High harmonics illuminate the movement of atoms and electrons

Laser light can radically change the properties of solid materials, making them superconducting or magnetic within a millionth of a billionth of a second. The intense light causes fundamental, instantaneous changes in a solid by “shaking” the atomic lattice structure and moving electrons around. But what exactly happens at that elementary level? How do those atoms and electrons actually move?

Now a theory team from the Max Planck Institute for the Structure and Dynamics of Matter in Hamburg has found a new way to illuminate those atomic motions. In PNAS, the researchers outline how a laser pulse generates light emission at higher frequencies from the material, the so-called higher harmonics. However, this high-energy light does not stay the same, but changes with every movement of the schedule† As the high harmonics change in intensity, they provide “snapshots” of the movements of the atoms and electrons at any exact moment.

The team studied a monolayer of hexagonal boron nitride (hBN) just one atom thick, whose lattice can be excited to vibrate on timescales of tens of femtoseconds. A first “pump” laser pulse hits the material, causing the atoms to move together. Then a second infrared laser pulse excites the electrons even further, causing them to emit light at new frequencies – the high harmonics. These contain the underlying information about the lattice vibrations (also called phonons). Analyzing them gives scientists detailed new insights into those atomic motions.

Published in Proceedings of the National Academy of SciencesThe team’s findings represent a major step forward in understanding the fundamental changes in a solid material while being irradiated by an intense laser. It’s also a very efficient method because until now researchers needed much more sophisticated light sources to detect those elemental movements.

In addition, the team showed that once the atoms start to vibrate, the interaction between the material and the initial laser pulse changes with the phase of the laser itself. This means scientists can determine exactly which movement in the grid was caused by which phase in the laser’s optical cycle, as if they were running a stopwatch at that particular moment. In other words, the team’s work has yielded a highly advanced spectroscopic technique with extreme temporal resolution. Within this approach, lattice movements can be mapped down to a single femtosecond, but without the need for high energy X-rays or attosecond pulses, which are much more difficult to use.

“The main impact of this work is that we provide a starting point for understanding how phonons play a role in nonlinear interactions of light matter,” said lead author Ofer Neufeld of the MPSD theory department. “This approach allows us to investigate femtosecond structural dynamics in solids, including phase transitionsclothed phases of matter, and also coupling between electrons and phonons.”