Physicists sculpt 3D light fields to steer electrons into new quantum states

Physicists at the University of Oldenburg in Germany have found a way to make light do something it rarely does in the laboratory: behave fully three-dimensionally. By making two ultrashort laser pulses of different colours converge from different directions and superimpose them at a single point, the team generated light fields whose electric fields oscillate in all three spatial directions at once. With these carefully sculpted three-dimensional light fields, the researchers were able to excite electrons in atoms into quantum states that had previously existed only in theory, and then to make those states spatially visible. The work, published in the journal Physical Review Research, expands the experimental optics toolkit with an entirely new class of light fields and opens fresh avenues for studying chiral molecules, controlling light-matter interactions and producing tailored electronic quantum states.

The man behind the project, Prof. Dr Matthias Wollenhaupt, who leads the Ultrafast Coherent Dynamics research group at Oldenburg, describes the achievement as a genuine extension of what experimentalists can do. “With our method, we can generate electronic quantum states that have previously only been described theoretically and also make them spatially visible,” he explains. “We have thus expanded the experimental optics toolkit to include a new class of three-dimensional light fields.” That statement captures the essence of the result: for decades, theorists have predicted that fully three-dimensional light fields should enable new forms of quantum control, but creating such fields in the laboratory and using them to steer electrons has remained out of reach until now.

The technical trick at the heart of the experiment lies in the combination of two specially shaped femtosecond laser pulses. Femtosecond pulses are bursts of light lasting only a few millionths of a billionth of a second, so short that within the duration of a single pulse, chemical bonds barely have time to begin vibrating. The researchers used an interferometer to split their laser light into two beams of different colours, shaped each pulse with great precision, and then directed the two beams so that they intersected inside a vacuum chamber at a single point. Where the beams overlapped, their electromagnetic fields added together, producing a three-dimensional interference pattern whose geometry the team could control by adjusting the colours, phases, polarizations and relative timing of the two pulses.

“The fields oscillate in all three spatial directions, opening up new possibilities for investigating and controlling specific light-matter interactions,” explains Darius Köhnke, one of the two lead authors of the study and a PhD student in Wollenhaupt’s group. Conventional laser experiments typically rely on light that oscillates in one or two dimensions, which limits the kinds of quantum states that can be reached. A truly three-dimensional field, by contrast, can drive an electron in ways that no one-dimensional or two-dimensional field can, coupling the electron’s motion along all three axes simultaneously. This is precisely what allows the method to access quantum states that are forbidden or invisible to conventional excitation schemes.

To demonstrate the power of the technique, the team applied their three-dimensional light fields to potassium atoms, a workhorse of atomic physics experiments. They used the fields to selectively excite electrons within the atoms into higher-energy states, known as excited states, and then to release those electrons from the atoms entirely, a process called ionization. The properties of the liberated electrons carry a detailed fingerprint of the quantum states from which they were ejected, allowing the researchers to reconstruct what happened inside the atom. Crucially, by varying the parameters of the light field, they could choose which excited states to populate and which pathways the electrons would follow on their way out.

The experiment also functioned like an ultra-high-speed camera for quantum processes. Because the laser pulses are so short, the researchers were able to observe the changes in the electron states at closely spaced time intervals, capturing successive snapshots of the evolving quantum system. The principle resembles stroboscopic flash photography, in which a rapidly flashing lamp freezes fast motion into a sequence of still images. Stitched together, these snapshots form a movie of the electron states as they evolve under the influence of the three-dimensional field. This time-resolved view is what allowed the team to confirm that the theoretically predicted states were indeed being created and to make their structure spatially visible for the first time.

Beyond its fundamental appeal, the method is particularly promising for one of the most consequential problems in modern chemistry: the detection and control of chiral molecules. Chiral molecules come in two forms that are mirror images of each other but cannot be superimposed, much like a person’s left and right hands. This handedness is not a minor detail. Many biomolecules, including amino acids, carbohydrates and the active ingredients of medicinal products, are chiral, and the two mirror-image forms, called enantiomers, frequently have strikingly different biological effects. The body may metabolize one form beneficially while the other is inert or even harmful.

The dangers of ignoring molecular handedness are written into pharmaceutical history. The active ingredient thalidomide, marketed in the late 1950s and early 1960s under the brand name Contergan, exists in two enantiomeric forms: one causes severe birth defects when taken during pregnancy, while the other is comparatively harmless. Because the two forms are chemically almost identical, separating or distinguishing between them can be extremely difficult, and in the case of thalidomide the drug was sold as a mixture of both. A technique that could reliably sense and even selectively manipulate one handedness over the other would therefore be a powerful tool for drug development, quality control and fundamental biology alike.

Three-dimensional light fields may offer exactly that capability. “Theoretical studies show that three-dimensional light fields can also possess chiral properties,” Wollenhaupt explains. In other words, the light itself can be handed, twisting through space in a way that distinguishes left from right. Such chiral light fields could interact differently with the two enantiomers of a chiral molecule, providing a far more sensitive and selective probe than conventional methods, which typically rely on weak magnetic-dipole effects. The opportunities that three-dimensional light fields open up for investigating and controlling molecular chirality were recently highlighted by physicist Prof. Dr Olga Smirnova of the Max Born Institute for Nonlinear Optics in Berlin in an article published in the journal Science entitled “A New Age of Molecular Chirality”. The Oldenburg team writes that their current study lays an important foundation for such applications, bridging the gap between theoretical proposals and laboratory reality.

The work, reported under the title “Multiphoton ionization with three-dimensional light fields,” signals a broader shift in how physicists think about controlling matter with light. Rather than treating a laser beam as a simple one-dimensional oscillating field, the Oldenburg approach treats the full spatial structure of light as a designable parameter, something to be engineered pulse by pulse in order to write specific quantum states onto atoms and molecules. The same interferometric superposition scheme that produced the new states in potassium atoms can, in principle, be adapted to more complex systems, from chiral molecules in solution to surfaces and nanostructures. If the promise holds, the coming years could see three-dimensional light fields become a standard instrument in the quest to read out, and perhaps even exploit, the handedness of nature at the molecular scale, turning a long-standing theoretical prediction into a practical technology for chemistry, biology and medicine.

Subject of Research: Generation of three-dimensional light fields from superimposed femtosecond laser pulses to control electron quantum states and probe chiral molecules

Article Title: Using three-dimensional light fields to control electrons

Article References: Using three-dimensional light fields to control electrons. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: three-dimensional light fields, femtosecond laser pulses, quantum states, multiphoton ionization, chiral molecules, light-matter interactions, potassium atoms, quantum control, ultrafast optics, chiral sensing, Physical Review Research, University of Oldenburg