Sculpted laser pulses steer electron flow in 860-attosecond bursts

Watching an electron move through a material demands two things at once: the ability to see individual atoms and the ability to resolve motion on extremely short timescales. A team led by Prof. Michael Krüger of the Technion-Israel Institute of Technology has now brought both capabilities together in a single instrument. In a study published in Nature Photonics, the researchers used carefully shaped laser pulses inside a scanning tunneling microscope to control which direction electrons travel across the tiny gap between the microscope’s needle tip and a sample, while reconstructing current bursts lasting just 860 attoseconds. One attosecond is one billionth of a billionth of a second.

The scanning tunneling microscope at the heart of the work relies on an exceptionally sharp needle brought very close to a surface. At that separation, electrons can cross the gap through quantum tunneling, and the resulting current allows researchers to map minute features of the surface with atomic-scale sensitivity. Following the rapid movement of electrons within that gap itself has been a major challenge, and the Technion team attacked it with light.

The researchers directed infrared laser pulses into the tip-sample gap, with each pulse containing two colors of light. By adjusting the relative timing of the two colors, they changed the shape of the combined light wave, and with it the direction of the tunneling current: electrons could be steered from the needle tip to the sample, or back in the opposite direction. Crucially, the team varied the timing of the two colors instead of repeatedly switching the laser on and off. That choice helped them distinguish the genuine ultrafast current from noise signals caused by laser heating of the junction.

“I fondly remember the day when our students Daniel Davidovich and Boyang Ma and Dr. Adi Goldner came to my office with a new and exciting measurement,” said Krüger. “We could finally prove without any doubt that we can control the direction of the current.” The measurement did not come easily. “We tested three different laser systems and rebuilt the optical setup many times until we could clearly distinguish between ultrafast current and noise,” said Dr. Goldner, a staff scientist in the Krüger lab. The experimental work was carried out in collaboration with theoretical physicist Andrei Borisov of CNRS Université Paris-Saclay and the Donostia International Physics Center.

By comparing the measurements with several theoretical models, the researchers reconstructed current bursts lasting 860 attoseconds and clarified how the light field drives an electron across the gap. According to the models, the optical wave makes the tunneling barrier easier to pass through, accelerates the electron once it is free, and drives it into the surface. This sequence, in which the same oscillating field lowers the barrier and then propels the electron, is what allows the direction of the current to be set by the timing of the two colors.

Importantly, the ultrafast operation did not come at the cost of the microscope’s spatial performance. Even while the laser pulses were driving the current, the instrument retained a sideways resolution of about two nanometers and could detect changes in surface height smaller than one ångström. The measurements were performed at room temperature and in air, conditions that matter for how readily the approach can be extended to other labs and other materials.

The result is a step toward observing how electrons move at specific locations in a material as that motion unfolds, combining the spatial acuity of tunneling microscopy with temporal resolution approaching that of attosecond science. According to Krüger, the work could also inform future electronics in which light waves control currents at scales and speeds beyond those of today’s devices. Those applications remain a goal for future research; the present study establishes precise control of electron flow in the microscope’s nanoscale gap rather than a working device.

The research was funded by the European Research Council under the ATTIDA project, by the Israel Science Foundation through Grant No. 1504/20, and by the Helen Diller Quantum Center at the Technion. The experimental study demonstrates that the direction and timing of tunneling currents can be set by sculpted optical waveforms, and that this control can coexist with ångström-level topographic sensitivity, marking a concrete advance toward microscopes that can track electron motion in both space and time.

Subject of Research: Attosecond control and timing of tunneling electron currents in a scanning tunneling microscope

Article Title: Sculpted light steers ultrafast electron flow in an atomic-scale microscope

Article References: Sculpted light steers ultrafast electron flow in an atomic-scale microscope. Original publication (Bibliographic details unavailable.)

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Keywords: attosecond science, scanning tunneling microscopy, ultrafast optics, lightwave electronics, electron tunneling, laser pulses, nanoscale imaging, Technion, Nature Photonics, quantum transport, two-color laser field, electron dynamics