When light strikes a metal, its energy can excite electrons, causing them to jump from lower to higher energy levels. This process, known as photoexcitation, is crucial to a wide range of technologies, including solar cells, photodiodes, and light-emitting diodes (LEDs). However, the exact sequence of events that occur during photoexcitation has remained elusive.
Now, the researchers have captured a detailed sequence of these events in real time, providing a direct observation of how light excites electrons in a metal. The team carried out the experiments at SLAC's Stanford Synchrotron Radiation Lightsource (SSRL) and used an ultrafast laser to excite electrons in a thin film of metal. They then used a time-resolved photoemission spectrometer to measure the energy and momentum of the excited electrons as a function of time.
The results, published in the journal Nature, reveal that photoexcitation occurs in a series of steps. First, the light is absorbed by the metal, creating an electron-hole pair. The electron and hole then quickly accelerate in opposite directions due to the electric fields created by the light wave. Finally, the electron and hole recombine, emitting a photon of light.
The researchers were able to directly observe this process by using an ultrashort laser pulse to excite the electrons. This allowed them to capture the dynamics of the photoexcitation process on a timescale of femtoseconds (10-15 seconds).
"We can now see exactly what happens when light hits a metal," said Philip Heimann, a professor of applied physics at Stanford University and a co-author of the study. "This is a fundamental understanding of a process that is essential to many optoelectronic devices."
The team's findings could lead to the development of new optoelectronic devices that are more efficient and have faster response times. They could also help researchers understand how light interacts with other materials, such as semiconductors and insulators.
