Tools to glimpse how “helicity” impacts matter and light

Tokyo, Japan – Researchers from Tokyo Metropolitan University have succeeded in detecting laser-assisted electron scattering (LAES) using circularly polarized light for the first time. The use of circularly polarized light promises valuable insights into how atomic scale “helicity” impacts how electrons interact with matter and light. Using synchronized femtosecond laser pulses and electron pulses directed at argon atoms, they succeeded in detecting a LAES signal showing excellent agreement with theory.

Laser-assisted electron scattering (LAES) is a cutting-edge tool for understanding how electrons interact with matter under the influence of strong fields. When electrons are fired at atoms or molecules, they are scattered in all directions; the presence of strong light can change the way in which the scattering takes place due to an exchange of energy with the surrounding light field. The amount of energy exchanged is precisely governed by quantum mechanics and yields characteristic shifts in the energies held by scattered electrons. Recent experiments have used LAES measurements to show how strong electromagnetic fields can fundamentally change the behavior of matter. Examples include “light-dressing,” where strong laser light modifies the distribution of electrons around atoms.

A key challenge in recent work on LAES is the use of circularly polarized light. As a wave of oscillating electric and magnetic fields, light can have a “polarization” direction in which the electric fields oscillate. In circularly polarized light, the polarization direction rotates as the wave propagates forward. Since it has a characteristic “handedness,” of whether it rotates left or right, the way in which it interacts with matter may be sensitive to any intrinsic “chirality” in the structure, the “handedness” of the structure itself. Importantly, measuring the difference between left and right-handed light in LAES gives access to the “phase” of scattered electrons, an inaccessible quantity using linearly polarized light. However, the gold standard measurement of LAES using circularly polarized light in single atoms has not yet been achieved.

Now, a team led by Professor Reika Kanya from Tokyo Metropolitan University has successfully demonstrated LAES in argon using femtosecond circularly polarized laser pulses in the near-infrared range, and simultaneous electron pulses. They were able to measure the energy and angular distributions of electrons scattered by argon in the presence of femtosecond laser pulses, finding peaks characteristic of the LAES process. While the signal was weaker than with linearly polarized light, the scattering signal agreed with a form expected from seminal extensions to LAES theory (Kroll-Watson theory). At this point, they were unable to measure the minute difference between LAES with left and right-handed light.

While there is further work to be done to improve detection efficiency and statistical accuracy, the team’s work demonstrates how LAES with circularly polarized light might illuminate new aspects of electron-matter interaction in strong fields.

This work was supported by JSPS KAKENHI Grant Numbers JP20H00371, 21H01892, JP23KJ1805, 24K21757, 25K03392, and 25H01631, the MEXT Quantum Leap Flagship Program (MEXT Q-LEAP) Grant Number JPMXS0118068681, and JST PRESTO Grant Number JPMJPR2007.

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