Light-matter Discovery Challenges Understanding of Photon Momenta

Although bulk silicon does not naturally emit light, silicon that is porous and nanostructured can produce detectable light after being exposed to visible radiation. This phenomenon, although not fully understood, fueled a study on photon momentum in silicon.

A research team at the University of California, Irvine (UC Irvine), working with researchers at Kazan Federal University, investigated structural photoemission in silicon glass. They found that photons can gain substantial momentum when they are confined to nanometer-scale spaces in silicon, in a process similar to how momentum is obtained by electrons in solid materials. The discovery could make silicon a more effective medium for optoelectronics applications like solar energy and semiconductor lasers.

“Silicon is Earth’s second-most abundant element, and it forms the backbone of modern electronics,” said Dmitry Fishman, director of Laser Spectroscopy Labs at UC Irvine. “However, being an indirect semiconductor, its utilization in optoelectronics has been hindered by poor optical properties.”

The work of the UC Irvine-led team builds on the earlier discoveries of physicists Arthur Compton and C.V. Raman. In 1923, Compton discovered that gamma photons possessed sufficient momentum to strongly interact with free or bound electrons. “In our experiments, we showed that the momentum of visible light confined to nanoscale silicon crystals produces a similar optical interaction in semiconductors,” Fishman said.

Professor Eric Potma (left) and professor Dmitry Fishman made a breakthrough discovery regarding the way light interacts with solid matter in silicon. Their work could lead to improved efficiency in solar electric systems, semiconductor lasers, and other advanced optoelectronic technologies. Courtesy of Lucas Van Wyk Joel/UC Irvine.

Raman’s investigations in 1928 into inelastic scattering in liquids and gases led to what is now known as the vibrational Raman effect and the spectroscopy technique Raman scattering.

For their investigation, the researchers produced silicon glass samples ranging in clarity from amorphous to crystalline. They exposed a 300 nm-thick silicon film to a tightly focused, continuous wave laser that was scanned to write an array of straight lines. In areas of the film where the temperature did not exceed 500 °C, a homogeneous, cross-linked glass was formed. In areas where the temperature surpassed 500 °C, a heterogeneous semiconductor glass was created instead. The differences in the silicon film allowed the team to observe how the electronic, optical, and thermal properties of silicon glass varied on the nanometer scale.

Based on their experiments, the researchers surmised that light emission in silicon glass is the result of electronic Raman scattering.

“Our discovery of photon momentum in disordered silicon is due to a form of electronic Raman scattering,” said Eric Potma, a professor at UC Irvine. “But unlike conventional vibrational Raman, electronic Raman involves different initial and final states for the electron, a phenomenon previously only observed in metals.”

The researchers attribute photoemission in the disordered system specifically to the presence of an excess electron density of states within the forbidden gap (Urbach bridge) where electrons occupy trapped states. Transitions from gap states to the conduction band are enabled through electron-photon momentum matching. The researchers found that this momentum matching resembles Compton scattering, but is observed for visible light and driven by the enhanced momentum of a photon confined within nanostructured spaces.

The researchers’ experiments demonstrate the role of photon momentum in the optical response of solids that display disorder on the nanoscale.

“This work challenges our understanding of light and matter interaction, underscoring the critical role of photon momenta,” Fishman said. “In disordered systems, electron-photon momentum matching amplifies interaction — an aspect previously associated only with high-energy gamma photons in classical Compton scattering."

“Ultimately, our research paves the way to broaden conventional optical spectroscopies beyond their typical applications in chemical analysis, such as traditional vibrational Raman spectroscopy, into the realm of structural studies — the information that should be intimately linked with photon momentum,” he said.

The ability to induce photon momentum in silicon glass could lead to greater efficiency in solar power systems, LEDs, and other technologies.

“This newly realized property of light no doubt will open a new realm of applications in optoelectronics,” Potma said. “The phenomenon will boost the efficiency of solar energy conversion devices and light-emitting materials, including materials that were previously considered not suitable for light emission.”

The research was published in ACS Nano (www.doi.org/10.1021/acsnano.3c12666).