Electrons In The Fast Lane

Using ultrafast laser flashes, a research group from the University of Rostock and the Max Planck Institute for Solid State Physics in Stuttgart has generated and measured the shortest electron pulse to date. The electrons were released from a tiny metal tip with the help of lasers, which took only 53 attoseconds, i.e. 53 billionths of a billionth of a second. With this study, the results of which will be published in the journal Nature, the researchers set a new speed record for artificially controlling electric currents in solid materials.

Have you ever wondered what determines the speed of your computer and other electronic devices? It's the length of time it takes for electrons - some of the smallest particles in our microcosm - to stream out of tiny contacts inside the transistors of electronic microchips. Methods to accelerate this process are central to advancing electronics and their applications to the ultimate limits of performance. But what is the shortest possible flow time for electrons from a tiny metal line in an electronic circuit?

A research team headed by Professor Eleftherios Goulielmakis, head of the Extreme Photonics working group at the Institute for Physics at the University of Rostock, and employees at the Max Planck Institute for Solid State Physics in Stuttgart have investigated this question. With extremely short flashes of light, generated using state-of-the-art laser technology, the researchers shot electrons out of a tungsten nanotip and thus generated the shortest electron pulse to date. The results of the study have now been published in Nature (January 25, 2023, DOI: https://www.nature.com/articles/s41586-022-05577-1 ).

In fact, it has been known for a long time that light can release electrons from metals through the so-called photo effect – for the explanation of which Albert Einstein was honored with the Nobel Prize in 1921. Yet this process remains extremely difficult to manipulate to this day, as the light's electric field reverses its direction about a million billion times per second. This makes it extremely difficult to control how and when it rips the electrons out of a metal's surface.

To overcome this difficulty, the Rostock scientists and their collaborators used the technology they had previously developed, known as light field synthesis. This allows them to condense a flash of light to less than a full cycle of its own field. The researchers fired such flashes of light at the tip of a tiny tungsten needle in order to hurl electrons into the vacuum. "Using light pulses that cover only a single cycle of the field, it is now possible to give the electrons a precisely controlled kick, so that they are ejected from the tungsten tip within a very short time interval," explains Eleftherios Goulielmakis.

However, the challenge of generating the shortest electron pulses to date could only be mastered after the scientists also found a way to determine the duration of the generated pulses. To do this, the team developed a novel camera that can take snapshots of the electrons during the ultra-short period of time in which they are transported from the nanotip into the vacuum by the laser. "The trick was to use a second, very weak flash of light," says Dr. Hee-Yong Kim, lead author of the study. "This second laser flash can slightly modify the energy of the generated electron pulse, allowing us to figure out what it looked like over time," he adds. "It's a bit like the game 'What's in the box?' where you try to identify an object without looking,

But how could this new methodology be used in electronics? "As modern technology advances rapidly, it is expected that in the future microscopic electronic circuits will be developed in which the electrons travel in a vacuum between densely packed conductors to avoid obstacles that slow them down," says Goulielmakis. "Using light to detach electrons from these conduits and move them between them could accelerate future electronics by thousands of times their current speed," he explains.

However, the researchers are of the opinion that their newly developed methodology can also be used directly scientifically. "Emission of electrons from a metal within a fraction of the cycle of a light field dramatically simplifies the interpretation of the experiments and allows us to use advanced theoretical methods to understand the emission of electrons in a way that was never possible before," says Professor Thomas Fennel, co-author of the new study. "Since our electron pulses provide excellent resolution for snapshots of electronic and atomic motions in materials, we want to use them to gain a deep understanding of complex materials and thus facilitate their application for future technologies," Goulielmakis concludes.