Scientists at the University of Konstanz have developed a method that uses femtosecond flashes to generate pulses of electrons lasting about five attoseconds. This breakthrough provides a higher time resolution than light waves, paving the way for observing ultrafast phenomena such as nuclear reactions. This is also one of the shortest signals ever produced by physicists.
Molecular or solid-state processes in nature can sometimes occur on time scales as short as femtoseconds (four billionths of a second) or attoseconds (five billionths of a second). Nuclear reactions are even faster. Now, scientists Maxim Tsarev, Johannes Thurner and Peter Baum of the University of Konstanz are using a new experimental setup to achieve signals of attosecond duration, i.e. one billionth of a nanosecond, opening up new prospects in the field of ultrafast phenomena.
Even light waves cannot achieve such time resolution because a single oscillation takes too long. Electrons are a remedy, as they can greatly improve temporal resolution. In their experimental setup, the Konstanz researchers used a pair of femtosecond flashes from a laser to generate extremely short pulses of electrons in a free-space beam. The findings were published in the journal Nature Physics.
How do scientists do this?
Similar to water waves, light waves can also be superimposed to create the crests and troughs of standing or traveling waves. Physicists chose the angle of incidence and frequency so that resonant electrons traveling in a vacuum at half the speed of light overlap with the crests and troughs of light waves traveling at exactly the same speed. The so-called "thinking power" pushes the electrons in the direction of the next wave trough. Therefore, after a brief interaction, a series of extremely short electron pulses are produced - especially in the middle of the pulse sequence, where the electric field is very strong.
For a short time, the electron pulse lasts only about five attoseconds. To understand this process, the researchers measured the velocity distribution of electrons after compression. Physicist Johannes Tourner explains: "The speed of the output pulse is not very uniform, but has a very broad distribution, which is the result of a strong deceleration or acceleration of some electrons during the compression process. What's more: this distribution is not smooth. Instead, it consists of thousands of velocity steps, since only an integer number of pairs of light particles can interact with the electrons at a time."
Research significance
The scientist said that from a quantum mechanical point of view, this is the time superposition (interference) of electrons with themselves after experiencing the same acceleration at different times. This effect is related to quantum mechanical experiments - such as the interaction of electrons with light.
It is also remarkable that plane electromagnetic waves like light beams are generally unable to induce permanent velocity changes in electrons in a vacuum because the total energy and total momentum of massive electrons and light particles (photons) with zero rest mass cannot remain constant. However, this problem can be solved by the simultaneous presence of two photons in a wave slower than the speed of light (Capizza-Dirac effect).
For Peter Baum, Professor of Physics at the University of Konstanz and head of the Light and Matter Group, these results are clearly still basic research, but he emphasizes the huge potential for future research: "If a material is shocked by two of our short pulses at different time intervals, the first pulse can induce changes and the second pulse can be used for observation - similar to the flash of a camera."
He believes that the biggest advantage is that no materials are involved in the experimental principle, and everything is conducted in free space. In principle, lasers of any power could be used for stronger compression in the future. "Our new two-photon squeezing technique allows us to enter new dimensions of time and even film nuclear reaction processes," Baum said.