New Technique to Reveal How Light Influences Materials

Physicists have created a new method to establish the electronic properties of thin gold films after they interact with light. Details of the new method have been published in the Nature Communications, which increase the understanding of the fundamental laws that rule the interaction of electrons and light.

Surprisingly, up to now there have been very limited ways of determining what exactly happens with materials after we shine light on them. Our finding may pave the way for improvements in devices such as optical sensors and photovoltaic cells.

Hayk Harutyunyan, Assistant Professor of Physics, Emory University and lead author of the study

From cameras and cell phones to solar panels — to seeing with your eyes — the interaction of photons of light with electrons and atoms is universal. “Optical phenomenon is such a fundamental process that we take it for granted, and yet it’s not fully understood how light interacts with materials,” Harutyunyan says.

One problem with comprehending the details of these interactions is their complexity. When the energy of a light photon is shifted to an electron in a light-absorbing material, the photon is ruined and the electron is excited from one level to another. But so many photons, electrons, and atoms are involved — and the process takes place so fast — that laboratory modeling of the process is computationally difficult.

For the Nature Communications paper, the physicists began with a comparatively simple material system — ultra-thin gold layers — and conducted experiments on it.

“We did not use brute computational power,” Harutyunyan says. “We started with experimental data and developed an analytical and theoretical model that allowed us to use pen and paper to decode the data.”

Harutyunyan and Manoj Manjare, a post-doctoral fellow in his lab, designed and carried out the experiments. Stephen Gray, Gary Wiederrecht, and Tal Heipern — from the Argonne National Laboratory — provided the mathematical tools required. The Argonne physicists also prepared the theoretical model, together with Alexander Govorov from Ohio University.

The nanolayers of gold were placed at specific angles for the experiments. Light was then aimed at the gold in two, sequential pulses.

These laser light pulses were very short in time — thousands of billions of times shorter than a second. The first pulse was absorbed by the gold. The second pulse of light measured the results of that absorption, showing how the electrons changed from a ground to excited state.

Hayk Harutyunyan

Generally, gold absorbs light at green frequencies, reflecting all the spectrum’s other colors, which makes the metal look yellow. However, in the form of nanolayers, gold is capable of absorbing light at longer wavelengths, in the infrared region of the spectrum.

“At a certain excitation angle, we were able to induce electronic transitions that were not just a different frequency but a different physical process,” Harutyunyan says. “We were able to track the evolution of that process over time and demonstrate why and how those transitions happen.”

Using the technique to better comprehend the interactions behind light absorption by a material may result in ways to tweak and manage these interactions.

Photovoltaic solar energy cells, for example, are presently only capable of absorbing a tiny percentage of the light that reaches them. Photocatalysts used in chemistry and optical sensors used in biomedicine are other instances of devices that could possibly be enhanced by the new technique.

While the Nature Communications paper provides proof of concept, the scientists plan to carry on refining the technique’s use with gold while also testing with a variety of other materials.

“Ultimately, we want to demonstrate that this is a broad method that could be applied to many useful materials,” Harutyunyan says.