Mixing Laser And X-rays

A team of researchers from the Max Born Institute and DESY has observed a novel wave mixing process with soft X-rays.

Unlike fictional lightsabers, real laser beams do not interact with each other when they cross - unless the beams meet in a suitable material that allows for non-linear light-matter interaction. In such a case, the mixing of waves can result in rays with changed colors and directions.

Wave mixing processes between different light beams are a cornerstone of the field of nonlinear optics, which has been firmly established since the widespread availability of lasers. In a suitable material, eg certain crystals, two laser beams can "sense" each other. Energy and momentum can be exchanged, so that additional laser beams emerge from the interaction zone in different directions and with different frequencies, which are perceived as different colors in the visible spectral range. These effects are often used to design and implement new laser light sources. Equally important, the analysis of the emitted light rays during wave mixing phenomena provides information about the nature of the material, where the wave mixing process takes place. Such wave mixing-based spectroscopy allows researchers to understand intricacies of a sample's electronic structure and the ways in which light can excite and interact with the material. So far, however, these approaches have hardly been used outside the visible or infrared spectral range.

A team of researchers from the Max Born Institute (MBI), Berlin, and DESY, Hamburg, has now observed a novel wave mixing process with soft X-rays. By superimposing ultra-short pulses of soft X-rays and infrared radiation in a lithium fluoride (LiF) single crystal, the team was able to observe how the energy of two infrared photons is transferred to one X-ray photon and thus how the "color" of the X-ray radiation changes in what is known as a third-order nonlinear process changes. Not only has this particular process been observed for the first time using X-rays, but the efficiency of mixing in changing the color of the incident X-rays has also been measured. It turned out that the mixed signals can only be detected if when an electron in the innermost shell of a lithium atom is excited into a state where it is tightly bound to the vacancy left—a state known as an exciton. Furthermore, comparison with theory shows that an otherwise “optically forbidden” transition of an inner-shell electron contributes to the wave mixing process.

By analyzing this resonant four-wave mixing process, the researchers get a detailed picture of where the optically excited electron is going during its very short lifetime. "Only when the excited electron is in close proximity to the hole it left behind do we observe the four-wave mixing signal," says Robin Engel, a PhD student involved in the work, "and since we used a specific color of X-rays, we know that this hole is very close to the nucleus of the lithium atom." Because X-rays can selectively excite inner-shell electrons on the different atomic species in a material, this approach allows researchers to track electrons moving in molecules or solids, after being excited by an ultrafast laser pulse. Precisely such processes - electrons moving to different atoms after being excited by light - are crucial steps in photochemical reactions or applications, such as e.g. B. photovoltaics or direct solar fuel production. "Since our approach of wave mixing spectroscopy on X-ray lasers can be scaled to much higher photon energies, many different types of atoms in the periodic table can be selectively excited. We therefore expect that in the future it will be possible to track the short-term presence of electrons on the different atoms of a more complex material, which will provide new insights into these important processes," explains Daniel Schick, researcher at MBI. that move to different atoms after being excited by light—are crucial steps in photochemical reactions or applications, such as e.g. B. photovoltaics or direct solar fuel production. "Since our approach of wave mixing spectroscopy on X-ray lasers can be scaled to much higher photon energies, many different types of atoms in the periodic table can be selectively excited. We therefore expect that in the future it will be possible to track the short-term presence of electrons on the different atoms of a more complex material, which will provide new insights into these important processes," explains Daniel Schick, researcher at MBI. that move to different atoms after being excited by light—are crucial steps in photochemical reactions or applications, such as e.g. B. photovoltaics or direct solar fuel production. "Since our approach of wave mixing spectroscopy on X-ray lasers can be scaled to much higher photon energies, many different types of atoms in the periodic table can be selectively excited. We therefore expect that in the future it will be possible to track the short-term presence of electrons on the different atoms of a more complex material, which will provide new insights into these important processes," explains Daniel Schick, researcher at MBI. Photovoltaic or direct solar fuel generation. "Since our approach of wave mixing spectroscopy on X-ray lasers can be scaled to much higher photon energies, many different types of atoms in the periodic table can be selectively excited. We therefore expect that in the future it will be possible to track the short-term presence of electrons on the different atoms of a more complex material, which will provide new insights into these important processes," explains Daniel Schick, researcher at MBI. Photovoltaic or direct solar fuel generation. "Since our approach of wave mixing spectroscopy on X-ray lasers can be scaled to much higher photon energies, many different types of atoms in the periodic table can be selectively excited. We therefore expect that in the future it will be possible to track the short-term presence of electrons on the different atoms of a more complex material, which will provide new insights into these important processes," explains Daniel Schick, researcher at MBI.