October 22, 2025Open Access

Giant counter-rotating oscillations on the attosecond timescale

JBJakob Nicolai BruhnkeLund University EOEdvin OlofssonLund University ASAxel StenquistLund University

Key Points

Giant population oscillations occur on the attosecond timescale, indicating novel effects in atomic interactions with light.
The oscillations arise from the interplay of energy-conserving and nonconserving photon processes via Rydberg states.
Floquet and Hamiltonian theories were utilized to validate our findings against simulated results.
The implications of this research extend past two-level atom scenarios, showing unique behaviors in the excited state.

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 simulations.

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