Ultrafast lasers have shone a light on the femtosecond-scale atomic motions involved in chemical reactions. But the dynamics of individual electrons, which happen on the scale of tens of attoseconds (10–18 seconds), have proved too fast to capture. Now, a multinational team has developed a technique, involving a novel approach involving multiple beams from a free-electron laser (FEL), that the scientists claim can provide temporal resolution on the order of 3 attoseconds—fast enough to provide control of electron dynamics (Nat. Photon., doi: 10.1038/nphoton.2016.13).

A “radically different” way to tune EUV phase

One reason coherent control at electron timescales has been so tough lies in the great difficulty in controlling phase at the very high frequencies and short wavelengths—in the extreme ultraviolet (EUV) range—that are required to access those short timescales. Coherent control of a quantum system requires manipulating the phase and wavelength of laser light to nudge the system to the desired final state. But in contrast to optical wavelengths, for which phase differences can be easily generated with nonlinear materials or mechanical delay lines, such techniques become far more difficult to implement at EUV frequencies.

The scientists in the newly reported experiment—members of an international team from Italy, Australia, Japan, Russia, Germany, Slovenia and the United States—decided to attack the problem with what they call “a radically different approach” to phase tuning. Rather than attempting to manipulate and delay the phase of the EUV light beam itself, they manipulate phase differentials in the system that generates the beam: the electron bunches in a free-electron laser.

Tweaking FEL wiggles

FELs create ultrafast coherent radiation by passing a high-intensity, relativistic electron beam through a magnetic array called an undulator; the array introduces wiggles into the electron beam that result in electron “bunches.” Those bunches are converted, through the process of self-amplified stimulated synchrotron emission, into pulsed, coherent ultrafast EUV and X-ray beams (see “Sources and Science of Attosecond Light,” OPN, May 2015).

The research team reasoned that, at attosecond timescales, the best way to control the phase difference between two EUV beams would be to tune not the emitted light, but the electron wiggles that lead to the light’s emission. To do so, they used the FERMI FEL facility in Trieste, Italy, to create EUV beams at two different wavelengths: a 63.0-nm beam and its first harmonic at 31.5 nm. They then used an electron delay line to control the phase difference of the electron bunches giving rise to the first-harmonic beam relative to the bunches for the 63-nm beam.

Three-attosecond control

The ability for a tunable delay in the electron bunches allowed creation of two EUV beams with a tightly controllable phase difference—a feat impractical if one attempts to control the EUV beams themselves, rather than indirectly through the electron bunches. By manipulating the light phase in this way, and firing the beams into neon gas, the team found that they could control the angular distribution of photoelectrons generated in the gas by adjusting the phase, with a 3-attosecond temporal distribution.

The researchers believe that the result “opens the way for unique experiments in the EUV and soft X-ray regions,” to illuminate ultrafast phenomena such as the behavior of electron wave packets. “The main conclusion we can draw based on this experiment,” noted study coauthor Elena Gryzlova, a senior researcher at the D.V.Skobeltsyn Institute of Nuclear Physics, Moscow State University, “is that control over quantum processes with a precision of several attoseconds is possible at all.”