CAMBRIDGE, Mass. — You can tell a lot about a material based on the type of light you shine on it. Optical light illuminates the surface of the material, X-rays reveal the material’s internal structure, and infrared light captures the material’s radiant heat.

Now, MIT physicists have used terahertz light to uncover previously unobservable quantum oscillations inherent in superconducting materials.

Terahertz light is a type of energy that lies between microwave and infrared light on the electromagnetic spectrum. Vibrations occur over 1 trillion times per second. This is just the right pace for atoms and electrons to vibrate naturally within matter. Ideally, this would make terahertz light the perfect tool to investigate these movements.

But while the frequency is correct, the wavelength (the distance the wave repeats in space) is not. Terahertz waves have wavelengths several hundred microns long. Terahertz beams cannot be tightly confined because the smallest spot on which any type of light can be focused is limited by its wavelength. As a result, the focused terahertz beam is physically too large to interact effectively with the microscopic sample and simply washes away these small structures without revealing them.

in a paper published in a magazine naturescientists report that they have developed a new terahertz microscope that compresses terahertz light down to microscopic dimensions. This pinpoint terahertz light will be able to reveal previously inaccessible quantum details in matter.

The researchers used a new microscope to beam terahertz light into a sample of bismuth strontium calcium copper oxide (BSCCO, pronounced “BIS-co”), a material that is superconducting at relatively high temperatures. Using a terahertz scope, the researchers observed a frictionless “superfluid” of superconducting electrons collectively swinging back and forth at terahertz frequencies within the BSCCO material.

“This new microscope allows us to see a new mode of superconducting electrons that no one has seen before,” said Nou Gedik, Donner Professor of Physics at the Massachusetts Institute of Technology.

Using terahertz light to probe BSCCO and other superconductors will allow scientists to better understand the properties that could lead to long-sought room-temperature superconductors. The new microscope will also help identify materials that emit and receive terahertz radiation. Such materials could be the basis for future terahertz-based wireless communications, which could transmit more data and faster compared to today’s microwave-based communications.

“There’s a big movement 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.”

In addition to Gedik and von Hoegen, co-authors of the study include Tommy Tai, Clifford Allington, Matthew Yeung, Jacob Pettine, Alexander Kossak, Byunghun Lee, and Geoffrey Beach of MIT and Harvard University, the Max Planck Institute for Structure Dynamics of Matter, the Max Planck Institute for the Physics of Complex Systems, and Brookhaven National Laboratory.

reach the limit

Terahertz light is a promising yet largely untapped imaging tool. Terahertz radiation occupies a unique spectral “sweet spot.” Like microwaves, radio waves, and visible light, terahertz radiation is non-ionizing, meaning it does not have enough energy to cause harmful radiation effects and can be safely used in humans and biological tissues. At the same time, like X-rays, terahertz waves can penetrate a wide range of materials, including fabrics, wood, cardboard, plastics, ceramics, and even thin brick walls.

Because of these unique properties, terahertz light is being actively researched for applications in security inspections, medical image processing, wireless communications, and more. In contrast, much less effort has been expended on applying terahertz radiation to microscopy and illumination of microscopic phenomena. The main reason for this is a fundamental limitation common to all forms of light: the diffraction limit, where spatial resolution is approximately limited to the wavelength of the radiation used.

With wavelengths on the order of a few hundred microns, terahertz radiation is much larger than atoms, molecules, and many other microstructures. As a result, the ability to directly resolve microscale features is fundamentally limited.

“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 range, you’re going to miss all of these quantum phases that have distinctive fingerprints.”

zoom in

The research team has found a way to circumvent the diffraction limit of terahertz by using spintronic emitters, a new technology that generates sharp pulses of terahertz light. Spintronic emitters are made from multiple ultrathin metal layers. When a laser shines on a multilayer structure, the light causes a cascading effect on the electrons within each layer, ultimately causing the structure to emit pulses of energy at terahertz frequencies.

By holding the sample close to the emitter, the researchers captured the terahertz light before it spread out, essentially trapping it in a space much smaller than its wavelength. In this region, light can bypass the diffraction limit and resolve features that were previously too small to see.

The MIT team applied this technique to observing microscopic, quantum-scale phenomena. For the new study, the team developed a terahertz microscope using a spintronic emitter connected to a Bragg mirror. This multilayer structure of reflective coatings continuously filters out certain unwanted wavelengths of light while allowing others to pass through, protecting the sample from the “harmful” lasers that cause terahertz radiation.

As a demonstration, the research team used the new microscope to image a small sample of atomically thin BSCCO. They placed the sample very close to a terahertz source and imaged it at temperatures close to absolute zero, hot enough for the material to become a superconductor. To create the images, they scanned a laser beam and sent terahertz light through the sample, looking for specific traces left by superconducting electrons.

“You can see that the main pulse is followed by 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.”

After further analysis, the team concluded that the terahertz microscope was observing natural collective terahertz oscillations of superconducting electrons within the material.

“What we see shaking is this superconducting gel,” von Hoogen says.

Although this oscillating superfluid was expected, it had never been directly visualized before. The research team is now applying the microscope to other two-dimensional materials, hoping to capture more terahertz phenomena.

“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.

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Written by: Jennifer Chu, MIT News