the photon pair compresses the electron beam into a short pulse

June 19, 2023

(Nanowerk News) Physicists at the University of Konstanz produced one of the shortest signals ever produced by humans: Using paired laser pulses, they managed to compress a series of electron pulses to a numerically analyzed duration of just 0.00000000000000000005 seconds.

Processes in nature that occur in molecules or solids sometimes run on a time scale of one millionth (femtosecond) or one millionth (attosecond) second. Nuclear reactions are even faster. Now, Maxim Tsarev, Johannes Thurner and Peter Baum, scientists from the University of Konstanz, use a new experimental setup to achieve a signal of attosecond duration, which is one billionth of a nanosecond, which opens new perspectives in the field of ultrafast phenomena.

Not even light waves could attain such time resolution, as a single oscillation takes too long for that. Electron provides a solution here, as it allows much higher time resolution. In their experimental setup, the Konstanz researchers used a pair of femtosecond light flashes from a laser to generate very short pulses of electrons in a beam of free space.

The results are reported in a journal Natural Physics (“Nonlinear-optical quantum control of matter waves of free electrons”). Green and infrared (red) laser pulses, shown as their overlapping electric fields form the peaks of the traveling waves (pink) traveling at the same speed (v) as the electrons (shown as blue clouds). The slow-motive force (little pink arrow) pushes the electron toward the next trough. Thus a series of pulses of electrons is generated, which are very short in time – especially in the middle of the series of pulses, where the electric field is strongest. (Image: Johannes Thurner)

How do scientists do it?

Similar to water waves, light waves can also stack to create standing or traveling crests and troughs of waves. Physicists choose the angle of incidence and frequency so that the co-propagating electrons, which fly through a vacuum at half the speed of light, overlap the peaks and troughs of optical waves at exactly the same speed.

What are known as ponderomotive forces then push the electrons toward the trough of the next wave. So, after a brief interaction, a series of electron pulses are generated in a very short time – especially in the middle of the pulse sequence, where the electric field is very strong.

For such a short time, the temporal duration of the electron pulse is only about five attoseconds. To understand the process, the researchers measured the velocity distribution of the electrons remaining after compression.

“Instead of a very uniform speed of the output pulse, you see a very wide distribution resulting from the strong deceleration or acceleration of some electrons during compression”, explains physicist Johannes Thurner. “But that’s not all: Its distribution isn’t smooth. Instead, it’s made up of thousands of speed steps, since only a limited number of pairs of light particles can interact with electrons at any one time.”

Significance for research

Quantum mechanically, says the scientist, this is a temporary superposition (interference) of electrons with themselves, having experienced the same acceleration at different times. This effect is relevant for quantum mechanics experiments – for example, on the interaction of electrons and light.

What’s also remarkable: Field electromagnetic waves such as light beams ordinarily cannot cause a permanent change in the speed of electrons in a vacuum, because the total energy and total momentum of massive electrons and zero rest mass light particles (photons) are not conserved. However, having two photons simultaneously in a wave traveling slower than the speed of light solves this problem (Kapitza-Dirac effect).

For Peter Baum, professor of physics and head of the Light and Matter Group at the University of Konstanz, these results are clearly still basic research, but he emphasizes the huge potential for future research: “If a material is hit by two of our short waves, at variable time intervals , the first pulse can trigger changes and the second pulse can be used for observation – similar to a camera flash.”

In his view, the big advantage was that no matter was involved in the experimental principle and everything happened in free space. Lasers of any power can in principle be used in the future for stronger compression. “Our new two-photon compression will allow us to travel to new dimensions of time and perhaps even film nuclear reactions,” said Baum.

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