X-ray lasers were invented in the 1970s when physicists realized that laser beams amplified using ions would have far higher energy than beams amplified with gases. Nuclear explosions were also proposed as a means of powering these high-energy lasers. A pulse of light strikes a target in an X-ray laser, removing electrons from its atoms to produce ions and pumping energy into the ions.
Real laser beams do not interact with one other when they cross, unlike mythical laser swords, unless they meet within a suitable material that allows for nonlinear light-matter interaction. Wave mixing in this situation can result in beams with varying hues and orientations.
Wave-mixing processes between various light beams are a cornerstone of the field of nonlinear optics, which has been firmly established since the widespread availability of lasers. Two laser beams can “sense each other’s existence” within a suitable material, such as certain crystals. Energy and momentum can be exchanged during this process, resulting in additional laser beams emanating from the interaction zone in different directions and with varied frequencies, observable as different colors in the visible spectrum range.
Only if the excited electron is localized in the immediate vicinity of the hole it has left behind do we observe the four-wave mixing signal, and because we used a specific color of x-rays, we know that this hole is very close to the atomic nucleus of the lithium atom.
Robin Engel
These effects are frequently employed in the design and realization of novel laser light sources. Moreover, the investigation of the emerging light beams in wave mixing events provides information about the nature of the material in which the wave mixing process happens. Researchers can use wave-mixing spectroscopy to learn about the complexities of a specimen’s electrical structure and how light can excite and interact with it. However, these technologies have rarely been applied outside of the visible and infrared spectral ranges.
A team of researchers from Max Born Institute (MBI), Berlin, and DESY, Hamburg, has now observed a new kind of such wave mixing process involving soft x-rays. Overlapping ultrashort pulses of soft x-rays and infrared radiation in a single crystal of lithium fluoride (LiF), they see how energy from two infrared photons is transferred to or from the x-ray photon, changing the x-ray “color” in a so-called third-order nonlinear process.
They were not only able to witness this process using x-rays for the first time, but they were also able to map out its effectiveness while changing the color of the incoming x-rays. It turns out that the mixing signals are only observable when an inner-shell electron from a lithium atom is promoted into a state known as exciton, in which the electron is securely connected to the vacancy it left behind. Furthermore, comparison with theory reveals that an otherwise “optically forbidden” inner-shell electron transition contributes to the wave mixing process.
The researchers acquire a thorough picture of where the optically stimulated electron moves in its extremely brief lifetime by analyzing this resonant four-wave mixing process. “Only if the excited electron is localized in the immediate vicinity of the hole it has left behind do we observe the four-wave mixing signal,” says Robin Engel, a PhD student involved in the work, “and because we used a specific color of x-rays, we know that this hole is very close to the atomic nucleus of the lithium atom.”
The presented method allows researchers to follow electrons traveling around in molecules or solids after they have been excited by an ultrafast laser pulse because x-rays may selectively activate inner shell electrons at distinct atomic species in a material. Such processes, in which electrons move towards various atoms after being activated by light, are critical steps in photochemical reactions or applications such as light harvesting, for example, via photovoltaics or direct solar fuel generation.
“Many different atoms of the periodic table can be selectively stimulated since our wave-mixing spectroscopy approach can be scaled to considerably higher photon energy at x-ray lasers. We anticipate that tracking the transitory presence of electrons at many distinct atoms of a more complicated material will be achievable in this manner, providing fresh insight into these critical processes” MBI researcher Daniel Schick adds.
