Physicists produce signals with attosecond duration

Processes in nature that occur in molecules or solids sometimes run on a time scale of quadrillionths (femtoseconds) or quintillionths (attoseconds) of a second. Nuclear reactions are even faster. Now, a team of researchers from the University of Konstanz are using an experimental set-up to achieve signals of attosecond duration to open up new perspectives in the field of ultrafast phenomena.

Not even light waves can achieve such a time resolution, because a single oscillation takes too long for that. Electrons provide a solution here, as they enable higher time resolution. In the experimental set-up, the Konstanz researchers used pairs of femtosecond light flashes from a laser to generate their short electron pulses in a free-space beam. The results were reported in the journal Nature Physics.

Much like water waves, light waves can superimpose to create standing or travelling wave crests and troughs. The physicists chose the incidence angles and frequencies so that the co-propagating electrons, which fly through vacuum at half the speed of light, overlap with optical wave crests and troughs of exactly the same speed. What is known as ponderomotive force then pushes the electrons in the direction of the next wave trough. Thus, after a short interaction, a series of electron pulses is generated which are short in time — especially in the middle of the pulse train, where the electric fields are strong.

For a short time, the temporal duration of the electron pulses is only about five attoseconds. In order to understand that process, the researchers measured the electrons’ velocity distribution that remained after compression. Physicist Johannes Thurner said instead of a uniform velocity of the output pulses, the researchers observed a broad distribution resulting from the strong deceleration or acceleration of some electrons in the course of compression.

“But not only that: The distribution is not smooth. Instead, it consists of thousands of velocity steps, since only a whole number of light particle pairs can interact with electrons at a time,” Thurner said.

Quantum mechanically, this is a temporal superposition (interference) of the electrons with themselves, after experiencing the same acceleration at different times. This effect is relevant for quantum mechanical experiments, such as the interaction of electrons and light. Plane electromagnetic waves like a light beam normally cannot cause permanent velocity changes of electrons in vacuum, because the total energy and total momentum of the massive electron and a zero rest mass light particle (photon) cannot be conserved. However, having two photons simultaneously in a wave travelling slower than the speed of light solves this problem (Kapitza–Dirac effect).

Peter Baum, physics professor at the University of Konstanz, said that although the results are clearly basic research, there is great potential for the future. “If a material is hit by two of our short pulses at a variable time interval, the first pulse can trigger a change and the second pulse can be used for observation — similar to the flash of a camera,” Baum said.

Baum said that the key advantage is that no material is involved in the experimental principle and everything happens in free space. Lasers of any power could in principle be used in the future for stronger compression. “Our new two-photon compression allows us to move into new dimensions of time and perhaps even film nuclear reactions,” Baum said.

Image caption: The compressor from the experimental set-up. Image credit: Johannes Thurner