The properties of solid materials can be dramatically altered by laser light. They can become superconducting and magnetic in a matter of milliseconds. Intense light can cause fundamental changes in a material by "shaking" the atomic structure of the material and moving electrons around. What exactly happens at the elementary level? What is the actual movement of atoms or electrons?
A theory team from the Max Planck Institute for the Structure and Dynamics of Matter, Hamburg has discovered a new way of illuminating these atomic motions. The researchers describe how a laser pulse emits light at higher frequencies, or higher harmonics, from the material. However, this high-energy light does not remain the same. It changes with every movement of the lattice. The high harmonics provide "snapshots," which are snapshots of the electrons and atoms' movements at any given moment, as they change in intensity.
The team examined a monolayer hexagonal boron (hBN), which is just one atom thick. Its lattice can vibrate in timescales of tens to femtoseconds. The material is excited by a first "pump", laser pulse that hits it. This causes the atoms to move in unison. A second infrared pulse is then applied to the material, exciting the electrons further. This causes the emission of light at higher frequencies, the high harmonics. These include the information necessary to understand the lattice vibrations, also known as phonons. Scientists gain new insight into these atomic motions by analysing them.
The team's findings were published in Proceedings of the National Academy of Sciences. They represent a significant step forward in understanding the fundamental change in solid materials while they are being irradiated with an intense laser. This method is also highly efficient, as it allows researchers to observe elementary motions using far more advanced light sources than they currently have.
The team also demonstrated that once the atoms start to vibrate, the interaction of the material with the initial laser pulse changes according to the phase of laser. Scientists can now pinpoint which lattice movement was caused by which phase of the laser's optical cycling, just as if they were setting the stopwatch for that moment in time. This means that the team has developed a high-tech spectroscopic technique that can be used to pinpoint lattice movements with exceptional temporal resolution. This approach allows lattice movements to be charted down as low as one femtosecond, but without the need to use high-energy Xrays or attosecond pulses which can be much more difficult.
Ofer Neufeld, a lead author from the MPSD Theory Department, stated that "the main impact of this research is that we are creating a starting point for understanding how phonons can play a role as nonlinear light material interactions." This approach allows us to probe femtosecond structural dynamics of solids, such as phase transitions and dressed phases, and also coupling between electrons or phonons.