Laser-Driven Electrons Observed in Real Time

Accelerated electron bunches and the plasma wave produced by the strong laser light that drives them have been observed for the first time in real time.

The discovery will advance the development of new x-ray sources, the resolution of which will be much higher than current devices allow, according to physicists at the Laboratory of Attosecond Physics (LAP) at Max Planck Institute for Quantum Optics (MPQ) and Ludwig Maximilians University of Munich (LMU), in cooperation with colleagues from Friedrich Schiller University Jena.

Helium atoms flowing from a small nozzle are ionized by a laser pulse, forming a plasma channel from helium ions and free electrons. In this channel, the flash of light accelerates a small portion of the electrons almost to the speed of light. (Image: Thorsten Naeser)

In the researchers' experiments, when short laser pulses irradiate helium atoms, their structure is heavily disturbed. If the light is strong enough, electrons are pulled out of the atoms, and the helium atoms become ions. In this mixture, the electrons are much lighter than the helium ions and, as a result, are pushed aside.

Although the laser pulse sweeps across the system, the ions remain stationary and the released electrons oscillate around one location. Together, the particles form wave structures (electron plasma waves). In laser physics, this process and these waves are used under special conditions to rapidly accelerate a small number of the electrons to close to the speed of light and to control them.

In the plasma wave, gigantic electric fields are formed, which are 1000 times stronger than those generated in the world’s largest particle accelerators. A small number of the electrons take advantage of these fields, flying as a swarm behind the laser pulse in its slipstream and accelerating to close to the speed of light. In this process, every accelerated electron has almost the same energy.

An intense light pulse (yellow-orange) produces a plasma wave (white, modulated surface) from oscillating electrons and stationary helium ions. Some electrons leave the plasma wave and fly close to the speed of light as a swarm (red spheres) behind the laser pulse. (Image: Christian Hackenberger)

Physicists have long been aware of this phenomenon, and it has been demonstrated in earlier experiments, but until now, it has been possible to individually observe only the electron swarm or the whole plasma wave with reduced resolution.

The laser physicists, including Ferenc Krausz and his employees Laszlo Veisz and Alexander Buck of LAP, succeeded in recording both phenomena with a high-resolution image of the plasma wave. The process was documented in snapshots with the same light pulse responsible for accelerating the electrons. The physicists previously split the laser pulse so that a small portion of it illuminated the system of free electrons and ions perpendicularly to the electron beam. The periodic structure of the plasma wave refracts and partially deflects the light.

"We observe the deflection and thereby image the plasma wave as a modulation of brightness onto a camera," said Veisz, the research group leader of the LAP team.

In doing so, the researchers can achieve a unique spatial and temporal resolution in the femtosecond range. The electron swarm produces strong magnetic fields that they also can record to determine its position and duration. Eventually, a film describing the acceleration of the electrons results from the combination of both measurement methods.

"The obtained improved knowledge about laser-driven electron acceleration helps us in the development of new x-ray sources of unprecedented quality, not only for basic research, but also for medicine," Krausz said.

The physicists describe their results in the scientific journal Nature Physics.

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