Wikipedia Deep Dive

Terahertz radiation

12 min read

The Light We Cannot See

There's a kind of light that passes through your clothes, your walls, and even your skin—but you've never seen it, and neither has anyone else. It's invisible to the naked eye, undetectable by most instruments, and until recently, almost impossible to generate or measure with any precision. Scientists call it terahertz radiation, and for decades it occupied a strange no-man's-land in physics: too fast for electronics, too slow for optics, and frustratingly out of reach for practical applications.

That's changing.

Terahertz radiation sits in a peculiar position on the electromagnetic spectrum—that grand continuum that includes everything from radio waves to gamma rays. It nestles between microwaves (the kind that heat your food) and infrared light (the kind that night-vision goggles detect). One terahertz equals one trillion cycles per second, or about a thousand times faster than the gigahertz frequencies your WiFi router uses.

To understand what makes this slice of the spectrum special, imagine the electromagnetic spectrum as a piano. Radio waves are the deep bass notes on the left, rumbling at frequencies you can almost feel. Visible light sits in the middle octaves, the notes your eyes can perceive. X-rays and gamma rays scream at the far right, at frequencies so high they can punch through matter and damage DNA.

Terahertz radiation plays somewhere around middle C—not the dramatic crash of X-rays, not the comfortable thrum of radio. It's an in-between frequency that, as it turns out, has some remarkable properties.

The Terahertz Gap

For most of the twentieth century, terahertz radiation was more theoretical than practical. Engineers had mastered the art of generating and detecting radio waves, microwaves, and even infrared light. But terahertz? The technology simply didn't exist.

The problem was fundamental. Radio waves are created by making electrons oscillate back and forth in an antenna—essentially, by moving charge in a controlled, rhythmic pattern. This works beautifully up to a few hundred gigahertz, but beyond that, electrons can't keep up. Their mass becomes a problem. They're too heavy to oscillate a trillion times per second.

On the other side of the spectrum, infrared and visible light are generated by quantum jumps—electrons hopping between energy levels in atoms and molecules. But these quantum transitions produce light at frequencies that are typically too high for terahertz. You can't easily coax atoms into making the small, precise jumps needed for this middle frequency.

This left a gap. Scientists called it, with uncharacteristic directness, the "terahertz gap." It was a region of the electromagnetic spectrum where neither electronic nor optical techniques worked particularly well. For decades, it remained largely unexplored—a blank spot on the map.

What Terahertz Can Do

Despite the difficulty of generating it, scientists eventually discovered that terahertz radiation has some extraordinary properties. And these properties suggest applications that could transform medicine, security, and manufacturing.

First, terahertz waves pass through many materials that block visible light. Clothing, paper, cardboard, wood, plastic, ceramics—terahertz radiation slips through them all. This is similar to how microwaves penetrate materials, but with an important difference: terahertz wavelengths are much shorter, which means they can form sharper images.

Second, terahertz radiation is stopped by water and metal. This might sound like a limitation, but it's actually useful. The human body is mostly water, which means terahertz waves don't penetrate deeply into tissue. They can see through your shirt but not through your skin—or more precisely, they can see a few millimeters into tissue with low water content, like fat, but are absorbed by anything more hydrated.

Third—and this is crucial—terahertz radiation is non-ionizing. Unlike X-rays, which carry enough energy to knock electrons off atoms and damage DNA, terahertz photons are gentle. They don't break chemical bonds. They don't cause cancer. They just... bounce around and reveal information.

Put these properties together and you get something remarkable: a form of radiation that can see through clothes and packaging, create detailed images, and do it all without the health risks of X-rays.

Seeing Through Walls (Sort Of)

The most obvious application is security screening. Airports currently use millimeter-wave scanners to check passengers for concealed weapons. Terahertz imaging could do the same thing, potentially with better resolution and the ability to identify specific materials.

Here's why material identification matters. Different molecules absorb and reflect terahertz radiation in characteristic ways—they have unique "fingerprints" at these frequencies. A plastic explosive has a different terahertz signature than a block of cheese. In principle, a terahertz scanner could not only see that you're hiding something under your coat but also determine what that something is made of.

In 2002, a team at the Rutherford Appleton Laboratory in the United Kingdom created the first passive terahertz image of a human hand. "Passive" means they didn't blast the hand with terahertz radiation; they simply detected the natural terahertz emissions that all warm objects produce. Everything above about two Kelvin—two degrees above absolute zero—radiates some terahertz energy. The amount is tiny, but with sensitive enough detectors, it can be measured.

By 2004, a company called ThruVision had built a compact terahertz camera for security applications. The prototype successfully imaged guns and explosives concealed under clothing. Unlike traditional body scanners, which essentially show a nude image of the subject, terahertz scanners tuned to specific frequencies could theoretically detect only contraband materials, leaving everything else invisible—a privacy improvement, at least in principle.

The New York Police Department announced in 2013 that it was experimenting with terahertz scanning to detect concealed weapons. A privacy advocate promptly sued, arguing that looking through someone's clothing without a warrant violated reasonable expectations of privacy. By 2017, the department said it had no plans to deploy the technology.

The Medical Promise

The potential medical applications of terahertz imaging might be even more significant than security screening. Because terahertz radiation is absorbed differently by tissues with different water content, it can reveal contrasts that other imaging techniques miss.

Consider cancer detection. Tumors often have different water content than surrounding healthy tissue. Terahertz imaging might be able to spot these differences, particularly in epithelial cancers—cancers of the skin and the linings of organs. The technique would be non-invasive, painless, and safe.

Dentistry offers another application. Terahertz waves can penetrate tooth enamel and create three-dimensional images of dental structures. Early studies suggest this might be more accurate than traditional X-rays for detecting cavities and other problems, without any radiation exposure.

When the COVID-19 pandemic created urgent demand for rapid screening methods, researchers proposed terahertz spectroscopy as a potential tool. The idea was that infected tissue might have a different terahertz signature than healthy tissue, allowing for quick, non-contact screening.

The Art of Looking Back

One of the more unexpected applications of terahertz radiation involves art history. Many ancient buildings have paintings hidden beneath layers of plaster or newer paint—frescoes that were covered up rather than destroyed. Terahertz radiation can penetrate these covering layers and reveal what lies beneath, all without damaging the artwork.

This is possible because terahertz waves interact with pigments and binding materials in characteristic ways. Different paints absorb different amounts of terahertz radiation. By scanning a plastered-over wall with terahertz radiation and measuring what reflects back, art historians can reconstruct images of hidden murals—ghosts of paintings that haven't been seen for centuries.

The Coldest Stars

Before terahertz radiation found applications on Earth, it was already important in astronomy. Cold objects in space—interstellar dust clouds, the gas between galaxies, the frigid outer reaches of stellar systems—emit most of their radiation in the terahertz range.

This follows from a fundamental law of physics discovered by Max Planck in 1900. Hot objects glow at high frequencies; cold objects glow at low frequencies. The sun, at nearly 6,000 Kelvin, radiates mostly visible light. A human body, at about 310 Kelvin, radiates infrared. And cosmic dust, at temperatures of just 10 to 20 Kelvin, radiates in the terahertz band.

To study this cold universe, astronomers have built telescopes specifically designed to detect submillimeter waves—another name for terahertz radiation, reflecting the fact that its wavelengths are shorter than a millimeter. The James Clerk Maxwell Telescope and the Submillimeter Array in Hawaii, the Herschel Space Observatory, and the Atacama Large Millimeter Array in Chile all observe at these frequencies.

There's a catch, though. Earth's atmosphere absorbs terahertz radiation strongly. Water vapor in the air acts like a fog, blocking most submillimeter waves from reaching the ground. That's why these telescopes are built at extreme altitudes—Hawaii's Mauna Kea, Chile's Atacama Desert—or launched into space. Only in the driest, highest, or vacuum environments can astronomers glimpse the cold universe.

Closing the Gap

The terahertz gap is finally closing. Over the past two decades, researchers have developed a growing toolkit of sources and detectors that work in this once-inaccessible frequency range.

Some sources are exotic. Gyrotrons use spinning electrons in magnetic fields to generate terahertz waves. Free-electron lasers accelerate electrons to nearly the speed of light and wiggle them through magnetic fields, producing intense terahertz radiation. Synchrotrons—the giant particle accelerators used for physics research—also emit terahertz light as a byproduct of bending electron beams around curves.

Other sources are surprisingly mundane. In 2009, researchers discovered that peeling adhesive tape generates terahertz radiation. As you pull the tape away from a surface, you create static electricity—the same effect that makes your hair stand up when you pull off a wool sweater. This "tribocharging" builds up electric charge, which then discharges in tiny sparks. Those sparks, it turns out, produce terahertz pulses.

The most promising commercial sources are solid-state devices—semiconductor chips that can generate terahertz waves electronically. Quantum cascade lasers, developed in the 1990s, work by cascading electrons through a precisely engineered series of quantum wells, emitting a photon at each step. Originally, these devices required cryogenic cooling to work, but by 2008, engineers at Harvard had achieved room-temperature operation.

Graphene, the atom-thick sheets of carbon that have fascinated materials scientists for two decades, offers another route. Researchers have proposed building tiny antennas from graphene strips just 10 to 100 nanometers wide. These nano-antennas could emit and receive terahertz signals, potentially enabling new kinds of wireless communication at frequencies far above today's WiFi and cellular bands.

The Biological Resonance

Here's a fact that sounds almost too strange to be true: the natural vibrations of biological molecules happen at terahertz frequencies.

Proteins flex and twist. Enzymes close around their substrates. DNA unzips and reforms. These molecular motions occur on timescales of picoseconds—trillionths of a second. And a picosecond corresponds to exactly one terahertz: one trillion cycles per second, or one cycle per trillionth of a second. The math works out perfectly.

This means terahertz radiation resonates with the natural rhythms of life's machinery. At sufficiently low intensities—not enough to heat tissue or ionize atoms—terahertz waves might be able to nudge or modulate biological processes. Some researchers have reported effects on neural function in laboratory settings, though this work remains preliminary and controversial.

The implications are unsettling and exciting in equal measure. If terahertz radiation can influence biological systems without heating or damaging them, it opens possibilities for new medical treatments—but also new concerns about exposure. As terahertz technology becomes more common, understanding its biological effects will become increasingly important.

The Wireless Future

Perhaps the most transformative application of terahertz technology lies in communications. The electromagnetic spectrum is crowded. Radio, television, cellular phones, WiFi, Bluetooth, satellite links—all compete for the same limited frequencies below about 100 gigahertz. As demand for wireless bandwidth continues to grow, engineers are looking higher.

Terahertz frequencies offer an enormous amount of virgin spectrum. And higher frequencies mean more bandwidth, which means more data. A terahertz wireless link could, in principle, carry vastly more information than today's 5G networks.

There's a problem, though. Remember that Earth's atmosphere absorbs terahertz radiation? In air, most terahertz energy is attenuated within a few meters. You can't build a continent-spanning terahertz cell network.

But you could build short-range, high-bandwidth links. Inside buildings, where distances are measured in meters rather than kilometers, terahertz wireless networks could provide data rates measured in terabits per second—fast enough to transfer an entire movie in less than a second. For indoor applications, data centers, or point-to-point links across short distances, the terahertz band could be transformative.

Still Emerging

Despite all this promise, terahertz technology remains in its adolescence. Most devices are still expensive, bulky, or both. Many require cryogenic cooling. Mass production is limited, which keeps costs high.

The phrase "terahertz gap" is still used, though it increasingly describes a technology gap rather than a physical one. The physics is understood. The applications are clear. What remains is the engineering work of making terahertz devices cheap, compact, and robust enough for everyday use.

That work is underway. In laboratories around the world, researchers are shrinking terahertz sources, developing room-temperature detectors, and designing integrated circuits that operate at frequencies once thought impossible for electronics. The gap is closing year by year.

Sometime in the next decade or two, terahertz technology will likely transition from exotic laboratory curiosity to everyday tool—just as microwave technology did in the mid-twentieth century, or laser technology did in the late twentieth. When that happens, the strange, invisible light that passes through your clothes will start passing through security checkpoints, medical scanners, and wireless networks.

The light we cannot see is about to become very useful indeed.