#348 "The Terahertz Revolution: Opening the Door to an 'Invisible World'" (Exploration Explosion Days #62)
Exploration Explosion Days is a series where I share one exploration per day. It is an inquiry series where I use AI to the fullest to 'explode' the overflowing curiosity and inquisitive spirit of Kumemura as an entrepreneur and manager, leading to new wisdom.
If you can use AI to the fullest, once you have a question, you research, research, research, and then get lost in the forest of inquiry, where you try to apply what you have learned to a framework of thinking.
Triggered by daily questions and interests, I delve into diverse themes such as history, invention, innovation, technology, taboos, management, subculture, and science fiction, delivering 'intellectual stimulation' where multifaceted ideas, unexpected discoveries, and ideas chain together.
This time, it's about terahertz waves.
Imagine a world where you can be seen through while wearing clothes. What if there were technology that could check the contents of a sealed box without harmful radiation like X-rays? What if a medical revolution occurred that could detect the early stages of cancer non-invasively? Furthermore, what if a future where you could instantly determine the freshness of food with a smartphone camera were just around the corner?
What makes all of this possible is an electromagnetic wave called a 'terahertz wave.' Once dubbed the 'last frontier of the electromagnetic spectrum,' this mysterious wave sits between light and radio waves. This uncharted territory, which has long troubled scientists, is now on the verge of transforming into a revolutionary technology that will change the way we see our world.
The true nature of the wave that sees the 'invisible'
Our lives are supported by a large family of electromagnetic waves—radio and television waves, microwaves in microwave ovens, infrared rays felt from the sun, visible light that illuminates objects, ultraviolet rays that develop photographs, and X-rays for medical diagnosis. In this electromagnetic family, the terahertz wave has long been treated as the 'problem child that exists but cannot be used.'
The frequency of terahertz waves is located between 0.1 and 10 terahertz. 'Tera' means 'one trillion,' and this wave vibrates one trillion times per second. Converted to wavelength, it is 3 millimeters to 30 micrometers, and you can see its fineness even when compared to a human hair (about 100 micrometers).
This frequency band was once called the 'terahertz gap.' It was like an unexplored land on the map of science and technology. Why was it called a 'gap'? Because it was located between the two approaches of conventional electronics, where the frequency was too high, and optical technology, where the wavelength was too long. It was a 'difficult-to-handle electromagnetic wave' that was hard to generate or detect efficiently.
However, this 'troublesome wave' hid astonishing characteristics. The ability to penetrate non-metallic materials such as plastic, paper, cloth, and ceramics. Non-ionizing properties that do not harm the human body, unlike X-rays. Characteristics that react strongly to moisture. And analytical capabilities that provide a substance-specific 'fingerprint spectrum.' These characteristics are the reason why scientists have continued to challenge the terahertz gap.
Until the 'unusable wave' causes a revolution
The path for terahertz waves, which were long only present in laboratories, to evolve into practical technology was not smooth.
Early terahertz research was driven by astronomy, which captures weak signals from space. Research on the Cosmic Microwave Background (CMB) discovered in 1965 accelerated the development of technology in the terahertz band. In 1970, the rotational transition line of carbon monoxide molecules from the Orion Nebula was detected, opening a new window to explore the 'cold universe.'
The true turning point came in the 1990s. Rapid advances in femtosecond laser technology and the maturation of semiconductor technology brought about a breakthrough that overcame the terahertz gap.
A device called a photoconductive antenna (PCA) generates terahertz waves by irradiating a micro-gap on a semiconductor substrate with an ultra-short optical pulse laser and accelerating the photo-generated carriers. Although this method was initially thought to be weak, in 1998, Nobuhiko Sarukura and others at the Institute for Molecular Science in Japan discovered a significant increase in intensity by applying a magnetic field, opening the way to a practical light source.
In addition, the quantum cascade laser (QCL) invented by Jérôme Faist and Federico Capasso of Bell Labs in 1994 is an innovative light source that utilizes intersubband transitions within semiconductor heterostructures. QCL emerged as a compact semiconductor light source capable of generating relatively high-output coherent terahertz light. While early versions required cryogenic cooling with liquid helium, researchers at Harvard University succeeded in terahertz oscillation at room temperature in 2008.
Japan has conducted world-leading research and development in this field. The National Institute of Information and Communications Technology (NICT) is a pioneer in terahertz research in Japan. The development of the world's first practical handheld real-time terahertz camera in 2009 in collaboration with NEC was groundbreaking. RIKEN has also made significant contributions, such as developing a broadband wavelength-tunable parametric terahertz light source in 2001.
To overcome the triple hardship of being 'expensive, large, and difficult to use,' the development of small semiconductor-based devices has been progressing. It is reported that the latest electronic device called a resonant tunneling diode (RTD) is less than one-tenth the price of conventional methods and has been miniaturized to less than one-thousandth the volume. The barrier to practical application is collapsing rapidly right now.
A world changed by the magic of see-through
The most attractive capability that terahertz waves bring is 'see-through.' This capability is creating revolutionary applications in various fields.
Security that sees through hidden things
Ensuring safety in airports and public spaces is a critical challenge in modern society. Terahertz waves can penetrate clothing to detect hidden weapons, plastic explosives, and powdered drugs. They offer higher resolution than conventional millimeter-wave scanners and do not carry the radiation concerns associated with X-ray scanners.
Of particular note is the "passive system." This method passively detects weak terahertz waves naturally emitted by the human body and the surrounding environment. Because it emits no radio waves at all, it has no impact on health and, in terms of operation, is similar to standard surveillance cameras. Systems using technology from the European Space Agency (ESA) allow for remote scanning while passengers are walking, without the need to stop. While the images are low-resolution to respect privacy, they can reliably detect concealed items.
It is also highly effective for mail inspection. It can detect powders (such as narcotics, explosives, and anthrax), liquids, and electronic devices hidden inside envelopes or boxes without opening them. RIKEN has a track record of developing devices for detecting narcotics inside envelopes, contributing to safety assurance.
A Pharmaceutical Revolution: Visualizing Invisible Quality
Quality control in pharmaceutical manufacturing is directly linked to patient safety. Terahertz technology enables 100% inspection of tablets on a production line without destroying them. Compared to conventional destructive testing via sampling, it improves quality assurance levels while reducing time and costs.
The thickness and uniformity of tablet coatings, as well as internal porosity and moisture content, can be measured non-destructively. The UK-based company TeraView provides systems capable of inspecting tens of thousands of tablets per hour to pharmaceutical companies, contributing to more efficient quality control.
"Polymorphs," which have the same chemical composition but different crystal structures, significantly affect a drug's solubility and efficacy. Terahertz spectroscopy can identify these differences in crystal structure as a "fingerprint spectrum," making it useful for quality control in manufacturing processes. This is a "visualization" of quality that could never be captured by visual inspection or conventional testing methods.
Non-Destructive Inspection: Seeing Through Industrial Products
Technology that non-destructively detects defects such as broken wiring, voids, cracks, and delamination inside resin-encapsulated semiconductor chips contributes significantly to improving the quality of electronic devices. It is also possible to map dopant concentration distribution and sheet resistance in silicon wafers.
In the automotive industry, it is used to measure paint film thickness and investigate the state of corrosion under the paint. Because it can detect corrosion that progresses over time at an early stage, it contributes to improved safety.
It is also possible to detect delamination and fiber orientation within composite materials such as carbon fiber reinforced plastic (CFRP) used in the aerospace sector. By detecting defects that are difficult to find with conventional inspection methods, it contributes to improving aircraft safety.
The core value of terahertz technology in industrial non-destructive testing lies in its ability to non-destructively penetrate optically opaque dielectric materials to obtain internal information. X-rays may not provide sufficient contrast for low-density defects in polymers and composite materials, and ultrasonic testing requires a couplant. Terahertz waves hold the potential to resolve these limitations.
"Seeing Through" to Protect Food Safety
Technology for detecting foreign objects (insects, plastic fragments, glass shards, etc.) in packaged food is essential for ensuring food safety. It also enables the evaluation of freshness and internal quality (sugar content, ripeness, etc.) of fruits and vegetables, measurement of moisture content, detection of foreign matter in powders, and detection of mold and mycotoxins. Unlike X-rays, it is non-ionizing, which is a major advantage as there are fewer concerns regarding irradiation of food.
In the future, the day may come when consumers can check the freshness of food and the presence of foreign objects themselves using terahertz sensors built into smartphones. A world where you can check the internal quality of fruit before putting it in your shopping basket is no longer a fantasy.
Uncovering the Secrets of History: Cultural Property Investigation
Non-destructive analysis using terahertz waves is also bringing a revolution to the field of cultural property protection. It enables the detection of underdrawings in paintings, visualization of hidden layers, pigment identification, and evaluation of degradation states. It is also used for investigating the internal structures of artifacts and mummies.
It contributes to the preservation of globally important cultural properties, such as evaluating the state of the plaster layers of the Kitora Tomb murals in Nara Prefecture and investigating Leonardo da Vinci's "The Last Supper." NICT is a pioneer in this field, and in 2012, during an investigation of Ogata Korin's "Irises" screen, they discovered the fact that the entire surface had been covered in gold leaf. This was an important discovery for art history as well.
Because terahertz waves can investigate the interior of cultural properties without damaging them, research that was previously impossible is now being conducted. A new window has been opened to elucidate the production techniques and material choices of artists from the past.
A New Eye for Examining the Human Body
The biological safety of terahertz waves and their specific interactions with water and biomolecules are fueling active research into their applications in the medical field.
A New Diagnostic Method to "See Through" Cancer
There are differences in water content, cell density, and structure between cancerous and healthy tissues. Research is underway to use terahertz waves to detect these differences and identify cancer. Various types of cancer are being studied, including skin cancer, breast cancer, brain tumors, colorectal cancer, laryngeal cancer, stomach cancer, and oral cancer.
Of particular promise is its application in evaluating surgical resection margins. By more accurately identifying the boundary between tumor tissue and healthy tissue, it may be possible to reduce the amount of cancer left behind while preventing the excessive removal of healthy tissue. This is a technology that directly leads to improvements in a patient's quality of life.
The development of in vivo imaging systems, such as confocal systems using quantum cascade lasers, is also progressing, bringing us closer to realizing non-invasive cancer diagnosis. It holds the potential to estimate the nature of lesions without patient burden, prior to conventional definitive diagnosis via biopsy.
A Precision Revolution in Dentistry
It may be possible to detect early-stage tooth decay under the enamel at a stage where it is difficult to detect with X-rays or visual inspection. Research is also being conducted on the development of intraoral devices to evaluate the depth of tooth decay and prevent excessive cutting of healthy tooth structure during treatment.
Terahertz technology, which enables "minimally invasive" precision dental treatment, has the potential to fundamentally change the quality of dental care. It will contribute to the realization of ideal dental medicine—providing necessary and sufficient treatment while preserving as much of the patient's healthy tooth structure as possible.
The Evolution of Aesthetic Medicine Through Skin Visualization
The properties of terahertz waves are also being utilized in the field of dermatology, including skin moisture measurement (evaluation of hydration status), assessment of burn depth, and characterization of scar tissue and monitoring of the healing process.
A major characteristic of terahertz waves is their strong sensitivity to water. This is a double-edged sword. While strong absorption by water in biological tissue acts as a constraint for application to deep tissues, this very water sensitivity provides the primary contrast mechanism (difference in water content) between lesions such as cancer or burns and healthy areas.
In the world of aesthetic medicine, it will become possible to verify the effectiveness of cosmetics and quantitatively evaluate skin improvement. In the future, terahertz sensors might be integrated into home beauty devices, realizing the "visualization" of skincare effects.
A Future Transformed by Ultra-High-Speed Communication and Sensing
The vast, unused frequency bands in the terahertz range, particularly the sub-terahertz band (100 GHz and above), are expected to be the key to realizing ultra-high-speed, large-capacity wireless communication for next-generation mobile communication systems (6G) and beyond. Data transmission speeds on the order of terabits per second (Tbps) are predicted to be possible, which is more than 100 times the speed of current 5G communication.
However, terahertz communication faces several challenges. Because the frequency is high, free-space propagation loss (proportional to the square of the distance) and absorption loss by molecules in the atmosphere (especially water vapor) are significant. Furthermore, non-line-of-sight propagation is difficult, and it is easily affected by shielding from objects such as the human body.
To overcome these challenges, technology development is underway for beamforming, which uses numerous antenna elements to concentrate radio waves in a specific direction, as well as Massive MIMO and Reconfigurable Intelligent Surfaces (RIS).
With the advancement of autonomous driving technology, terahertz radar is attracting attention as a technology that complements existing millimeter-wave radar, LiDAR, and cameras. It has the potential to perform better than optical sensors, especially in adverse weather conditions such as fog, snow, and dust. This is a technology that will significantly improve the safety of autonomous driving.
Sensing using terahertz waves will enable not only the detection of road conditions and obstacles but also the measurement of road surface freezing and snow depth. This will bring us closer to realizing autonomous driving under all weather conditions.
The Big Challenge of Small Light Sources: Technical Issues and Latest Trends
For the widespread adoption of terahertz technology, there are several key challenges. The biggest barriers are cost, size, and output/sensitivity.
One of the primary focuses of research and development is to reduce reliance on expensive femtosecond lasers and cryogenic cooling components, shifting instead toward integrated semiconductor solutions. There are reports of new resonant tunneling diode (RTD) devices aiming for costs less than one-tenth of conventional methods.
Miniaturization of devices is essential for integration into portable scanners, smartphones, drones, and satellites. It is claimed that the new RTD devices have reduced their volume to less than 1/1000th of previous versions. CMOS integration is the key.
In terms of performance, improvements in light source output (especially room-temperature continuous-wave), detector sensitivity and response speed, system dynamic range, and bandwidth are being continuously pursued. Efforts are also underway to overcome the fundamental limitations caused by the terahertz gap.
Leaps in evolution brought about by new materials
The discovery and application of new materials are contributing significantly to the performance improvement and functional expansion of terahertz technology. Graphene is being actively researched for applications such as wavelength-tunable terahertz modulators, polarizers, detectors, and sensors, leveraging its unique properties of electrically controllable conductivity and strong interaction with terahertz waves.
Graphene, often called a "dream material," is a carbon sheet structure only one atom thick. Due to its unique electronic properties, it exhibits strong interaction with terahertz waves. Development of wavelength-tunable terahertz modulators and detectors utilizing these properties is progressing.
Artificially designed microstructures known as metamaterials and metasurfaces exhibit electromagnetic responses not found in nature (such as perfect absorption at specific frequencies, light focusing, polarization control, and phase modulation). This enables new devices that were difficult to realize with conventional optical components, such as thin lenses, high-sensitivity sensors, and perfect absorbers.
The appeal of metamaterials lies in their artificial design flexibility. Through appropriate structural design, special electromagnetic responses that cannot be obtained with natural materials can be realized. This technology contributes significantly to the realization of high-performance optical components that can bend terahertz waves in arbitrary directions or selectively transmit, reflect, or absorb specific frequencies.
An analytical revolution brought about by fusion with AI
Processing and interpreting terahertz data is also a major challenge. The development of signal processing and analysis algorithms is essential to extract useful information from raw terahertz data and obtain practical insights.
The use of machine learning (ML) and artificial intelligence (AI) technologies is advancing rapidly, with ML algorithms being applied to noise reduction in spectra and images, feature extraction, classification and identification of defects and substances, anomaly detection, and quantitative prediction of physical properties. This is expected to lead to the automation, acceleration, and higher precision of analysis.
The combination of terahertz waves and AI is particularly powerful in medical diagnosis and security screening. By utilizing AI, high-precision identification becomes possible even for non-experts, which will accelerate practical application in a wider range of fields.
Social transformation brought about by the "See-Through Revolution"
Terahertz technology has overcome the former "gap" and developed into a vibrant field where practical applications are being demonstrated in diverse areas. Its unique physical properties (transparency to specific substances, non-ionizing safety, and molecular identification capability) provide powerful advantages that meet specific needs difficult to address with existing technologies.
The long-term vision for terahertz technology is to move beyond specialized industrial equipment and scientific instruments to be seamlessly integrated into everyday devices and infrastructure such as smartphones, automobiles, and communication networks. As seen with GPS and cameras, the progress of miniaturization and cost reduction utilizing semiconductor platforms will make integration into consumer products a reality.
An explosively growing market
The terahertz technology market is expected to see significant growth. While forecasts vary by research firm, a high compound annual growth rate (CAGR) of 15.5% to 21.82% is predicted. Some forecasts suggest the market size will reach 4.08 billion USD by 2032, up from approximately 780 million USD in 2023.
Supporting this growth is demand from a wide range of industrial sectors, including semiconductors, electronics, pharmaceuticals, food, security, and communications. In particular, application fields such as non-destructive testing, quality control, and security screening are expected to drive the market.
Social impact requiring a balance
The widespread adoption of terahertz technology has the potential to have various impacts on society. It is necessary to strike a balance between the improved safety provided by security screening and non-destructive testing, and privacy concerns regarding devices such as body scanners.
Terahertz technology is non-ionizing and may have advantages from a privacy protection perspective, as it may depict less of the body's shape than conventional X-ray backscatter systems. However, as the performance of see-through technology improves, social and ethical debates will likely intensify.
Ultra-high-speed, large-capacity communication from 6G onwards could accelerate the digitalization of society and create new services and lifestyles. Earlier and less invasive diagnostic technologies will likely contribute to the advancement of personalized medicine and the extension of healthy life expectancy.
One of the greatest changes brought about by the spread of terahertz technology will be the 'visualization' of the invisible world. Internal structures and physical properties that were previously inspected only by experts using specialized equipment will become easily verifiable by the general public. This has the potential to expand our very act of 'seeing' and fundamentally change how we perceive the world.
Toward a future of seeing the invisible world
Terahertz technology literally provides society with a new window to 'see the invisible.' It holds the potential to have a major impact over the coming decades in fields such as communications, sensing, security, industry, and medicine. The 'see-through revolution' opened up by terahertz is currently underway, and its future is filled with immeasurable possibilities.
What was once called the 'last frontier of the electromagnetic spectrum,' this uncharted territory is now transforming into a revolutionary technology that will change our daily lives. The future that this wave brings to us is no longer the world of science fiction, but a reality unfolding before our eyes.
Checking food safety with a small smartphone, performing non-invasive early detection of cancer, safely operating autonomous vehicles even in fog, and uncovering the hidden secrets of cultural assets—the 'see-through revolution' opened up by terahertz waves pushes the limits of our perception and democratizes access to the invisible world. Scientific progress is once again about to fundamentally change our experience of 'seeing'.
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