Light momentum turns pure silicon from an indirect to a direct bandgap semiconductor

UC Irvine-led research reveals that the optical properties of materials can be dramatically enhanced—not by changing the materials themselves, but by giving the light new properties.

The researchers demonstrated that by manipulating the momentum of incoming photons, they could fundamentally change how light interacts with matter. One striking example from their findings is that the optical properties of pure silicon, a widely used and essential semiconductor, can be enhanced by an astonishing four orders of magnitude.

This breakthrough holds great promise to transform solar energy conversion and optoelectronics at large. The study, featured as the cover story of the September issue of ACS Nano, was 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 adjunct professor of chemistry. “By giving light new properties, we can fundamentally reshape how it interacts with matter.

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

“This photonic phenomenon stems directly from the Heisenberg uncertainty principle,” said Eric Potma, co-author and professor of chemistry. “When light is confined to scales smaller than a few nanometers, its momentum distribution widens. The momentum increase is so substantial, that it surpasses that of free-space photons by a factor of a thousand, making it comparable to the electron momenta in materials.”

Ara Apkarian, a distinguished professor of chemistry, expanded on this, saying, “This phenomenon fundamentally changes how light interacts with matter. Traditionally, textbooks teach us about vertical optical transitions, where a material absorbs light with the photon changing only the electron’s energy state.

“However, momentum-enhanced photons can change both the energy and momentum states of electrons, unlocking new transition pathways we hadn’t considered before. Figuratively speaking, we can ’tilt the textbook’ as these photons enable diagonal transitions. This dramatically impacts a material’s ability to absorb or emit light.”

Fishman continued, “Take silicon, for example—the second most abundant element in Earth’s crust and the backbone of modern electronics. Despite its widespread use, silicon is a poor absorber of light, which has long limited its efficiency in devices like solar panels.

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

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

Potma emphasized, “With the escalating effects of climate change, it’s more urgent than ever to shift from fossil fuels to renewable energy. Solar energy is key in this transition, yet the commercial solar cells we rely on are falling short.

“Silicon’s poor ability to absorb light means these cells require thick layers—almost 200 micrometers of pure crystalline material—to effectively capture sunlight. This not only drives up production costs but also limits efficiency due to increased carrier recombination.

“Thin-film solar cells are widely seen as the solution to both of these challenges. While alternative materials like direct bandgap semiconductors have demonstrated thin solar cells with efficiencies exceeding 20%, these materials are often prone to either rapid degradation or come with high production costs, making them impractical at the moment.”

“Guided by the promise of Si-based thin-film photovoltaics, researchers have been searching for ways to improve light absorption in silicon for more than four decades,” Apkarian added. “But a true breakthrough has remained elusive.”

Fishman continued, “Our approach takes a radically different step forward. By enabling diagonal transitions through momentum-enhanced photons, we effectively transform pure silicon from an indirect 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 door to ultra-thin devices and solar cells that could outperform current technologies at a fraction of the cost. Moreover, because the phenomenon does not require any changes to the material, the approach can be integrated into existing fabrication technologies with little to no modifications.”

Apkarian concluded, “We are just beginning to explore the wide 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 enhanced photon momentum has the potential to revolutionize energy conversion and optoelectronics.”

Co-authors on this study included Jovany Merham, a UC Irvine junior specialist in chemistry, Kazan Federal University researchers Sergey Kharintsev, Aleksey Noskov, Elina Battalova, and Tel Aviv University researchers Liat Katrivas and Alexander Kotlyar.