Oldenburg physicists cross two laser pulses to reach electron states known only from theory

The part of this experiment I like best is the readout. According to the university's release ^[11], the group aimed its field at potassium atoms, lifted electrons into excited states and then freed them from the atoms, recording how the electron states changed at short intervals along the way ^[8]. The release compares this to stroboscopic flash photography. Successive frames of the different electron states were assembled into a movie of their evolution ^[8].

"With our method, we can generate electronic quantum states that have previously only been described theoretically and also make them spatially visible," said Matthias Wollenhaupt, who leads the team ^[4].

The field comes from two specially shaped femtosecond pulses of different colors, crossed at a single point ^[6]. Each pulse lasts a few millionths of a billionth of a second ^[6], or a few times 10^-15 seconds ^[12]. The team could control the shape of the combined field ^[6]. "The fields oscillate in all three spatial directions, opening up new possibilities for investigating and controlling specific light-matter interactions," said Darius Koehnke, a PhD student in the Ultrafast Coherent Dynamics group and one of the study's two lead authors ^[7].

The paper's title, "Multiphoton ionization with three-dimensional light fields," describes the measurement accurately ^[3]. The release does not report ionization yields, how cleanly each target state was populated, or a comparison with a conventional pulse arrangement. Those figures would show how selective the excitation was.

The application the researchers emphasize is chirality ^[9]. Chiral molecules come in two mirror-image forms that cannot be superimposed, like left and right hands ^[9]. Many amino acids, carbohydrates and drug ingredients are chiral, and their two forms often have different properties ^[9]. Separating or distinguishing the forms can be very difficult ^[10].

"Theoretical studies show that three-dimensional light fields can also possess chiral properties," Wollenhaupt said ^[13]. Olga Smirnova of the Max Born Institute for Nonlinear Optics in Berlin recently set out what such fields could do for the study and control of molecular chirality, in a Science article titled "A New Age of Molecular Chirality" ^[14]. The Oldenburg team writes that its study lays an important foundation for those applications ^[15].

The demonstration and the application use different targets. The only target named in the release is potassium atoms ^[8]. By Wollenhaupt's own wording, the chirality case still rests on theoretical studies ^[13]. So far the group has shown a field it can shape in three dimensions, and electron states that experiments could not previously reach ^[2]. I think that is a real result on its own terms, and a narrower one than the chiral-sensing framing implies. "We have thus expanded the experimental optics toolkit to include a new class of three-dimensional light fields," Wollenhaupt said ^[5].