New method for generating ultrashort electron pulses developed

Scientists at the University of Konstanz have achieved a groundbreaking feat in generating incredibly short signals. Through the utilization of paired laser pulses, physicists compressed a series of electron pulses to a remarkably brief duration of only 0.000000000000000005 seconds. This remarkable achievement opens up new possibilities in the study of ultrafast phenomena.

In the realm of natural processes occurring in molecules and solids, timescales can often be measured in femtoseconds or even attoseconds. Nuclear reactions operate at an even faster pace. The team of Maxim Tsarev, Johannes Thurner, and Peter Baum from the University of Konstanz has developed an innovative experimental setup to produce attosecond-duration signals, offering novel perspectives in the field of ultrafast phenomena.

Even light waves are unable to achieve such high time resolution since their single oscillation requires too much time. However, electrons prove to be a solution as they enable significantly greater time resolution. In their experimental configuration, the researchers at Konstanz employ pairs of femtosecond light flashes from a laser to generate remarkably short electron pulses in a free-space beam. The findings of this study have been published in the prestigious journal Nature Physics.

How did the scientists go about it?

In a manner akin to water waves, light waves also possess the ability to combine and create either stationary or traveling wave crests and troughs. The physicists carefully selected the incidence angles and frequencies in such a way that the co-propagating electrons, moving through a vacuum at half the speed of light, aligned with the optical wave crests and troughs, matching their velocity.

The phenomenon known as ponderomotive force then propels the electrons toward the subsequent wave trough. As a result of this brief interaction, a sequence of electron pulses is generated, exhibiting an incredibly short temporal duration—particularly in the middle of the pulse train where the electric fields are exceptionally intense.

During this transient period, the electron pulses last for approximately five attoseconds. To gain insight into this process, the researchers analyze the velocity distribution of the electrons subsequent to compression.

Physicist Johannes Thurner elucidates, “Instead of observing a uniform velocity in the output pulses, we observe a broad distribution stemming from the significant deceleration or acceleration experienced by certain electrons during the compression process. Moreover, the distribution is not smooth; it comprises thousands of velocity steps since only a whole number of light particle pairs can interact with electrons simultaneously.”

Significance for research

From a quantum mechanical perspective, the scientists describe the phenomenon as a temporal superposition or interference of electrons with themselves, resulting from their identical acceleration at different points in time. This quantum effect holds significance for experiments involving the interaction of electrons and light.

What adds to the significance of this achievement is the fact that conventional plane electromagnetic waves, such as a light beam, typically do not induce permanent velocity changes in electrons within a vacuum. This is because the conservation of total energy and total momentum between the massive electron and the zero rest mass light particle (photon) poses a challenge. However, by employing two photons simultaneously in a wave that travels slower than the speed of light, this issue is resolved, known as the Kapitza-Dirac effect.

For Peter Baum, a physics professor and leader of the Light and Matter Group at the University of Konstanz, these findings primarily contribute to fundamental research. Nonetheless, he highlights their immense potential for future investigations. Baum explains, “If a material is exposed to two of our short pulses with an adjustable time interval, the first pulse can induce a change, while the second pulse can be utilized for observation, akin to the flash of a camera.”

He further emphasizes the advantage of this experimental approach being entirely conducted in free space without the need for any material. In theory, lasers of any power could be employed in the future to achieve even stronger compression. Baum envisions the application of their novel two-photon compression technique to explore new dimensions of time and potentially capture nuclear reactions on film.

Source: University of Konstanz

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