X-ray absorption spectroscopy is an important tool for materials analysis and continues to evolve with the advent of attosecond soft X-ray pulses. These pulses allow the material's entire electronic structure to be analyzed simultaneously, a breakthrough led by the ICFO team. A recent study shows that the conductivity of graphite can be manipulated through light-matter interactions, revealing potential applications in photonic circuits and optical computing. This advance in spectroscopy opens new avenues for studying many-body dynamics in materials, a key challenge in modern physics.
The advances made by ICFO researchers in attosecond soft X-ray spectroscopy have transformed materials analysis, particularly in the study of light-matter interactions and many-body dynamics, with profound implications for future technology applications.
X-ray absorption spectroscopy is an element-selective and electronic state-sensitive technique that is one of the most widely used analytical techniques for studying materials or substance compositions. Until recently, this approach required painstaking wavelength scans and did not provide the ultrafast time resolution to study electron dynamics.
Over the past decade, ICFO's Attosecond Science and Ultrafast Optics Group, led by ICREA Professor Jens Biegerth, has developed attosecond soft X-ray absorption spectroscopy into a new scanning-free analytical tool with attosecond time resolution.
Breakthrough progress in attosecond soft X-ray spectroscopy
The duration of attosecond soft X-ray pulses ranges from 23 seconds to 165 seconds, while the bandwidth of coherent soft X-rays ranges from 120 to 600eV, allowing the entire electronic structure of the material to be detected in one go.
The temporal resolution to detect electron motion in real time, combined with the coherence bandwidth to record where changes occur, provides a new and powerful tool for solid-state physics and chemistry.
One of the most important fundamental processes is the interaction of light with matter, for example, understanding how plants harvest solar energy or how solar cells convert sunlight into electricity.
An important aspect of materials science is the use of light to change the quantum state or functionality of materials or substances. This study of the many-body dynamics of materials addresses core puzzles in contemporary physics, such as what triggers any quantum phase transition, or how a material's properties arise from microscopic interactions.
Latest research from ICFO researchers
ICFO researchers Themis Sidiropoulos, Nicola DiPalo, Adam Summers, Stefano Severino, Maurizio Reduzzi, and Jens Biegert reported in a recent research report published in the journal Nature Communications that they observed a light-induced increase and control of the conductivity of graphite by manipulating the many-body state of graphite.
Innovative measurement technology
The researchers used carrier packet phase-stabilized sub-2-period light pulses with a wavelength of 1850 nanometers to induce a light-matter mixed state. They used attosecond soft X-ray pulses with a duration of 165 seconds to probe electron dynamics at the K-edge of graphitic carbon at 285 eV. Attosecond soft X-ray absorption measurements interrogate the entire electronic structure of the material with attosecond-spaced pump-probe delay steps. Pumping at a wavelength of 1850 nm induces a highly conductive state in the material. The existence of this state is entirely due to the interaction of light and matter, so it is called a light-matter hybrid.
Researchers are interested in such conditions because they promise to allow materials to develop quantum properties that do not exist in other equilibrium states, and these quantum states can be switched at the speed of light up to several terahertz.
However, it is currently unclear how these states behave inside the material. Therefore, there has been much speculation in recent reports on light-induced superconductivity and other topological phases. ICFO researchers used soft X-ray attosecond pulses for the first time to "observe" the behavior of light matter states inside materials.
ThemisSidiropoulos, first author of the study, noted: "The requirements for coherent detection, attosecond time resolution, and attosecond synchronization between pump and probe are completely novel and fundamental requirements for new research of this type enabled by attosecond science."
Electron dynamics in graphite
Experimenters physically manipulate the sample to observe changes in electronic properties. Unlike twisted electronics and twisted bilayer graphene, Sidiropoulos explains: "Instead of manipulating the sample, we optically excite the material with powerful light pulses, thereby exciting the electrons to high energy states, and observe how these electrons relax in the material, not just individually, but relax as a whole system, observing the interaction between these charge carriers and the crystal lattice itself."
In order to observe how electrons in graphite relax after being irradiated with strong pulsed light, they used a broad X-ray spectrum to first observe how each energy state relaxes individually, and secondly to observe how the entire electronic system is excited, thereby observing the many-body interaction between light, carriers and atomic nuclei at different energy levels. By looking at this system, they found that all charge carrier energy levels indicate that the material's photoconductivity increased at some point, showing signatures or memories of the superconducting phase.
Observing coherent phonons
How did they see this? In fact, in a previously published article, they observed the behavior of coherent (non-random) phonons, or the collective excitation of atoms inside a solid. Because graphite has a very strong (high-energy) phonon array, these phonons can efficiently transfer large amounts of energy out of the crystal without damaging the material through mechanical vibrations of the crystal lattice. Because these coherent phonons move back and forth like waves, electrons in the solid appear to ride on these waves, producing the artificial superconductivity the team observed.
Impact and Prospects
The findings suggest promising applications in photonic integrated circuits, or optical computing, where light can be used to manipulate electrons or light can be used to control and manipulate material properties. As Jens-Bigert concludes, "Many-body dynamics is at the heart of contemporary physics and arguably one of the most challenging problems. The results we achieve here open up new areas of physics and provide new ways to study and manipulate the relevant phases of matter in real time, which is crucial for modern technology."
Compiled source: ScitechDaily