Newly discovered interaction between light and matter
Scientists have reported photons trapped in nanometre-scale spaces in disordered silicon having significant momentum, which they say could unlock new optoelectronic capabilities
A research team headed by chemists at the University of California, Irvine (UC Irvine) has reported a previously unknown way in which light interacts with matter, a finding that they say could lead to improved solar power systems, light-emitting diodes, semiconductor lasers, and other technological advancements.
In a paper published recently in the journal ACS Nano, the scientists, joined by colleagues at Russia’s Kazan Federal University, explain how they learned that photons can obtain substantial momentum, similar to that of electrons in solid materials, when confined to nanometre-scale spaces in silicon.
“Silicon is Earth’s second-most abundant element, and it forms the backbone of modern electronics,” said lead author Dmitry Fishman, adjunct professor of chemistry at UC Irvine. “However, being an indirect semiconductor, its utilisation in optoelectronics has been hindered by poor optical properties.”
While silicon does not naturally emit light in its bulk form, Fishman explained that porous and nanostructured silicon can produce detectable light after being exposed to visible radiation. Scientists have been aware of this phenomenon for decades, but the precise origins of the illumination have been the subject of debate.
“In 1923, Arthur Compton discovered that gamma photons possessed sufficient momentum to strongly interact with free or bound electrons,” said Fishman. “This helped prove that light had both wave and particle properties, a finding that led to Compton receiving the Nobel Prize in physics in 1927. In our experiments, we showed that the momentum of visible light confined to nanoscale silicon crystals produces a similar optical interaction in semiconductors.”
In 1928, Indian physicist C.V. Raman, who won the 1930 Nobel Prize in physics, attempted to repeat the Compton experiment with visible light. However, he encountered a formidable obstacle in the substantial disparity between the momentum of electrons and that of visible photons. Despite this setback, Raman’s investigations into inelastic scattering in liquids and gases led to the revelation of what is now recognised as the vibrational Raman effect, and spectroscopy – a crucial method of spectroscopic studies of matter – has come to be known as Raman scattering.
“Our discovery of photon momentum in disordered silicon is due to a form of electronic Raman scattering,” said co-author Eric Potma, professor of chemistry 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.”
For their experiments, the researchers produced in their laboratory silicon glass samples that ranged in clarity from amorphous to crystal. They subjected a 300-nm-thick silicon film to a tightly focused continuous-wave laser beam that was scanned to write an array of straight lines. In areas where the temperature did not exceed 500 degrees Celsius, the procedure resulted in the formation of a homogenous cross-linked glass. In areas where the temperature exceeded 500 degrees Celsius, a heterogeneous semiconductor glass was formed. This “light-foamed film” allowed the researchers to observe how electronic, optical and thermal properties varied on the nanometre scale.
“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.”
Potma added: “This newly realised property of light no doubt will open a new realm of applications in optoelectronics. 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.”
Image credit: Lucas Van Wyk Joel / UC Irvine
