A research team led by chemists at the University of California, Irvine has discovered a previously unknown way in which light interacts with matter.
According to the authors, this finding may lead to better solar energy systems, light-emitting diodes, semiconductor lasers and other technological advances.
In a paper published in the journal ACS Nano, the scientists, along with colleagues at Kazan Federal University in Russia, explain how they learned that photons can gain substantial momentum, similar to that of electrons in solid materials, when confined in nanoscale spaces in silicon.
“Silicon is the second most abundant element on Earth and forms the backbone of modern electronics. However, as an indirect semiconductor, its use in optoelectronics has been hampered by poor optical properties,” the study said in a statement. Senior author Dmitry Fishman, associate professor of chemistry at Irvine.
He said that while silicon does not naturally emit light in its bulk form, porous, nanostructured silicon can produce detectable light after being exposed to visible radiation. Scientists have known about this phenomenon for decades, but the precise origin of the lighting has been the subject of debate.
“In 1923, Arthur Compton discovered that gamma photons possessed enough momentum to interact strongly with free or bound electrons. This helped demonstrate that light had both wave and particle properties, a finding that led to Compton receiving the Nobel Prize. of Physics in 1927,” Fishman said.
“In our experiments, we show that driving visible light confined to nanoscale silicon crystals produces a similar optical interaction in semiconductors.”
To understand the origin of the interaction it is necessary to go back to the beginning of the 20th century. In 1928, the Indian physicist CV Raman, who won the Nobel Prize in Physics in 1930, attempted to repeat Compton’s 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 investigations 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 of spectroscopic studies of matter, is 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, also a professor of chemistry at 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 silicon glass samples in their laboratory whose clarity ranged from amorphous to crystalline. They subjected a 300-nanometer-thick silicon film to a highly focused continuous-wave laser beam that was scanned to write a series of straight lines.
In areas where the temperature 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 C, a heterogeneous semiconductor glass was formed. This “light foam film” allowed the researchers to observe how the electronic, optical and thermal properties varied on the nanometer scale.
“This work challenges our understanding of the interaction of light and matter, underscoring the critical role of photon momentum,” Fishman said.
“In disordered systems, momentum coincidence between electrons and photons amplifies the interaction, an aspect previously associated only with high-energy gamma photons in classical Compton scattering. Ultimately, our research paves the way to extend conventional optical spectroscopy more beyond its typical applications in chemical analysis, such as traditional vibrational Raman spectroscopy in the field of structural studies: the information that should be closely linked to the photon momentum.
