Transparent ceramic could boost internet speeds and cut energy use
Researchers use advanced physics theory to solve performance mystery of the advanced material
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A new physics theory called zentropy could help explain why recently developed transparent ceramics control light far better than expected, a discovery that could lead to faster, smaller and more energy-efficient optical technologies used in communications, sensing and medical imaging.
view moreCredit: Zi-Kui Liu/Phases Research Lab
UNIVERSITY PARK, Pa. — A new class of ceramics are not only transparent, but they can control light with exceptional efficiency — better than any theories predicted. Now, an advanced theory put forth by researcher at Penn State may explain why this material is so good at light control, which could lead to large-scale manufacturing of these materials for faster, smaller and more energy efficient technologies used in high-speed communications, medical imaging and advanced sensing.
To solve the puzzle of why transparent ceramic’s electro-optic properties — the ability to change how they bend or transmit light when a voltage is applied — performed far better than predicted, Haixue Yan, reader in materials science and engineering from the Queen Mary University of London, reached out to Zi-Kui Liu, a Penn State professor of materials science and engineering. Liu previously developed an advanced theory of entropy, or the concept that systems trend towards disorder if no energy is applied to keep the chaos at bay. This advanced theory, known as zentropy theory, blends quantum mechanics, thermodynamics and statistical mechanics into a single predictive framework. Together, along with a team representing multiple institutions across six countries, they solved the mystery and published their work in the Journal of the American Chemical Society.
Ceramics offered major advantages for optical technologies because they are far cheaper to manufacture than single crystals, easier to scale into usable components and allow precise control of composition. However, to function in electro-optic devices, the material must be transparent so that light passes through it smoothly, a longstanding challenge that recent processing advances have finally overcome.
“Ceramics are much cheaper, easier to manufacture and allow precise control of the material’s chemical composition,” Liu said. “The challenge is that ceramics must be transparent, so the light can pass through them smoothly without distortion, before they can function as electro-optic materials.”
Researchers achieved transparency by using improved manufacturing techniques that smooth out the tiny imperfections inside the ceramic, the same imperfections that would normally scatter light and make the material look cloudy. These newer methods help the ceramic’s internal grains line up more evenly with much less defects, allowing light to pass straight through. The research team used these techniques to create the fully transparent ceramics used in the study. This, in turn, enabled the strong electro-optic results, which were a surprise to the researchers.
“There was no existing theory in the ferroelectrics community that could explain these results,” Liu said, explaining that Yan learned of his zentrophy theory and reached out to collaborate. Liu said the team was motivated by hints in the scientific literature that transparent ferroelectric single crystals with dense domain walls could show unusually strong electro-optic behavior. Scientists suspected that if unusual electro-optic behavior appeared in single crystals with many domain walls — the internal boundaries that separate differently oriented regions inside the material — the same underlying mechanism might also show up in ceramics, which naturally contain even richer domain structures.
From analyzing the transparent ceramic materials, Yan, Liu and the rest of the team found that the same mechanism did appear — and enabled a much stronger performance. The problem, they explained, was understanding why. To understand these results, the team zoomed farther into the material than scientists normally look. In typical ferroelectric materials, the electric charge is arranged into large “domains,” which are regions made of thousands of atoms that all line up and flip direction together when a voltage is applied. These big domains work well for technologies that operate at slower, radio-frequency speeds, but they simply can’t move fast enough to respond to the incredibly rapid light waves used in photonics. According to the researchers, big domains could not account for the unusually strong electro-optic effects they saw in transparent ceramics.
So, the team turned to high-resolution transmission electron microscopy and advanced computer simulations to look at the material on a much smaller scale. Instead of large, slow-moving domains, they found the material contained tiny pockets of polarization only a few atoms wide. These small, fast-responding structures, almost like “mini-domains,” helped explain the ultrahigh performance.
“These very small polar features have extremely fast relaxation times,” Liu said. “They can adjust their electronic polarization almost instantly under an applied field.”
He explained that these tiny polar regions are not static. Instead, they fluctuate continuously and are dynamic, which allows them to respond at optical speeds.
“This behavior is very different from typical ferroelectrics,” Yan said.
Liu’s zentropy theory helped the team make sense of why the new ceramics behaved so differently from what existing ferroelectric models predicted. Zentropy is designed to capture how atoms inside a material constantly shift, vibrate and rearrange — behavior that traditional theories often treat as background noise, Liu said. Through the lens of zentropy, the researchers mapped out all the tiny structural states the atoms can adopt and then calculated how those rapid fluctuations add up to influence the material’s overall performance. This approach is especially useful for ferroelectrics, whose internal structures are highly dynamic, particularly at the high frequencies used in photonics, according to the researchers.
They found that the theory of zentropy could explain why the small, fast-moving polar regions they observed were able to respond at optical speeds. When a material’s internal structure breaks down into these tiny, fluctuating units, the energy needed for the polarization to flip becomes extremely low. That means the material can adjust to an applied electric field almost instantly, producing the ultrahigh electro-optic response seen in the experiments. Traditional theories, which assume larger and slower-moving domain structures, simply couldn’t account for this behavior. Liu noted that zentropy showed that the remarkable performance was not a lucky accident but a natural consequence of the material’s atomic-scale dynamics.
“By breaking the larger system into smaller atomic units, the energy barrier for polarization changes becomes much lower,” Liu said. “That allows the response to be extremely fast.”
This understanding is key to being able to scale up future production of transparent ceramics, Liu said. The researchers have already demonstrated that their ceramics can be produced reliably at laboratory scale, and they are now working to scale production, evaluate long term reliability and develop safer lead-free versions for industry.
“With progress in these areas, we are optimistic that practical devices could follow in the near future,” Liu said.
Such practical devices could reshape key optical devices — fiber optic internet infrastructure to self-driving car guidance systems and precision medical diagnostics, to name a few examples —that power the modern digital economy, the researchers said, explaining that lithium niobate has been the standard material in these devices for decades. Applying electricity changes how lithium niobate bends light, but only by an amount so small, it is like nudging a ruler by the width of a few atoms. The ceramics developed in this new study demonstrate coefficients far beyond that level.
“These materials could pave the way for a new generation of electro-optic devices that are smaller, faster, more energy efficient and lower cost,” Yan said. “Potential applications include optical modulators, optical switches, communication components, sensors and integrated photonics.”
Liu’s contributions to this research were supported by the U.S. Department of Energy and the Dorothy Pate Enright Professorship at Penn State. Full collaborator and funding details may be found in the paper.
Journal
Journal of the American Chemical Society
Method of Research
Experimental study
Subject of Research
Not applicable
Article Title
Dynamic Atomistic Polar Structure Underpins Ultrahigh Linear Electro-Optic Coefficient in Transparent Ferroelectric Ceramics
