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arXiv:2012.11040 · Physical Review Research

Coherence control in helium-ion ensembles

Saad Mehmood, Eva Lindroth, Luca Argenti

Published Jun 22, 2021• 3 citations
arXivHTMLPDF

Abstract

The authors show how an attosecond XUV-pump IR-probe sequence can be used to ionize the helium atom and create a polarized ion whose coherence is controllable through the pump-probe delay and can be reconstructed from the picosecond fluctuations of the ionic dipole caused by fine-structure interactions.
DOI
10.1103/physrevresearch.3.023233

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The ionization of atoms with sequences of attosecond pulses gives rise to excited ionic states that are entangled with the emitted photoelectron. Still, the ionic ensemble preserves some coherence that can be controlled through the laser parameters. In helium, control of the 2s/2pHe+2s/2p\phantom{\rule{4pt}{0ex}}{\mathrm{He}}^{+}2s/2pHe+ coherence is mediated by the autoionizing states below the N=2N=2N=2 threshold [Phys. Rev. Res. 3, 023233 (2021)]. In the present work we study the role of the resonances both below and above the N=3N=3N=3 threshold on the coherence of the N=3He+N=3\phantom{\rule{4pt}{0ex}}{\mathrm{He}}^{+}N=3He+ ion, in the attosecond pump-probe ionization of the helium atom, which we simulate using the newstock ab initio code. Due to the fine-structure splitting of the N=3He+N=3{\mathrm{He}}^{+}N=3He+ level, the ionic dipole beats on a picosecond timescale. We show how, from the dipole beating, it is possible to reconstruct the polarization of the ion at its inception.
If another linearly polarized attosecond pulse (the probe pulse) centered at 100 eV doubly ionizes the helium atom in the mixed state as illustrated in the right graph of Figure 9.1, the ionization probability depends on the time delay between the pump and the probe pulses. The probability is higher when the two electrons are located near each other, as depicted in Figure 9.2. Experimentally, one can measure the doubly ionized helium ion (He2þ) signal as function of time delay between the probe pulse and the pump pulse. The He2þ yield as a function of the time delay would inform us how the electron separation changes with time. Hu and Collins’s simulation shows that the separation changes periodically with a 2 fs cycle, which can be observed with the attosecond pulses. The cycle is much larger than the 200 as corresponding to the energy difference between the ground state and the first excited state (1s2p) because other excited states are involved.
Metastable states are important actors in the ionisation of atoms and molecules. Sub-femtosecond extreme ultraviolet pulses can coherently populate several transiently bound states at once, thus starting the attosecond clocks which are required to monitor and control ultrafast electronic evolution above the ionisation threshold. Here we illustrate, from a theoretical point of view, the effects coherent superpositions of 1 P o doubly excited states in the helium atom have on channel-resolved photoelectron spectra as well as on the transient absorption spectrum of the atom in the extreme ultraviolet region, when they are created by a single-attosecond pulse in the presence of a strong few-cycle near-infrared/visible pulse which acts as a probe. Interference fringes varying rapidly with the pump-probe time delay are visible in both photoelectron and transient absorption spectra. From such fringes, the wave packet itself can conceivably be reconstructed. Conversely, all observables are modulated by the characteristic beating periods of the wave packet, so that control of partial ionisation yields, branching ratios, and light absorption or amplification can be achieved.
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.
Entanglement is one of the most intriguing aspects of quantum mechanics and lies at the heart of the ongoing second quantum revolution, where it is a resource that is used in quantum key distribution, quantum computing, and quantum teleportation. We report experiments demonstrating the crucial role that entanglement plays in pump-probe experiments involving ionization, which are a hallmark of the novel research field of attosecond science. We demonstrate that the degree of entanglement in a bipartite ion + photoelectron system, and, as a consequence, the degree of vibrational coherence in the ion, can be controlled by tailoring the spectral properties of the attosecond extreme ultraviolet laser pulses that are used to create them.