The kind of light used to examine a material can reveal very different details. Visible light shows what’s happening on a surface, X-rays expose what lies inside, and infrared detects heat being emitted.
Now, researchers at MIT have taken a major step forward by using terahertz light to uncover quantum-level vibrations inside a superconducting material. These subtle motions had never been directly observed before.
What Makes Terahertz Light Unique
Terahertz radiation sits between microwaves and infrared light on the electromagnetic spectrum. It pulses more than a trillion times per second, closely matching the natural vibrations of atoms and electrons within materials. In theory, this makes it an ideal way to study those movements.
However, there is a major challenge. The wavelength, or the distance between repeating peaks of the wave, is very long, measuring hundreds of microns. Because light cannot be focused into a spot smaller than its wavelength, terahertz beams are too large to clearly probe tiny structures. Instead of revealing fine details, they tend to wash over microscopic samples.
A New Terahertz Microscope Breakthrough
In a study published in Nature, MIT scientists report a solution. They created a new type of terahertz microscope that compresses this long-wavelength light into an extremely small region. This focused beam can now detect quantum-scale features that were previously out of reach.
Using this tool, the team examined a material called bismuth strontium calcium copper oxide, or BSCCO (pronounced “BIS-co”), which becomes superconducting at relatively high temperatures. The microscope allowed them to observe a frictionless flow of electrons behaving like a “superfluid,” moving together and oscillating at terahertz frequencies within the material.
“This new microscope now allows us to see a new mode of superconducting electrons that nobody has ever seen before,” says Nuh Gedik, the Donner Professor of Physics at MIT.
Why This Discovery Matters
Studying BSCCO and similar materials with terahertz light could help scientists better understand superconductivity and move closer to developing room-temperature superconductors. The technology may also help identify materials that can emit and detect terahertz radiation.
Such materials could play a key role in future wireless systems that operate at terahertz frequencies, potentially enabling much faster data transmission than current microwave-based technologies.
“There’s a huge push to take Wi-Fi or telecommunications to the next level, to terahertz frequencies,” says Alexander von Hoegen, a postdoc in MIT’s Materials Research Laboratory and lead author of the study. “If you have a terahertz microscope, you could study how terahertz light interacts with microscopically small devices that could serve as future antennas or receivers.”
The research team also included MIT scientists Tommy Tai, Clifford Allington, Matthew Yeung, Jacob Pettine, Alexander Kossak, Byunghun Lee, and Geoffrey Beach, along with collaborators from Harvard University, the Max Planck Institute for the Structure and Dynamics of Matter, the Max Planck Institute for the Physics of Complex Systems, and Brookhaven National Laboratory.
The Diffraction Limit Problem
Terahertz light has long been considered promising for imaging because it occupies a useful middle ground. Like radio waves and visible light, it is nonionizing and safe for biological tissues. At the same time, it can penetrate many materials, including fabrics, plastics, wood, and even thin walls, similar to X-rays.
Because of these advantages, terahertz radiation is being explored for security scanning, medical imaging, and communications. But its use in microscopy has been limited by a fundamental constraint known as the diffraction limit. This rule restricts how finely light can resolve details based on its wavelength.
Since terahertz wavelengths are much larger than atoms and molecules, they cannot normally resolve microscopic features.
“Our main motivation is this problem that, you might have a 10-micron sample, but your terahertz light has a 100-micron wavelength, so what you would mostly be measuring is air, or the vacuum around your sample,” von Hoegen explains. “You would be missing all these quantum phases that have characteristic fingerprints in the terahertz regime.”
Overcoming the Limit With Spintronic Emitters
To get around this limitation, the researchers used spintronic emitters, a newer technology that generates short bursts of terahertz radiation. These emitters are made from stacked ultrathin metal layers. When struck by a laser, they trigger a chain reaction in electrons that produces terahertz pulses.
By placing the sample extremely close to the emitter, the team captured the terahertz light before it could spread out. This effectively compressed the light into a region much smaller than its wavelength, allowing it to bypass the diffraction limit and reveal much finer details.
Imaging Quantum Motion in Superconductors
The team built their microscope by combining spintronic emitters with a Bragg mirror, a layered structure that filters out unwanted wavelengths while protecting the sample from the laser used to generate the terahertz light.
They tested the system on an ultrathin sample of BSCCO, cooling it to near absolute zero so it would enter its superconducting state. By scanning a laser across the sample, they sent terahertz pulses through it and measured how the signal changed.
“We see the terahertz field gets dramatically distorted, with little oscillations following the main pulse,” von Hoegen says. “That tells us that something in the sample is emitting terahertz light, after it got kicked by our initial terahertz pulse.”
Further analysis revealed that these signals came from the natural, collective oscillations of superconducting electrons.
“It’s this superconducting gel that we’re sort of seeing jiggle,” von Hoegen says.
A New Window Into Quantum Phenomena
Although scientists had predicted this kind of motion, it had never been directly observed until now. The team is already applying the microscope to other two-dimensional materials to explore additional terahertz-scale effects.
“There are a lot of the fundamental excitations, like lattice vibrations and magnetic processes, and all these collective modes that happen at terahertz frequencies,” von Hoegen says. “We can now resonantly zoom in on these interesting physics with our terahertz microscope.”
This work was supported in part by the U.S. Department of Energy and the Gordon and Betty Moore Foundation.