Real laser beams do not interact with each other when they cross. This is until the beams meet within a suitable material that allow for nonlinear light-matter interaction. In this case, wave mixing can give rise to beams with changed colors and directions.
Wave-mixing processes between different light beams are one cornerstone of the field of nonlinear optics. It has become firmly established since lasers have become hugely available. Within a suitable material such as particular crystals, two laser beams can “feel each other’s presence.”In this process, energy and momentum can be exchanged. It gives rise to additional laser beams emerging from the interaction zone in different directions and with different frequencies. These effects are commonly used to design and realize new laser light sources.
The analysis of the emerging light beams in wave mixing provides insights into the nature of the material in which the wave mixing process occurs. Such wave-mixing-based spectroscopy allows scientists to understand the intricacies of the electronic structure of a specimen and how light can excite and interact with the material. But these approaches have been hardly used outside of the visible or infrared spectral range.
Scientists from Max Born Institute (MBI) have observed a new kind of such wave mixing process which involve soft X-rays. Overlapping ultrashort pulses of soft X-rays and infrared radiation in a single crystal of lithium fluoride (LiF). They have shown how energy from two infrared photons was transferred to or from the X-ray photon.
Scientists were also able to map out its efficiency when changing the color of the incoming X-rays. It turns out that the mixing signals are only detectable when the process involves an inner-shell electron from a lithium atom being promoted into a state where this electron is tightly bound to the vacancy it left behind. This is a state known as exciton.