New solid-state material from Kyushu University turns visible light into high-energy UV at sunlight intensity, expanding solar energy potential (from www.kyushu-u.ac.jp)

Associate Professor Yoichi Sasaki
Faculty of Engineering

Fukuoka, Japan—Two cups of warm water don’t make one cup of boiling water. But in the quantum world, multiple low-energy photons can combine to produce a single, higher-energy photon.

A research team at Kyushu University has developed a solid-state molecular material that “upgrades” visible light into ultraviolet (UV) light under ordinary outdoor sunlight, achieving a conversion efficiency of 1.9%. The study was published in Nature Communications (doi.org/10.1038/s41467-026-73898-0) on June 23.

Harsh UV light is something most people try to avoid in summer, yet it is indispensable across fields ranging from air purification and resin curing in 3D printing to gel hardening in dental fillings and nail art. Despite its importance, UV accounts for only about 6% of the sunlight reaching Earth’s surface, with only a fraction of that being practically usable.

“What we do here is ‘add together’ the energy from two visible light photons to make one ultraviolet photon. It’s a fascinating process called photo upconversion,” explains Yoichi Sasaki, Associate Professor at Kyushu University’s Faculty of Engineering and the study’s corresponding author.

One mechanism that enables such upconversion is triplet-triplet annihilation (TTA). A “donor” molecule absorbs visible light and excites its electrons into a high-energy triplet state, then passes it to a neighboring “acceptor” molecule. When two triplets meet, they annihilate each other, releasing their combined energy as a UV photon. TTA works well in liquids, where molecules move freely, and triplets collide easily. But those systems often rely on toxic solvents and can evaporate, limiting their practical use. That is why scientists have long searched for solid alternatives.

“In solids, molecules are packed tightly, and the π electron clouds—regions of high electron density hovering above and below each molecular plane—can overlap,” says Sasaki. “When that happens, triplets easily fizzle out before they ever meet. Molecules must be close enough for energy to transfer but separated enough to prevent quenching of excitons.”

The team found their answer in an organic semiconductor called dihydroindenoindenedene (DHI). By attaching alkyl chains to DHI’s sp³ carbon atoms—which have four bonds pointing in fixed 3D directions—the researchers created precisely controlled gaps between neighboring molecules, keeping them close enough for energy transfer without unwanted strong electronic interaction.

The optimized material shows strong light emission, long-lived excited states, and efficient energy transfer, achieving a solid-state fluorescence quantum yield above 60%. With a donor molecule, the system reaches an upconversion efficiency of 1.9%.

“This means roughly two UV photons are produced for every hundred visible-light photons absorbed,” Sasaki adds. “It may sound low, but it runs on natural sunlight alone. Most solid-state materials cannot realize this even at much higher light intensity.”

The material has been filed for a patent. Beyond efficiency, it offers advantages for real-world use, including straightforward synthesis and low-cost starting materials. The team sees potential applications in solar-driven photocatalysis, indoor air purification, and low-intensity 3D printing.

For the research team, the work also carries personal weight.

In 2012, Nobuo Kimizuka, now Professor Emeritus at Kyushu University’s Research Center for Negative Emissions Technologies, pioneered research into photon upconversion via triplet energy migration in self-assemblies, seeking to establish a molecular systems chemistry where self-assembly performs useful functions. His team made steady progress in both solution and gel systems, yet developing efficient solid-state upconversion systems remained challenging. A breakthrough finally came in May 2024, less than a year before Kimizuka’s retirement.

What followed was a sprint driven as much by shared bonds and gratitude as by science. At that time, graduate students Naoyuki Harada, Hayato Shoyama, Nutnicha Boonmong, along with then-Assistant Professor Kiichi Mizukami of Kyushu University’s Faculty of Engineering, worked alongside Sasaki to compress years of work into one.

“We handed the draft to Professor Kimizuka just 11 days before he left the lab, which for us felt like a heartfelt retirement gift,” Sasaki notes.

“This discovery is the culmination of over 14 years of our research and marks a major milestone in photon-upconversion and molecular self-assembly research,” concludes Kimizuka.

###

For more information about this research, see “Sterically Protected π-Electron Systems for Efficient Solid-State Photon Upconversion,” Naoyuki Harada, Hayato Shoyama, Nutnicha Boonmong, Kiichi Mizukami, Yuya Watanabe, Pei Zhao, Masahiro Ehara, Yoichi Sasaki, Nobuo Kimizuka, Nature Communications, https://doi.org/10.1038/s41467-026-73898-0

Schematic of the dual-chamber configuration. Credit: Nature Communications (2026). DOI: 10.1038/s41467-026-74421-1

Fusion at high temperatures powers the sun and, if harnessed, could provide a potential source of energy here on Earth. But controlling fusion reactions has other benefits. The process also generates subatomic particles called neutrons that are used in a range of applications spanning medicine, research and national security.

Scientists at the University of California, Davis, and the Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) have found that the materials surrounding a fusion reaction can dramatically increase how often it occurs, particularly at low energies where fusion is rare. Their study is published in Nature Communications. The study’s first author is Micah Karahadian, a doctoral candidate in Munday’s lab at UC Davis.

Their approach establishes a way to study and engineer nuclear reactions within solid materials, opening a new field of “materials-driven fusion.” Instead of designing materials just to survive the harsh conditions of fusion, researchers might be able to design materials that boost the reaction under specific conditions, similar to the way catalysts speed up chemical processes.

“It gives you a new knob to turn that you didn’t have before,” said co-author Arun Persaud, head of the Fusion Science & Ion Beam Technology group in Berkeley Lab’s Accelerator Technology & Applied Physics Division (ATAP). “If we understand this effect better, it opens the door to engineering new materials that would affect the fusion rate under certain conditions. Someday future progress might enable more compact and efficient neutron generators, which have all kinds of applications, like cargo screening, planetary science and medical therapy and imaging.”

In the experiment, researchers used two different methods to pack deuterium—a heavy form of hydrogen often used in fusion—into thin foils of palladium and titanium. They then fired a beam of deuterium ions at the foils at different energies and measured how often fusion occurred. They compared the rates from the different materials and methods with the “bare” fusion reaction (not in a material).UC Davis Professor Jeremy Munday, left, and doctoral candidate Micah Karahadian, who led the study, stand next to the experimental setup at UC Davis. Credit: Marina Leite, UC Davis

A surprising fusion plateau

The team found that fusion rates depended on how the deuterium was loaded into the metal foils. The biggest effect was at the lowest energies, below 2.5 kiloelectronvolts (keV), where theory predicts fusion rates drop off sharply. Instead, researchers found a surprising plateau: Some samples showed elevated fusion rates roughly a quintillion times higher than bare fusion reactions. (A quintillion is a 1 followed by 18 zeros.)

Researchers aren’t exactly sure why that’s happening, though they have some ideas and ways to test them. The electrons and defects within the material might partially shield repulsive electrostatic forces between deuterium nuclei, making it easier for them to get close together and fuse. Tuning the electronic structure, defects and composition of materials could help make nuclear reactions happen more readily.

“It comes down to better understanding the mechanism so that we can try to enhance it,” said corresponding author Jeremy Munday, a UC Davis professor in the College of Engineering. “We’ve seen that we can increase fusion rates, but what is the limit? Can we bring it to lower temperatures or energies? There’s obviously a lot of interest and excitement about fusion, so if there’s something we can learn about the physics at these lower energy scales, maybe there’s something we can translate to other areas of nuclear science.”

The team plans to explore a wider range of materials and continue probing the unexpected fusion plateau at lower energies. Their work establishes a reproducible experimental platform to study how solid materials influence nuclear reactions, creating a new area of research that links fusion science with materials science and chemistry.

“The work shows conclusively that the material environment where fusion occurs at low temperatures is an active participant rather than a passive container,” said co-author Cameron Geddes, director of ATAP. “That adds a new dimension to fusion research.”