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arXiv:2510.09441 · Physical Review Research

Coherent control of ionization via stabilization by resonant pulse pairs

Edvin Olofsson, Evan Lovelle Fulton, Rezvan Tahouri, Mattias Bertolino, J. M. Ngoko Djiokap, Jan Marcus Dahlström

Published Jan 31, 2026• 5 citations
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Abstract

We study the nonlinear and resonant process of two-photon ionization of atoms (He and H) in a pump-probe scheme. The pump pulse prepares the quantum system in a superposition of the ground state and an excited bound state. By varying the phase difference between the pulses, we show how it is possible to coherently control the dressed-state population during the probe pulse. Our main result is that for certain laser parameters, the control over the dressed-state population leads to strong control of the ionization probability during the probe pulse. The effect arises due to one of the dressed states becoming stabilized against ionization. Contrasting effects from circular and linear polarized pulses demonstrate how such “bound states in the continuum” are sensitive to the degeneracy of the coupled continuum.

DOI
10.1103/4s8v-32r8

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We study the nonlinear and resonant process of two-photon ionization of atoms (He and H) in a pump-probe scheme. The pump pulse prepares the quantum system in a superposition of the ground state and an excited bound state. By varying the phase difference between the pulses, we show how it is possible to coherently control the dressed-state population during the probe pulse. Our main result is that for certain laser parameters, the control over the dressed-state population leads to strong control of the ionization probability during the probe pulse. The effect arises due to one of the dressed states becoming stabilized against ionization. Contrasting effects from circular and linear polarized pulses demonstrate how such “bound states in the continuum” are sensitive to the degeneracy of the coupled continuum.
It is shown that the rate and degree of ionization of an atom can be controlled by preparing a specific superposition of Rydberg states. This effect is due to the Raman interaction of Rydberg levels via resonant lower-lying levels of the atom. Depending on the initial population of an atom interacting with a strong laser field, the atom either ionizes completely or exhibits an extremely high degree of stability (in this case the probability of the atom remaining in a bound state when the laser pulse is switched off is close to 1). It is shown that states which are stable against ionization decay or, conversely, decay rapidly into the continuum can be prepared using double laser pulses with a controllable time delay.
A theoretical analysis of the phase control of a two-channel atomic ionization scheme is given. We report results on phase-selective excitation, both with square and smooth laser pulses. We show that the phenomenon of population trapping, which is known to occur in this system, can be greatly influenced by the phase difference of the lasers involved in the excitation. As a result, the time dependences of the ionization probability, the populations in the bound states and the form of the photoelectron energy spectra can all be controlled by varying the phase difference of the lasers. We also show that for specific atomic and phase parameters population can be entirely trapped within a superposition state, totally immune to decay, with no ionization whatsoever.
A theoretical analysis of the phase control of a two-channel atomic ionization scheme is given. We report results on phase-selective excitation, both with square and smooth laser pulses. We show that the phenomenon of population trapping, which is known to occur in this system, can be greatly influenced by the phase difference of the lasers involved in the excitation. As a result, the time dependences of the ionization probability, the populations in the bound states and the form of the photoelectron energy spectra can all be controlled by varying the phase difference of the lasers. We also show that for specific atomic and phase parameters population can be entirely trapped within a superposition state, totally immune to decay, with no ionization whatsoever.
Coherent control provides a quantum interference based method for controlling atomic and molecular dynamics. We report two experimental observations of different ways to achieve such a control in atomic photoionization continuum. One is the photoionization of sodium atoms, initially prepared in the 3P1/2 excited state, by a non resonant three-photon interaction with 1064-nm laser light and by a one-photon interaction with the third-harmonic field, at 355 nm. Interference between the two transition paths allows the control of the total ionization rate by varying the relative phase of the laser fields. The second experiment is the observation of the modification of the ionization continua of xenon through laser induced continuum structure (LICS). The multiphoton radiative decay of the xenon ground state into the two electronic continua, that correspond to the two fine structure levels of the ground state of the xenon ion, is varied by electromagnetically embedding a bound state of the atom into the two continua. Both experiments are performed on an atomic beam apparatus which uses a time-of-flight spectrometer for energy analyzing the emitted photoelectrons.
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.
We have investigated multiphoton-ionization passages of rubidium atoms initiated from the excited 5P3/25{P}_{3/2}5P3/2​ energy state. For this we used coherent control schemes based on femtosecond laser pulse shaping applied to cold atoms spatially isolated in a magneto-optical trap. With programed laser pulses of spectral $\ensuremath{\pi}$-phase step, of which location was varied within the laser spectrum, the sequential two-photon-ionization passage along the 5P3/25{P}_{3/2}5P3/2​-5D5D5D continuum was probed in terms of trap-loss spectroscopy. The coherent control two-photon-ionization method unveiled not only the resonantly enhanced two-photon ionization but also the asymmetric nature of the ionization profile structure given as a function of the spectral phase step location. Experimental results show good agreement with the second-order perturbation calculation of the constituent possible ionization passages.