Physicists from the University of Konstanz have produced one of the shortest signals ever created by humans. Molecular or solid-state processes in nature can sometimes occur on time scales as short as femtoseconds (quadrillionths of a second) or attoseconds (quintillionths of a second). Nuclear reactions occur even faster. Now scientists Maxim Tsarev, Johannes Turner and Peter Baum from the University of Konstanz are using a new experimental setup to obtain signals with a duration of attoseconds (i.e. billionths of a nanosecond), which opens new perspectives in the field of ultrafast phenomena.

Even light waves cannot achieve this time resolution because it would take too long for a single oscillation to achieve this. Electrons help here because they provide much higher time resolution. In their experimental setup, the Konstanz researchers use double femtosecond light flashes from a laser to produce extremely short pulses of electrons in a beam of free space. The results were published in the journal Nature Physics.

How did scientists get to this point?

Like water waves, light waves can overlap to form the crests and troughs of standing or traveling waves. Physicists chose their angle of incidence and frequency so that electrons propagating through a vacuum at half the speed of light coincide with the peaks and troughs of optical waves at exactly the same speed.

What is called the ponderomotive force pushes the electrons in the direction of the next wave. Thus, after a short interaction, a series of electron pulses that are extremely short in time are generated, especially in the middle of the pulse series where the electric fields are very strong.

For a brief period, the duration of electronic pulses is only about five attoseconds. To understand this process, researchers measure the electron velocity distribution that remains after compression. Physicist Johannes Turner explains: “Instead of a very uniform speed of the output pulses, you see a very wide dispersion resulting from the strong deceleration or acceleration of some electrons during compression.” “But not only that: The distribution is not uniform. Instead, it consists of thousands of speed steps because only a full number of pairs of light particles can interact with the electrons simultaneously.”

Importance for research

The scientist says that in terms of quantum mechanics, this is a temporary superposition (interference) of electrons with themselves after being subjected to the same acceleration at different times. This effect is relevant in quantum mechanical experiments, for example the interaction of electrons and light.

Also noteworthy: Plane electromagnetic waves, such as a beam of light, cannot normally cause permanent changes in the speed of electrons in space, because the total energy and total momentum of a large-mass electron and a light particle (photon) with zero rest mass must be conserved. However, the presence of two photons at the same time in a wave propagating slower than the speed of light solves this problem (Capitza-Dirac effect).

For Peter Baum, professor of physics and head of the light and matter group at the University of Konstanz, these results are still basic research, but they highlight the great potential for future research: “If the material is exposed to our two short pulses over a variable time interval, the first pulse can cause a change and The second pulse can be used for observation, like a camera flash.”

According to him, the biggest advantage is that, in the experimental principle, no materials are used and everything happens in free space. In principle, lasers of any power can be used for further compression in the future. “Our new two-photon compression allows us to go into new dimensions of time and possibly even capture nuclear reactions,” says Baum. Source