The type of light used to examine the material reveals completely different details. Visible light shows what’s happening on the surface, X-rays reveal what’s inside, and infrared detects the heat being emitted.
Now, researchers at MIT have made a major step forward by using terahertz light to reveal quantum-level vibrations inside superconducting materials. Such subtle movements have never been directly observed before.
Characteristics of terahertz light
Terahertz radiation falls between microwave and infrared radiation on the electromagnetic spectrum. It pulses more than 1 trillion times per second, closely matching the natural vibrations of atoms and electrons in matter. In theory, this would be an ideal way to study their movements.
However, there are major challenges. The wavelength, or the distance between the repeating peaks of a wave, is very long, reaching hundreds of microns. Because light cannot be focused to a spot smaller than its wavelength, terahertz beams are too large to clearly investigate small structures. It tends to wash out microscopic samples instead of revealing fine details.
New advances in terahertz microscopy
In a study published in natureScientists at MIT report a solution. They developed a new type of terahertz microscope that compresses this long-wavelength light into an extremely small area. This focused beam allows us to detect quantum-scale features that were previously inaccessible.
Using this tool, the team investigated a material called bismuth strontium calcium copper oxide (BSCCO, pronounced “BIS-co”), which becomes superconducting at relatively high temperatures. Using this microscope, they were able to observe frictionless flows of electrons that behave like a “superfluid,” moving together in matter and vibrating at terahertz frequencies.
“With this new microscope, we can now see a new mode of superconducting electrons that no one has seen before,” said Nou Gedik, Donner Professor of Physics at MIT.
Why is this discovery important?
Studying BSCCO and similar materials with terahertz light could help scientists better understand superconductivity and move them closer to developing room-temperature superconductors. This technique could also help identify materials that can emit and detect terahertz radiation.
Such materials could play a key role in future wireless systems operating at terahertz frequencies, potentially allowing much faster data transmission than current microwave-based technologies.
“There’s a big push to take Wi-Fi and telecommunications to the next level, to terahertz frequencies,” says Alexander von Hoogen, a postdoctoral fellow in the MIT Materials Institute and lead author of the study. “If you have a terahertz microscope, you can study how terahertz light interacts with microscopically small devices that could serve as future antennas or receivers.”
The research team included MIT scientists Tommy Tai, Clifford Arrington, Matthew Yang, Jacob Pettine, Alexander Kosak, Byung-Hoon Lee, and Jeffrey Beach, as well as 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.
Diffraction limit problem
Terahertz light has long been considered promising for imaging because it occupies a useful intermediate point. Like radio waves and visible light, it is non-ionic and safe for living tissues. At the same time, like X-rays, they can penetrate many materials, such as fabrics, plastics, wood, and even thin walls.
Because of these benefits, terahertz radiation is being investigated for security scanning, medical imaging, and communications. However, its use in microscopy has been limited by a fundamental constraint known as the diffraction limit. This rule limits how finely detailed light can resolve based on wavelength.
Because terahertz wavelengths are much longer than atoms and molecules, they typically cannot resolve minute features.
“Our main motivation is the problem that we might have a 10-micron sample, but the wavelength of terahertz light is 100 microns, so what we’re primarily measuring is air, or the vacuum around the sample,” von Hoogen explains. “In the terahertz region, you’re going to miss all of these quantum phases that have characteristic signatures.”
Overcoming limitations with spintronic emitters
To get around this limitation, the researchers used a new technology, spintronic emitters, that generate short bursts of terahertz radiation. These emitters are made by stacking extremely thin metal layers. When hit by a laser, it triggers a chain reaction of electrons, creating a terahertz pulse.
By placing the sample very close to the emitter, the researchers captured the terahertz light before it spread out. This effectively compressed the light into an area much smaller than its wavelength, allowing it to bypass the diffraction limit and reveal finer details.
Imaging quantum motion in superconductors
The research team built a microscope by combining spintronic emitters and Bragg mirrors. A Bragg mirror is a layered structure that filters out unwanted wavelengths while protecting the sample from the laser used to generate terahertz light.
They tested their system on ultrathin samples of BSCCO, which were cooled to near absolute zero to become superconducting. By scanning a laser across the sample, they delivered terahertz pulses to the sample and measured how the signal changed.
“You can see that after the main pulse there are small oscillations that dramatically distort the terahertz field,” von Hoogen says. “This indicates that something in the sample is emitting terahertz light after being stimulated by the first terahertz pulse.”
Further analysis revealed that these signals come from natural collective oscillations of superconducting electrons.
“What we see shaking is this superconducting gel,” von Hoogen says.
A new window into quantum phenomena
Scientists had predicted this type of movement, but it had never been directly observed before. The researchers are already applying the microscope to other two-dimensional materials to investigate further terahertz-scale effects.
“There are a lot of fundamental excitations, like lattice vibrations and magnetic processes, and there are all these collective modes that occur at terahertz frequencies,” von Hoogen says. “With terahertz microscopy, we can now zoom in on these interesting physics resonantly.”
This research was supported in part by the U.S. Department of Energy and the Gordon and Betty Moore Foundation.
