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  • Letter
  • Open Access

Giant counter-rotating oscillations on the attosecond timescale

Jakob Nicolai Bruhnke, Edvin Olofsson, Axel Stenquist, and Jan Marcus Dahlström*

  • *Contact author: marcus.dahlstrom@fysik.lu.se
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Phys. Rev. Research 7, L042019 – Published 20 October, 2025

DOI: https://doi.org/10.1103/jx8n-775g

Abstract

We predict an unexplored type of ultrastrong coupling between atoms and intense ultraviolet light that leads to giant population oscillations on the attosecond timescale. These counter-rotating oscillations stem from the interference of energy-conserving and nonconserving photon absorption and emission processes via Rydberg states. They can be of similar amplitude as the elementary femtosecond Rabi oscillations between the two strongly coupled states. The effect, which is beyond the two-level atom, is nonreciprocal: It only affects the excited state, while the ground state is unaffected. We propose that two-photon Rabi oscillations (1s2–1s3d) in helium are suitable for the generation of this type of ultrastrong coupling with realistic pulses. We use a combination of Floquet theory and effective Hamiltonian theory to test our predictions against ab initio simulations.

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Phys. Rev. Research 7, L042019– Published 20 October, 2025
  • Received 31 March 2025
  • Accepted 23 September 2025
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Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI. Funded by Bibsam.

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