07-Jul-2022 - Max-Planck-Institut für Struktur und Dynamik der Materie

High harmonics illuminate the movement of atoms and electrons

Detailed new insights into atomic motions

Laser light can radically change the properties of solid materials, making them superconducting or magnetic within millionths of a billionth of a second. The intense light causes fundamental, immediate changes in a solid by ‘shaking’ its atomic lattice structure and moving electrons about. But what exactly is happening at that elementary level? How do those atoms and electrons actually move?

Now a theory team at the Max Planck Institute for the Structure and Dynamics of Matter in Hamburg has found a new way to illuminate those atomic motions. Writing in PNAS, the researchers outline how a laser pulse generates light emission at higher frequencies from the material, so-called higher harmonics. This high energy light, however, does not stay the same but it changes with each movement of the lattice. As the high harmonics change in intensity, they provide ‘snapshots’ of the atoms’ and electrons’ movements at each 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, making the atoms move in unison. Subsequently, a second infrared laser pulse excites the electrons yet further, so that they cause the emission of light at new frequencies - the high harmonics. These contain the underlying information about the lattice vibrations (also known as phonons). By analysing them, scientists gain detailed new insights into those atomic motions.

The team’s findings represent a major step forward in understanding the fundamental changes in a solid material while it is being irradiated by an intense laser. It is also a highly efficient method because until now researchers needed far more advanced light sources to observe those elementary motions.

In addition, the team showed that, once the atoms begin to vibrate, the interaction between the material and the initial laser pulse changes with the phase of the laser itself. This means that scientists can pinpoint exactly which movement in the lattice was sparked by which phase in the laser’s optical cycle, as if they were setting a stopwatch to that particular moment in time. Put differently: The team’s work has produced a highly advanced spectroscopic technique with extreme temporal resolution. Within this approach, lattice movements can be charted down to a single femtosecond – but without the need for high-energy X-rays or attosecond pulses, which are far more difficult to employ.

“The main impact of this work is that we are forming a starting point to understand how phonons play a role in nonlinear light matter interactions,” says lead author Ofer Neufeld from the MPSD Theory Department. “This approach lets us probe femtosecond structural dynamics in solids, including phase transitions, dressed phases of matter, and also coupling between electrons and phonons.”

Facts, background information, dossiers
  • atoms
  • electrons
  • hexagonal boron nitride
More about Max-Planck-Institut für Struktur und Dynamik der Materie
More about Max-Planck-Gesellschaft
  • News

    How do we want to heat our homes in the future?

    The question of heating is becoming increasingly important in times of scarce resources and of global warming. Are oil heating systems still future-proof? Moreover, if so, can they be operated with efficient, sustainable fuels? Researchers are striving to make heating with oil more sustaina ... more

    225 million euros for start-ups of the Max Planck Society

    A successful business year for Max Planck Innovation includes increasing investment sums for the start-ups managed. Especially life sciences start-ups with a high degree of maturity provided a record sum of 225 million euros. Companies in the Max Planck Innovation investment portfolio raise ... more

    Stalactites and stalagmites in the battery?

    They are considered the "Holy Grail" of battery research: so-called "solid-state batteries". They no longer have a liquid core, as is the case with today's batteries, but consist of a solid material. This leads to several advantages: Among other things, these batteries are more difficult to ... more

  • Research Institutes

    Max-Planck-Gesellschaft zur Förderung der Wissenschaften e.V.

    The research institutes of the Max Planck Society perform basic research in the interest of the general public in the natural sciences, life sciences, social sciences, and the humanities. In particular, the Max Planck Society takes up new and innovative research areas that German universiti ... more