Scientists discover new property of light

A team of physicists and chemists has discovered a previously unknown way in which light interacts with matter, a finding that could lead to improved solar energy systems, light-emitting diodes, semiconductor lasers and other technological developments.

Charintsev et al. found that photons can gain significant momentum, comparable to that of electrons in solid materials, when confined to nanometer-scale spaces in silicon. Image credit: Kharintsev et al.

“Silicon is the second most abundant element on Earth and is the backbone of modern electronics,” says Dr. Dmitry Fishman, a chemist at the University of California, Irvine.

“However, because it is an indirect semiconductor, its use in optoelectronics is hampered by poor optical properties.”

“Although silicon in its bulk form does not naturally emit light, porous and nanostructured silicon can produce detectable light after exposure to visible radiation.”

Scientists have been aware of this phenomenon for decades, but the precise origins of enlightenment have been the subject of debate.

“In 1923, Arthur Compton discovered that gamma photons possessed enough momentum to interact strongly with free or bound electrons,” said Dr. 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.”

Understanding the origins of the interaction requires another journey back to the early 20th century.

In 1928, Indian physicist CV 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 significant disparity between the momentum of electrons and that of visible photons.

Despite this setback, Raman’s research into inelastic scattering in liquids and gases led to the revelation of what is now recognized as the vibrational Raman effect, and spectroscopy – a crucial method for spectroscopic investigation of matter – has become known as Raman scattering.

“Our discovery of photon momentum in disordered silicon is due to a form of electronic Raman scattering,” Professor Eric Potma of the University of California, Irvine.

“But unlike conventional vibrational Raman, electronic Raman involves different initial and final states for the electron, a phenomenon previously observed only in metals.”

For their experiments, the researchers produced silicon glass samples in their laboratory 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 a series of straight lines.

In areas where temperatures did not exceed 500 degrees Celsius, the procedure resulted in the formation of a homogeneous cross-linked glass.

In areas where the temperature exceeded 500 degrees Celsius, a heterogeneous semiconductor glass was formed.

This lightly foamed film allowed the scientists to observe how electronic, optical and thermal properties varied on the nanometer scale.

“This work challenges our understanding of the interaction between light and matter, and underlines the crucial role of photon momenta,” said Dr. Fishman.

“In disordered systems, matching electron-photon momentum enhances the interaction – an aspect previously only associated with high-energy gamma photons in classical Compton scattering.”

“Ultimately, our research paves the way to broaden conventional optical spectroscopy 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 to photon momentum.”

“This newly realized property of light will undoubtedly open up a new domain of applications in optoelectronics,” said Professor Potma.

“The phenomenon will increase the efficiency of solar energy conversion devices and light-emitting materials, including materials previously considered unsuitable for light emission.”

This research is described in an article in the journal ACS Nano.

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Sergey S. Kharintsev et al. 2024. Photon momentum-enabled electronic Raman scattering in silicon glass. ACS Nano 18(13):9557–9565