Although the electron is a quantum object, the classical description of its motion is appropriate for our experimental technique.

Strong-field physics fundamentally depends on high-harmonic generation, which converts optical or near-infrared (NIR) light into the extreme ultraviolet (XUV) regime. In the well-known three-step concept, the driving light field ionizes the electron by tunnel ionization, accelerates it away and back to the ionic core, where the electron recollides and emits XUV light if it recombines.

In this study, physicists replaced the first step with an XUV single-photon ionization, which has a twofold advantage: First, one can choose the ionization time relative to the NIR phase. Second, the NIR laser can be tuned to low intensities where tunnel ionization is practically impossible. This allows us to study strong-field-driven electron recollision in a low-intensity limiting case.

The interaction between electrons and light is the most fundamental interaction in physics. Scientists from Goethe University Frankfurt performed an experiment in which they observed the Kapitza-Dirac effect for the first time in full temporal resolution.

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First postulated almost nine decades ago, the Kapitza–Dirac effect is a quantum mechanical effect consisting of the diffraction of matter by a standing wave of light. In its original description, the effect is time-independent.

Researchers disproved previously valid assumptions.

This significantly reduced the limitations of internal loss and phase noise.