Shaping Electron Bunches with Terahertz Light Pulses

Although seeing atoms is nowadays possible with modern electron micro­scopes, tracking atomic movements requires ultra­short measure­ment periods in the femto­second and atto­second range as well. Such extremely fast camera shutter speeds can be reached through ultra­short electron pulses, which are shorter than the time scale of the motion. Important for experi­ments is a special shaping of the electron pulses in space and time, adjusted to the pro­perties of the substance in question. Konstanz physicist Peter Baum and his team now succeeded in spatially and temporally directing and control­ling ultra­short electron pulses directly by using the light cycles of laser light, instead of the previously applied micro­waves.

Scheme of extremely short and specifically-shaped electron pulses for materials studies in the femtosecond and attosecond range produced with terahertz light pulses. (Source: P. Baum, U. Konstanz)

The result is not only a shortened pulse duration, but the researchers were also able to tilt the pulses, that is, have them run in another direction than vertically to the pulse front. Tilted electron pulses provide a huge potential for materials studies in which the funda­mental changes last only femto­seconds or atto­seconds. These times correspond to the period of atomic oscil­lations in crystals and molecules, or to the period of an individual light oscil­lation. Tilted pulses are also highly relevant for free electron lasers for producing more intense and shorter X-ray flashes for analyzing ultra­fast processes. “Our results show that we can now shape and control electron pulses as elec­trically as laser pulses, at the imaging reso­lution of modern electron micro­scopy”, summarizes Peter Baum.

According to quantum mechanics, the properties of particles at the smallest scale come in pairs, such as position and momentum in the uncer­tainty principle. And in the case of tilting? In laser optics it has been known for quite some time that the different colours must run into different direc­tions. In their experi­ments the researchers from Konstanz and Munich now demon­strated that these old laws of laser optics equally apply to the matter wave of electrons, too, even though the electrons have a rest mass and are not coherent like laser light.

It is probable that these measured relations between pulse tilt and angular disper­sion are generally valid for all wave pheno­mena in physics. In that sense the spatial and temporal shaping of electron pulses that the researchers have now achieved is not only of practical use for ultra­fast materials research, but is also funda­mentally interesting for physics in general. (Source: U. Konstanz)

Reference: D. Ehberger et al.: Tilted Electron Pulses, Phys. Rev. Lett. 121, 094801 (2018); DOI: 10.1103/PhysRevLett.121.094801

Link: Ultrafast Electron Imaging (P. Baum), Ludwig-Maximilians-University Munich, Garching, Germany

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