The light pulse transforms pure silicon from an indirect bandgap semiconductor to a direct bandgap semiconductor

Research from UC Irvine reveals that the optical properties of materials can be dramatically improved, not by changing the materials themselves, but by giving light new properties.

The researchers demonstrated that by manipulating the momentum of incoming photons, they could fundamentally change the way light interacts with matter. A striking example of their findings is that the optical properties of pure silicon, a critical and widely used semiconductor, can be improved by four orders of magnitude.

This breakthrough promises great advances in solar energy conversion and optoelectronics in general. The study, featured on the cover of the September issue of ACS Nanowas conducted in collaboration with Kazan Federal University and Tel Aviv University.

“In this study, we challenge the traditional belief that light-matter interactions are solely determined by the material,” said Dmitry Fishman, senior author and assistant professor of chemistry. “By giving light new properties, we can fundamentally reshape how it interacts with matter.”

“As a result, existing or optically ‘underestimated’ materials can achieve capabilities we never thought possible. It’s like waving a magic wand: rather than designing new materials, we improve the properties of existing materials simply by modifying the incoming light.”

“This photonic phenomenon arises directly from the Heisenberg uncertainty principle,” explains Eric Potma, co-author and professor of chemistry. “When light is confined to scales smaller than a few nanometers, its momentum distribution broadens. The momentum increase is so large that it exceeds that of photons in free space by a factor of a thousand, making it comparable to the momentum of electrons in materials.”

Renowned chemistry professor Ara Apkarian explained: “This phenomenon fundamentally changes the way light interacts with matter. Traditionally, textbooks teach us about vertical optical transitions, where a material absorbs light and the photon only changes the energy state of the electron.”

“However, momentum-enhanced photons can change both the energy and momentum states of electrons, opening up new transition pathways that we had not previously considered. Figuratively speaking, we can ‘turn the tables’ because these photons enable diagonal transitions. This has a dramatic impact on a material’s ability to absorb or emit light.”

Fishman continues: “Consider silicon, the second most abundant element in the Earth’s crust and the backbone of modern electronics. Despite its widespread use, silicon absorbs light poorly, which has long limited its effectiveness in devices like solar panels.

“This is because silicon is an indirect semiconductor, meaning it relies on phonons (lattice vibrations) to enable electronic transitions. The physics of light absorption in silicon is such that while a photon changes the energy state of the electron, a phonon is simultaneously needed to change the momentum state of the electron.

“Since the probability of a photon, a phonon and an electron interacting at the same place and time is low, the optical properties of silicon are inherently weak. This has posed a significant challenge to optoelectronics and has even slowed progress in solar energy technology.”

Potma stressed: “With the growing impacts of climate change, it is more urgent than ever to transition from fossil fuels to renewable energy. Solar energy is essential to this transition, but the commercial solar cells we rely on are not up to the task.”

“Silicon’s poor ability to absorb light means that these cells require thick layers (nearly 200 micrometers of pure crystalline material) to efficiently capture sunlight. This not only increases production costs, but also limits efficiency due to increased carrier recombination.

“Thin-film solar cells are widely considered the solution to both of these challenges. While alternative materials such as direct bandgap semiconductors have demonstrated thin solar cells with efficiencies greater than 20%, these materials are often subject to rapid degradation or come with high production costs, making them impractical at present.”

“Driven by the promise of silicon-based thin-film photovoltaic cells, researchers have been looking for ways to improve light absorption in silicon for more than four decades,” Apkarian added. “But real breakthroughs remain elusive.”

Fishman added: “Our approach represents a radically different advance. By enabling diagonal transitions with enhanced momentum photons, we effectively transform pure silicon from an indirect bandgap semiconductor to a direct bandgap semiconductor, without altering the material itself. This leads to a dramatic increase in silicon’s ability to absorb light, by several orders of magnitude.”

“This means we can reduce the thickness of silicon layers by the same factor, opening the way to ultra-thin devices and solar cells that could outperform current technologies at a fraction of the cost. Furthermore, because the phenomenon requires no material modification, the approach can be integrated into existing manufacturing technologies with little or no modifications.”

Apkarian concludes: “We are only just beginning to explore the vast range of phenomena associated with light confinement at the nanoscale and beyond. The physics involved is rich with potential for fundamental and applied discoveries. However, the immediate impact is already clear.”

“Transforming silicon into a direct bandgap semiconductor through photonic pulse enhancement has the potential to revolutionize energy conversion and optoelectronics.”

Co-authors of the study included UC Irvine junior chemistry major Jovany Merham, Kazan Federal University researchers Sergey Kharintsev, Aleksey Noskov, Elina Battalova, and Tel Aviv University researchers Liat Katrivas and Alexander Kotlyar.