Optics research uses dim light to produce bright LEDs

Researchers at Princeton and North Carolina State University have developed a technique that substantially improves the ability to convert low-energy light into a high-energy version. The method has immediate applications in lighting and displays.

The research builds on a technique called triplet-fusion upconversion, which uses a combination of molecules to gather lower-energy light, such as green light, and convert it to a higher-energy version like blue or ultraviolet light. The molecules absorb energy from the incoming light and temporarily store it by shifting electrons into a higher orbital state. The excited molecules collide and release the stored energy as higher-energy versions of light. Upconversion already is known to work well in liquids, because the molecules move constantly, allowing them to interact and boost the light’s energy. In solids, excitations can’t move very easily. The solid systems that do exist generally solve that problem by using extremely intense light to generate lots of excited states to interact. But those older techniques require high-power input, limiting upconversion’s applicability.

Researchers led by Barry Rand, a Princeton professor of electrical and computer engineering and at the Andlinger Center for Energy and the Environment, proposed leveraging a phenomenon called plasmonics to boost upconversion on a thin metal film. Plasmonics is the interaction of electrons on the surface of metals and electromagnetic waves such as visible light. Some materials, mainly metals, have electrons that are not bound to specific atoms. Instead, they exist as free electrons in the metal. When light hits these free electrons, it sets off an oscillation that merges the light’s energy with the electrons’ movement. These oscillations, called plasmons, concentrate light and enhance the electromagnetic field.

In an article published in the journal Nature Photonics, the research team described how they used a silver film to generate surface plasmons by exposing the film to low-energy light. As the plasmons propagated across the silver film, they increased the absorption of light by the upconversion molecules by about ten times compared to previous setups. The researchers said that the increased absorption allowed the system to increase the concentration of molecules that had absorbed the light, drastically cutting the intensity of light needed to trigger upconversion. The experimental results showed that the technique reduced the power needed to drive the reaction by 19 times compared to a non-plasmonic system.

The researchers used a laboratory setup to conduct the experiment, but they also wanted to demonstrate an immediate practical application for the technology. They built an organic light-emitting diode (OLED), a device commonly used in portable displays, to test the method and demonstrate viability. The researchers used the plasmonic film to generate blue light, and they combined the blue light with green and red light from an existing OLED to generate white light. Blue OLEDs can be challenging to operate because generating blue light requires high energy and can lead to instability. With their demonstration, the researchers showed that the thin-film technology can serve as a blue-light source without the need for high-energy input or special materials.

The work not only advanced upconversion technology, it also provided valuable experience to four Princeton undergraduate students, Kelvin Green, Amélie Lemay, Yiling Li and Tersoo Upaa. Green and Lemay both graduated in 2024 with degrees in civil and environmental engineering and completed the Program in Sustainable Energy at the Andlinger Center for Energy and the Environment. Li and Upaa are seniors in the Department of Electrical and Computer Engineering and served as interns at the Andlinger Center.

Li said the work gave her the sense of working in a lab at a graduate level. She said her co-researchers made the experience “especially rewarding by encouraging me to ask challenging questions and helping me learn how to find the answers through our research.”

For Upaa, the success of the work brought new confidence. “This internship made me much more comfortable doing research on topics that I don’t yet have a full grasp on,” he said.

Jesse Wisch, who led the experimentation in Rand’s lab, mentored the students. He said the undergraduates’ questions “forced me to deepen my own understanding of the subject material.”

The paper notes that future work could include improvements in white OLEDs through the development of higher performing films and optical structures.

The article, Plasmon-enhanced ultralow-threshold solid-state triplet fusion upconversion, was published Oct. 24 in the journal Nature Photonics.  Besides Rand, authors include: Jesse Wisch, Kelvin Green, Amélie Lemay, Yiling Li, Tersoo Upaa Jr., Hui Taou Kok, and Seamus Lowe, of Princeton University; Evgeny Danilov and Felix Castellano of North Carolina State University, Raleigh. Support for the project was provided in part by BioLEC, an Energy Frontier Research Center funded by the U.S. Department of Energy under award number DE-SC0019370.