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arXiv:1506.01182

Sub-cycle symmetry breaking of atomic bound states interacting with a short and strong laser pulse in a time-domain picture

Veit Stooß, Andreas Kaldun, Christian Ott, Alexander Blättermann, Thomas Ding, Thomas Pfeifer

Published Jun 3, 2015• 0 citations

Abstract

In any atomic species, the spherically symmetric potential originating from the charged nucleus results in fundamental symmetry properties governing the structure of atomic states and transition rules between them. If atoms are exposed to external electric fields, these properties are modified giving rise to energy shifts such as the AC Stark-effect in varying fields and, contrary to this in a constant (DC) electric field for high enough field strengths, the breaking of the atomic symmetry which causes fundamental changes in the atom's properties. This has already been observed for atomic Rydberg states with high principal quantum numbers. Here, we report on the observation of effects linked to symmetry breaking in helium within the optical cycle of the utilized strong visible laser fields for states with principal quantum number n=2. These findings were enabled by temporally resolving the dynamics better than the sub-optical cycle of the applied laser field, utilizing the method of attosecond transient absorption spectroscopy (ATAS). We identify the spectroscopic fingerprint of instantaneous polarization and breaking of the symmetry of the atom in the intense visible femtosecond pulse used in an ATAS experiment and develop a time domain picture describing the dipole response that leads to these signatures. In the future, this general experimental approach can be used to measure strong-field induced symmetry-breaking effects in other atomic or molecular systems also consisting of two or more active electrons and thus to examine new routes to their bound-state and transition-state control by laser fields.

DOI: 10.48550/arxiv.1506.01182

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In any atomic species, the spherically symmetric potential originating from the charged nucleus results in fundamental symmetry properties governing the structure of atomic states and transition rules between them. If atoms are exposed to external electric fields, these properties are modified giving rise to energy shifts such as the AC Stark-effect in varying fields and, contrary to this in a constant (DC) electric field for high enough field strengths, the breaking of the atomic symmetry which causes fundamental changes in the atom's properties. This has already been observed for atomic Rydberg states with high principal quantum numbers. Here, we report on the observation of effects linked to symmetry breaking in helium within the optical cycle of the utilized strong visible laser fields for states with principal quantum number n=2. These findings were enabled by temporally resolving the dynamics better than the sub-optical cycle of the applied laser field, utilizing the method of attosecond transient absorption spectroscopy (ATAS). We identify the spectroscopic fingerprint of instantaneous polarization and breaking of the symmetry of the atom in the intense visible femtosecond pulse used in an ATAS experiment and develop a time domain picture describing the dipole response that leads to these signatures. In the future, this general experimental approach can be used to measure strong-field induced symmetry-breaking effects in other atomic or molecular systems also consisting of two or more active electrons and thus to examine new routes to their bound-state and transition-state control by laser fields.

In this work, the strong-field-modified dipole response at the ionization threshold of helium is studied. The dipole response is induced by an attosecond pulse in the extreme ultraviolet spectral range and is manipulated by an ultrashort and strong femtosecond pulse in the near-infrared. To probe the response, the transient absorption spectrum of helium is recorded for different time delays between both pulses and different intensities of the femtosecond pulse. From the spectra, the dipole response of the ionization threshold is reconstructed, which is linked to the dynamics of excited electrons with energies in the transition region from bound to free. To identify the underlying processes of light-matter interaction leading to the observed structures in the time and spectral domain, different quantum-mechanical model simulations are conducted. As a result, the measured dipole response reveals light-induced energy shifts of the photoelectron’s kinetic energy close to the parent ion, signatures for field-driven recollisions of a photoelectron into the parent ion, and a temporal amplitude and phase gating mechanism. With the latter, the build-up dynamics of complex spectral structures are temporally resolved, which are the time-dependent separation and line-shape modification of the doubly excited Rydberg series as well as the temporal build-up of the ionization threshold.

Strong-field manipulation of autoionizing states is a crucial aspect of electronic quantum control. Recent measurements of the attosecond transient absorption spectrum (ATAS) of helium dressed by a few-cycle visible pulse [C. Ott et al , Nature 516, 374 (2014)] provide evidence of the inversion of Fano profiles. With the support of accurate ab-initio calculations that reproduce the results of the latter experiment, here we investigate the new physics that arise from ATAS when the laser intensity is increased. In particular, we show that (i) previously unnoticed signatures of the dark 2 p 2 1 S doubly excited state are observed in the experimental spectrum, (ii) inversion of Fano profiles is predicted to be periodic in the laser intensity, and (iii) the ac-Stark shift of the higher terms in the sp + 2, n autoionizing series exceeds the ponderomotive energy, which is the result of a genuine two-electron contribution to the polarization of the excited atom.

Strong-field manipulation of autoionizing states is a crucial aspect of electronic quantum control. Recent measurements of the attosecond transient absorption spectrum (ATAS) of helium dressed by a few-cycle visible pulse [C. Ott et al., Nature (London) 516, 374 (2014)] provide evidence of the inversion of Fano profiles. With the support of accurate ab initio calculations that reproduce the results of the latter experiment, here we investigate the new physics that arise from ATAS when the laser intensity is increased. In particular, we show that (i) previously unnoticed signatures of the dark

2{p}^{2}{\phantom{\rule{0.16em}{0ex}}}^{1}S

doubly excited state are observed in the experimental spectrum, (ii) inversion of Fano profiles is predicted to be periodic in the laser intensity, and (iii) the ac Stark shift of the higher terms in the

s{p}_{2,n}^{+}

autoionizing series exceeds the ponderomotive energy, which is the result of a genuine two-electron contribution to the polarization of the excited atom.

We employ an extreme ultraviolet (XUV) pulse to impulsively excite dipole polarization in atoms or molecules, which corresponds to coherently prepared superposition of excited states. A delayed near infrared (NIR) pulse then perturbs the fast evolving polarization, and the resultant absorbance change is monitored in dilute helium, dense helium, and sulfur hexaﬂuoride (SF6) molecules. We observe and quantify the time-dependence of various transient phenomena in helium atoms,includinglaser-inducedphase(LIP),time-varying(AC)Starkshift,quantumpathinterference, and laser-induced continuum structure. In the case of dense helium targets, we discuss nonlinear macroscopic propagation effects pertaining to LIP and resonant pulse propagation, which accoun tfor the appearance of new spectral features in transient lineshapes. We then use tunable NIR photons to demonstrate the wavelength dependence of the transient laser induced effects. In the case of molecular polarization experiment in SF6, we show suppression of XUV photoabsorption corresponding to inter-valence transitions in the presence of a strong NIR ﬁeld. In each case, the temporal evolution of transient absorption spectra allows us to observe and understand the transient laser induced modiﬁcations of the electronic structure of atoms and molecules.

The reconstruction of the full temporal dipole response of a strongly driven time-dependent system from a single absorption spectrum is demonstrated, only requiring that a sufficiently short pulse is employed to initialize the coherent excitation of the system. We apply this finding to the time-domain observation of Rabi cycling between doubly excited atomic states in the few-femtosecond regime. This allows us to pinpoint the breakdown of few-level quantum dynamics at the critical laser intensity near 2 TW/cm^{2} in doubly excited helium. The present approach unlocks single-shot real-time-resolved signal reconstruction across timescales down to attoseconds for nonequilibrium states of matter. In contrast to conventional pump-probe schemes, there is no need for scanning time delays in order to access real-time information. The potential future applications of this technique range from testing fundamental quantum dynamics in strong fields to measuring and controlling ultrafast chemical and biological reaction processes when applied to traditional transient-absorption spectroscopy.