Electrons are among the simplest yet most crucial fundamental particles in the universe. Achieving direct observation of the phase of electrons has long been a challenge for scientists.
Schematic Diagram of Momentum Spectrometer (Provided by Zhejiang University)
On March 29th, the journal "Science" published a research paper by Zhejiang University researcher Lin Kang, Professor Reinhard Dörner from Goethe University Frankfurt in Germany, and their collaborators. In their experiment, they discovered for the first time the "Ultrafast Kapitsa-Dirac Effect." This introduces a novel technique for studying electron properties, allowing direct observation of electron phase information by capturing diffraction patterns generated by electron pulses passing through standing-wave pulses at different times.
"This research extends the traditional Kapitsa-Dirac effect into the time domain, achieving ultrafast time resolution of phase evolution during electron motion," Lin Kang told the "Chinese Science News."
When a beam of light passes through a grating, diffraction occurs. Physicists Kapitsa and Dirac proposed in 1933 that when an electron beam passes through a sustained standing-wave optical field, diffraction also occurs, known as the traditional Kapitsa-Dirac effect. Due to technological limitations, this phenomenon was not experimentally confirmed until 2001 by American scientists.
The intriguing aspect of the traditional Kapitsa-Dirac effect lies in its elucidation of the wave-particle duality, where the roles of particles and waves interchange twice: electrons transition from particles to waves, while the grating shifts from solid material to a non-material optical field.
In this study, Lin Kang and Reinhard Dörner proposed expanding the traditional Kapitsa-Dirac effect using a pump-probe scheme. The new approach resets the observation target and method: first, neutral atoms are ionized to produce electron pulses using a femtosecond pulse, followed by diffraction of these electron pulses using a femtosecond standing wave with a time delay. "We transformed the observation target from a continuous electron beam to electron pulses and changed the observation method from continuous photon standing waves to instantaneous photon pulses," said Lin Kang.
The experiment can be understood as taking photographs of athletes. The upgraded approach adds a "continuous shooting" function, where each pulse of the photon standing wave acts as a "shutter," allowing for multiple pulses to "capture" the electron pulse. The "athlete" here refers to the electron pulse, and its continuously changing "movements" can be recorded by the shutter. Thus, we obtain a series of images of the athlete at different moments. Here, time acts as a ruler, visually dividing the process of electron pulse motion into multiple frames, achieving ultrafast dynamical resolution.
Ultimately, the ultrafast Kapitsa-Dirac effect was clearly demonstrated before the research team. The continuous shooting images perfectly display the images of electron pulses diffracted by photon standing-wave pulses at different times. Among them, the photon pulse standing wave has a "shutter" of 60 femtoseconds, with intervals of 100 femtoseconds for "continuous shooting," recording the evolution of electron pulses over time. In comparison, the traditional Kapitsa-Dirac effect lacks a time shutter, presenting a static image.
The journal reviewers found the paper's data extremely exciting, and the observed results are new and important. An important prospect of these experiments is the use of Kapitsa-Dirac interferometers as precise diagnostic tools for the phase of electron wave packets.
Related paper information: https://doi.org/10.1126/science.adn1555
