technological power. Of course, antenna line-of-sight requirements of THz limit the distance a radar can see. This horizon limitation of these ground-based systems would still allow low observable platforms to penetrate a considerable distance before being detected. As an example, based on simple mechanics and geometry, a ground-based radar system that is 40 feet tall would discover a LO strike package consisting of F-35 fighter planes and B-2 bombers ingressing to a target at 40,000 feet at range of roughly 300 miles.
The advent of THz on airborne and space assets could also be a shift in the way the United States conducts ISR, as it can overcome line-of-sight issues and ‘see-through’ structures. Countries that use camouflage, concealment, and deception techniques might no longer be able to depend on current strategies. Specifically, actors that use subterranean structures or bunkers to hide weapon systems will no longer enjoy the benefit of obscurity. In places like Kangwon, located in a remote area of the Democratic People’s Republic of Korea, airbases and surface-to-air weapon systems are commonly protected inside mountains and terrain.
ISR products developed from air or space assets would significantly improve the United States’ ability to find and strike critical military targets. Ideally, these products will have to be analyzed with advanced predictive analytics to develop target sets further. Figure 3 is an open-source example of proposed ISR targets.
Figure 3. Kangwon Underground Runway (DPRK) and Fordow Fuel Enrichment Plant (Iran)
THz imaging can detect resonant chemical signatures and molecular characteristics. Most of these molecules have specific absorption/dispersion characteristics that can supply unique information (similar to fingerprints). This ability to ‘look through’ hangar doors of Kangwon mountain could possibly reveal aircraft, fuel and maintenance equipment, and anti-aircraft artillery systems based on their chemical composition and absorption level. Impressively, THz can discriminate between distinct types of explosive material, fabrics, and metals inside this mountain. THz imaging could also be able to help find vertical ventilation and potential access ports on the top and sides of the mountain more easily than using raw imagery.
This ‘fingerprint’ capability also has application in locations such as the Fordow Fuel Enrichment Plant (FFEP) located near the Iranian city of Qom. THz technology might help in instances where diplomatic efforts fail or an adversary’s intention is unclear. It will be able to cross physical barriers and provide the intelligence community (IC) or decision-makers with valuable information. For example, Iran’s disclosure of the Qom facility to the International Atomic Energy Agency (IAEA) in 2009 was ambiguous, stating that the facility was enriching uranium for “medical purposes.” However, an Iranian Nuclear Archive document seized by Israel explains that Fordow’s intended purpose was to produce weapon-grade uranium for one to two weapons per year. Using THz capability, we could more confidently determine the capacity and ability of an adversary to produce highly enriched, weapons-grade uranium by imaging the subterranean layers of the bunker. This can help understand the facility’s layout which may be key to deducing what potential targets are inside the facility. These ISR targets could be cataloged and developed for future target nominations if kinetic responses or options are needed. In the distant future, THz could be used to determine specific locations and quantities of hidden materials to help targeteers determine optimal weapon solutions and find/fix future military targets.
THz can also assist in personnel recovery (PR) missions for downed aircrew or groups of people in unfortunate situations that require assistance or rescue. In degraded visual environments such as dust storms in the Middle East, THz’s operating frequencies and smaller wavelength size make it an ideal choice for finding and tracking human-sized targets of interest for recovery. In addition, THz could help rotary-wing aircrew perform field landings by detecting terrain and other obstacles such as power lines and vertical development in obscurant weather that could pose safety hazards.
Another military application for THz radiation can be found in the targeting process. Most fourth and fifth-generation strike aircraft use a combination of static synthetic aperture radar (SAR) images and electro-optical/infrared (EO/IR) pod video feeds for targeting objects on the ground. These devices are critical not only in finding and tracking targets, but also play an essential role in the targeting process. Both SAR and EO/IR pods are used to cue precision munitions like GPS or munitions or laser-guided bombs (sometimes in congested environments with an elevated risk of collateral damage).
As a very general rule, and for the purposes of this work, SAR has many uses but is preferred in situations where weather or atmospheric conditions such as smoke or haze exist in the environment. SAR is also primarily used for targets that cannot relocate like buildings and terrain. Conversely, EO/IR pods are ideal for clear weather. EO/IR pods can also better target dynamic entities that are moving. Often these sensors are used in concert in both planned and dynamic targeting situations such as close air support or derivative counter land missions such as strike coordination and reconnaissance (SCAR). Developing a THz payload capable of working in concert with this targeting capability could lead to significant improvements for both targeting methodologies.
First, THz radiation could be layered over SAR images in real-time to provide operators enhanced situational awareness of specific target composition. These images are snapshots in time and do not stream or update with a refresh rate like their EO sensor counterparts. SAR images can help in instances where targets are located inside of buildings, shallow bunkers, or exist behind any sort of barrier and are non-moving or ‘static.’
A cockpit-selectable filter function overlaid onto a plane’s radar screen is shown below in Figure 4. Semtex and Wool filters are selected as overlays on the SAR screen. In the SAR screenshot captured below, operators could easily find the standard plastic explosive Semtex collocated at an entry gate of a major military installation. In the graphic, the yellow pixels are seen to be in or beneath the road leading into a compound. The filter also informs the sensor operator that multiple traces of wool fabric are found several meters away in a tree line and identified by green pixels.
Detecting the presence of the plastic improvised explosive device coupled with several indications of people hiding under tree canopies could be indicative of an imminent attack. But keep in mind—while SAR is an elegant solution for radar targeting in degraded visual situations, its THz overlays will not work in mist, fog, or areas of precipitation until SWaP-C solutions are available to accommodate such a significant stand-off range.
Figure 4. SAR Map and EO/IR pod video image with THz Filter Capability (Hypothetical by Author)
Second, THz radiation could also work in parallel with EO/IR sensors to find and strike targets while moving. EO sensors are the primary cueing mechanism for laser-guided weapons and ideal for destroying high-speed targets. In the EO/IR video image in Figure 4, the pod operator can use a THz overlay to find and target a vehicle known to be transporting weapon-making material. This ability to detect chemical ‘fingerprints’ will help expedite finding targets in dense urban environments when discrimination can be difficult. Hypothetically, it will be possible to outfit an airplane’s SAR and EO/IR pod with a vast catalog of THz filters. Based on mission needs, (multiple) different filter settings will be selectable in any given situation.
The communication advantage of THz over conventional methods also needs discussion. As mentioned in the introduction, communication speeds in the THz band will enable 6G speeds. Though these signals degrade quickly in the atmosphere, there is a significant utility in space where the signals experience almost zero loss. Compared to modern spacecraft S-band, Ka-band, and Ku-band systems that use complex architecture to achieve high data rates, a THz wireless system is much less complex, the components are modular and light weight compared to Ku-band hardware. Satellites or spacecraft using this lightweight payload could be able to communicate easily while on orbit. As the next evolution in competition arises, THz enabled communication would be helpful on Lunar and Martian surfaces due to the lack of atmosphere and moisture.
The prospects of THz technology are attractive, and its military application is broad, ranging from ground-based and airborne security and intelligence, surveillance, and reconnaissance to terminally guiding precision weapons and enabling near-instant communication. To operationalize THz, several technological improvements must occur. Most importantly, THz imaging and communication components will need to be miniaturized while still considering the economics of size, weight, and power output. While we are years away from these achievements, we must begin to explore and discuss applications in the future security environment. Our adversaries are beginning to explore this region of the electromagnetic spectrum and are making considerable progress. If we are not forward-thinking about THz and its usefulness, then we are assured of losing this silent arms race before it even begins.
Damien Johnson is currently a student in the Multi-Domain Operational Strategist concentration at Air Command and Staff College. He is a B-1 Instructor Weapon Systems Officer and Electronic Warfare Officer with eleven years of operational experience in multiple combatant commands and over 1,000 combat hours.
Disclaimer: The views expressed are those of the author and do not necessarily reflect the official policy or position of the Department of the Air Force or the United States Government.