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arXiv:2506.10183

Laser Stabilised Ionising Transitions

Erika Cortese, Simone De Liberato

Published Jun 11, 2025• 0 citations
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Abstract

We investigate a ionising electronic transition under resonant pumping. We demonstrate that, above a critical value of the pump intensity, a novel metastable electronic bound state is created, which can decay into the free electron continuum by two-photon ionization. We calculate the system's resonant fluorescence spectrum, finding results qualitatively different from the Mollow triplet expected in a bound-to-bound transition. The fluorescent emission can be used to measure the time-resolved population of the novel metastable state. Contrary to Kramers-Hennenberger atoms, stabilised by non-perturbative, non-resonant laser pulses, the physics we observe is inherently resonant and relies on perturbative level repulsion. In analogy to how the AC-Stark shift is a semiclassical version of the single-photon Rabi splitting observed in photonic cavity, the phenomenon we describe is better understood as a semiclassical version of recently observed excitons bound by a single cavity photon. Our results demonstrate a novel way to stabilise electronic states with intense laser fields, increasing our capability to design and engineer non-classical states of matter.

DOI
10.48550/arxiv.2506.10183

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Stabilizing atoms exposed to intense laser fields has been a central topic in atomic physics for decades, inspiring several theoretical frameworks that describe the phenomenon under different resonance conditions and coupling regimes. Here, we theoretically investigate an ionizing electronic transition driven by a resonant pump field, in contrast to Kramers–Henneberger atoms, which are stabilized by non-perturbative, non-resonant laser pulses. We show that, above a critical pump intensity, the drive creates a metastable electronic bound state emerging from the continuum, which decays via two-photon ionization. We calculate the resulting resonant fluorescence spectrum and find a qualitatively different structure from the familiar Mollow triplet associated with bound-to-bound transitions. This fluorescence provides a time-resolved probe of the population in the metastable state. In analogy to how the AC Stark shift is a semiclassical counterpart of the single-photon Rabi splitting observed in optical cavities, the phenomenon we describe is best understood as a semiclassical analog of recently observed excitons bound by a single cavity photon.
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The energy levels of atoms and molecules exposed to intense, short, high-frequency laser pulses undergo time-dependent distortion known as dynamic Stark shift (DSS). These level shifts are induced by the coupling with nonresonant (nonessential) states and follow the temporal intensity profile of the laser pulse. Owing to the different DSSs of the individual atomic levels, transient resonance suppressions and enhancements significantly modify the multiphoton ionization pathway, leaving clear fingerprints (asymmetry, splitting, and shifting) in the energy spectrum of emitted photoelectrons. Here we investigate this phenomenon by solving the time-dependent Schr\"odinger equation of the valence electron of Na and Li in increasing levels of complexity: (i) developing minimal models for two-photon transition and (2+12+12+1)-photon ionization, (ii) applying a spectral method without continuum-continuum couplings, and (iii) propagating the electron wave packet on a large spatial grid accounting for all possible couplings. We show that appropriately detuned transform limited pulses provide a high level of control in strong-field excitation, when the atomic levels are dynamically shifted. Furthermore, we demonstrate the role of DSSs in the multipeak structure of the Autler-Townes doublet, found when several Rabi oscillations are induced during the strong-field resonant ionization.