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Chinese Physics Letters

Attosecond Time Delay in Bound–Bound Transitions Probed by Fano Resonances

Zhigang Zheng, Li Wang, Di Wang, Ming-Yue Wang, Fan Xiao, Jing Zhao, Xiaowei Wang, Zengxiu Zhao

Published Aug 1, 2025• 3 citations
Publisher

Abstract

Abstract This letter presents a method for probing the attosecond time delay between two radiatively resonant transitions from Fano structures, which arise from interference between the extreme ultraviolet free induction decay (XFID) emission and high-order harmonics. The ellipticity dependence of the Ne+ XFID yield confirms that the ionic excited-state populations originate from inelastic recollision between tunneling electrons and parent ions. Subsequent extraction of relative phases from Fano structures enables the determination of the time delay (∼ 22 as) between the two decay pathways. This work provides an experimental approach to probe the attosecond time delay between different XFID channels and contributes to a deeper understanding of the tunneling-plus-rescattering model in strong laser fields.

DOI
10.1088/0256-307x/42/9/090302

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Abstract This letter presents a method for probing the attosecond time delay between two radiatively resonant transitions from Fano structures, which arise from interference between the extreme ultraviolet free induction decay (XFID) emission and high-order harmonics. The ellipticity dependence of the Ne+ XFID yield confirms that the ionic excited-state populations originate from inelastic recollision between tunneling electrons and parent ions. Subsequent extraction of relative phases from Fano structures enables the determination of the time delay (∼ 22 as) between the two decay pathways. This work provides an experimental approach to probe the attosecond time delay between different XFID channels and contributes to a deeper understanding of the tunneling-plus-rescattering model in strong laser fields.
Attosecond chronoscopy enables the exploration of correlated electron dynamics in real time. One key observable of attosecond physics is the determination of "time zero" of photoionization, the time delay with which the wave packet of the ionized electron departs from the ionic core. This observable has become accessible by experimental advances in attosecond streaking and reconstruction of attosecond beating by interference of two-photon transitions (RABBIT) techniques. In this Letter, we explore photoionization time delays by strong extreme ultraviolet fields beyond the linear-response limit. We identify novel signatures in time delays signifying strong coupling between atoms and light fields and the light-field dressing of the ion. As a prototypical case, we study the interelectronic coherence and entanglement in helium driven by a strong extreme ultraviolet field. By the numerical solution of the time-dependent Schrödinger equation in its full dimensionality, we show that the time delay of the photoionized electron allows one to monitor the ultrafast variations of coherence dynamics and entanglement in real time.
In the present paper, we investigate the time-resolved transient absorption spectroscopy of doubly excited states of helium atoms by solving the time-dependent two-electron Schr\"odinger equation numerically based on a one-dimensional model. The helium atoms are subjected to an extreme ultraviolet (XUV) attosecond pulse and a time-delayed infrared (IR) few-cycle laser pulse. A superposition of doubly excited states populated by the XUV pulse is identified, which interferes with the direct ionization pathway leading to Fano resonance profiles in the photoabsorption spectrum. In the presence of an IR laser, however, the Fano line profiles are strongly modified: A shifting, splitting, and broadening of the original absorption lines is observed when the XUV attosecond pulse and infrared few-cycle laser pulse overlap in time, which is in good agreement with recent experimental results. At certain time delays, we observe symmetric Lorentz, inverted Fano profiles, and even negative absorption cross sections indicating that the XUV light can be amplified during the interaction with atoms. We further prove that the above pictures are general for different doubly excited states by suitably varying the frequency of the IR field to coherently couple the corresponding states.
The age of attosecond physics was ushered in by the invention of methods for probing phenomena on a time scale less than femtoseconds [F. Krausz, M. Ivanov, Rev. Mod. Phys. 81, 163 (2009)]. In photoemission, the Eisenbud-Wigner-Smith (EWS) time delay [R. Pazourek, S. Nagele, J. Burgdorfer, Rev. Mod. Phys. 87, 765 (2015)] is the interval between photon annihilation and emission of the electron, usually measured in attoseconds (10−1810^{-18}10−18 seconds), and varies with the kinetic energy and the angle of emission of the electron wave packet. We present a new interferometric method of measurement, using two phase-locked Extreme Ultraviolet pulses of frequency ω\omegaω and 2ω2\omega2ω, from a Free-Electron Laser. Phase differences between one- and two-photon ionization channels are extracted, and used to derive the relative EWS delay for one- and two-photon ionization of neon as a function of emission angle. Agreement with theory is excellent. Competing techniques require the use of an IR field, which necessitates corrections, but our method is correction-free. This new quantum clock is a powerful tool for investigating ultrafast processes in atoms and molecules.
Abstract We demonstrate that the extreme ultraviolet free induction decay emission can be significantly enhanced by employing isolated attosecond pulses. In this work, the near infrared pulses are applied to excite the neon atoms into Rydberg states coherently, and isolated attosecond pulses are used to manipulate the populations of the Rydberg states and the subsequent free induction decay process. The time resolved experimental measurement of the dependence of the resonance emission yield would help to understand the buildup dynamics of population of excited states. The enhancement assisted by attosecod pulses can serve as a mechanism to develop high-flux extreme ultraviolet light sources.
Strong field ionization injects a transient vacancy in the atom which is entangled to the outgoing photoelectron. When the electron is finally detached, the ion is populated at different excited states with part of coherence information lost. The preserved coherence of matter after interacting with intense short pulses has important consequences on the subsequent nonequilibrium evolution and energy relaxation. Here we employ attosecond transient absorption spectroscopy to measure the time-delay of resonant transitions of krypton vacancy during their creation. We have observed that the absorptions by the two spin-orbit split states are modulated at different paces when varying the time-delay between the near-infrared pumping pulse and the attosecond probing pulse. It is shown that the coupling of the ions with the remaining field leads to a suppression of ionic coherence. Comparison between theory and experiments uncovers that coherent Raman coupling induces time-delay between the resonant absorptions, which provides insight into laser-ion interactions enriching attosecond chronoscopy.
Laser-induced electron tunneling ionization from atoms and molecules plays as the trigger for a broad class of interesting strong-field phenomena in attosecond community. Understanding the time of electron tunneling ionization is vital to achieving the ultimate accuracy in attosecond metrology. We propose a novel attosecond photoelectron interferometer, which is based on the interference of the direct and near-forward rescattering electron wave packets, to determine the time information characterizing the tunneling process. Adding a weak perturbation in orthogonal to the strong fundamental field, the phases of the direct and the near-forward rescattering electron wave packets are modified, leading to the shift of the interferogram in the photoelectron momentum distributions. By analyzing the response of the interferogram to the perturbation, the real part of the ionization time, which denotes the instant when the electron exits the potential barrier, and the associated rescattering time are precisely retrieved. Moreover, the imaginary part of the ionization time, which has been interpreted as a quantity related to electron motion under the potential barrier, is also unambiguously determined.