Chinese and German scientists discover ultrafast Kapitza-Dirac effect

Recently, a collaboration team by Chinese and German scientists has unraveled the enigma of ultrafast phase evolution of an electron wavepacket. Prof. Reinhard Dörner from Goethe University, Prof. LIN Kang from Zhejiang University and their colleagues just published their findings entitled "Ultrafast Kapitza-Dirac effect" in Science on 29 March.

Fig. 1. Prof. Reinhard Dörner and Prof. LIN Kang

Electron is one of the simplest and most fundamental particles. Interestingly, the electron behaves more like a wave instead of a particle in the microscopic world. The direct observation of the electron phase has been a long-standing challenge for scientists. Now, the Chinese-German team has successfully observed the phase evolution of electrons by capturing the diffraction patterns produced by passing an electron pulse through a standing light wave pulse at different times.

The conventional Kapitza-Dirac effect, initially proposed by Kapitza and Dirac in 1933, describes that electrons will be diffracted when passing through a continuous standing light wave. Its beauty lies in the two-fold elucidation of the particle-wave duality, where the roles of particles and waves exchanges twice: the electron is a matter wave instead of a particle, and the grating is an immaterial standing light wave instead of a material one. However, due to the limitation of laser technology, this phenomenon was not experimentally verified until 2001 by American scientists.

Fig. 2. Schematic diagram of the momentum spectrometer

Differing from the conventional Kapitza-Dirac effect, the Chinese-German team used a pump-probe scheme to realize the ultrafast Kapitza-Dirac effect. A pump pulse is used to ionize neutral atoms to produce the electron pulse, followed by a time-delayed femtosecond standing light wave to diffract the evolving electron pulse. By adopting this strategy, they were able to capture the dynamic evolution of electron phase with high time resolution. "As compared to the conventional Kapitza-Dirac effect, we use an electron pulse instead of a continuous one, and we diffract the electron use a pulsed standing light wave rather than a continuous one," said Prof. Lin, "These two differences enable us to set a start time point, and then track the dynamics". The experiment could be simply understood as taking high-speed photos of a moving athlete, where each pulsed standing light wave serves as a shutter to take pictures of the moving electron.

Fig. 3. Observation of the ultrafast Kapitza-Dirac effect.

Fig. 4. Diffraction pattern as a function of time delay.

The implications of this discovery are profound. It extends the capability of the conventional Kapitza-Dirac effect into the time domain and provides a unprecedented tool for ultrafast electron spectroscopy, e.g. tracking the phase correlation between two electrons or the phase chirality of the electron wavepacket released from a chiral molecule.

More information: Kang Lin et al. Ultrafast Kapitza-Dirac effect, Science (2024) 10.1126/science.adn1555

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