Role of electron wavepacket interference in the optical response of helium atoms

Lucchini, Matteo; Herrmann, J.; Ludwig, André; Locher, Reto; Sabbar, Mazyar; Gallmann, L.; Keller, U.

doi:10.1088/1367-2630/15/10/103010

Cited by 24 publications

(26 citation statements)

References 27 publications

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“…More recently, processes involving four IR photons have been observed [13,19,22]. Earlier work [20,21] has addressed the polarisation dependence of the field dressing in the perturbative regime, but here we investigate the polarisation dependence of the dressing field on the atom in the strong field regime.…”

Section: W CMmentioning

confidence: 99%

Polarisation response of delay dependent absorption modulation in strong field dressed helium atoms probed near threshold

et al. 2016

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We present the first measurement of the vectorial response of strongly dressed helium atoms probed by an attosecond pulse train (APT) polarised either parallel or perpendicular to the dressing field polarisation. The transient absorption is probed as a function of delay between the APT and the linearly polarised 800 nm field of peak intensity´-1.3 10 W cm 14 2 . The APT spans the photon energy range 16-42 eV, covering the first ionisation energy of helium (24.59 eV). With parallel polarised dressing and probing fields, we observe modulations with periods of one half and one quarter of the dressing field period. When the polarisation of the dressing field is altered from parallel to perpendicular with respect to the APT polarisation we observe a large suppression in the modulation depth of the above ionisation threshold absorption. In addition to this we present the intensity dependence of the harmonic modulation depth as a function of delay between the dressing and probe fields, with dressing field peak intensities ranging from 2×10 12 to 2×10 14 -W cm 2 . We compare our experimental results with a full-dimensional solution of the single-atom time-dependent (TD) Schrödinger equation obtained using the recently developed abinitio TD B-spline ADC method and find good qualitative agreement for the above threshold harmonics.

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Section: W CMmentioning

confidence: 99%

Polarisation response of delay dependent absorption modulation in strong field dressed helium atoms probed near threshold

et al. 2016

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“…arrival time of the IR pulse. Depending on the position of the dark state, the transition can be the ladder type, where the coupled dark state is situated between the two bright states, corresponding to sub-cycle modulation features [5,[9][10][11][12][13][16][17][18], or a vee (lambda) type wherein the dark state is on the same side above (below) the two bright states in energy [19], corresponding to a slower modulation period in the absorption lines [6]. Therefore, the periodic modulation of the amplitude of a particular absorption line is generally recognized as the signature of population transfer.…”

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confidence: 99%

“…Feature (b) stems from the interplay between the hyperbolic lines (perturbed free induction decays) originating from two adjacent resonance states and can be used to quantify the ac Stark shift. Features (c) and (d) are due to population transfer, and they correspond to the ladder-type [5,[9][10][11][12][13][16][17][18] and vee-type transition schemes [6], respectively. The identification of all these features is based on the following model calculation.…”

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confidence: 99%

Attosecond transient absorption of argon atoms in the vacuum ultraviolet region: line energy shifts versus coherent population transfer

et al. 2016

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Using attosecond transient absorption, the dipole response of an argon atom in the vacuum ultraviolet (VUV) region is studied when an external electromagnetic field is present. An isolated attosecond VUV pulse populates Rydberg states lying 15 eV above the argon ground state. A synchronized fewcycle near infrared (NIR) pulse modifies the oscillating dipoles of argon impulsively, leading to alterations in the VUV absorption spectra. As the NIR pulse is delayed with respect to the VUV pulse, multiple features in the absorption profile emerge simultaneously including line broadening, sideband structure, sub-cycle fast modulations, and 5-10 fs slow modulations. These features indicate the coexistence of two general processes of the light-matter interaction: the energy shift of individual atomic levels and coherent population transfer between atomic eigenstates, revealing coherent superpositions. An intuitive formula is derived to treat both effects in a unifying framework, allowing one to identify and quantify the two processes in a single absorption spectrogram.Recently, attosecond transient absorption (ATA) spectroscopy has demonstrated great success in accessing ultrafast dynamics of bound and autoionizing systems in rare gas atoms [1-10] and diatomic molecules [11][12][13]. In this technique, dipole allowed states (bright states) are coherently populated by a broadband attosecond pulse, forming a wavepacket. The coherence between each individual excited state and the initial ground state forms a polarization dipole. Upon the arrival of a delayed short near infrared (NIR) pulse, these dipoles are subject to a complex amplitude change that occurs within a few femtoseconds, leading to novel absorption features in the transmitted spectrum of the attosecond pulse. Various dynamical aspects of the initially launched wavepacket are imprinted in the delay-dependent absorption spectra. Thus far, ATA studies have generally been carried out at photon energies ranging from 20 to 100 eV due to more efficient high harmonic generation above 20 eV. Here, we report ATA experiments on Ar atoms at photon energies around 15 eV, enabling a detailed study of electronic wavepacket dynamics for Rydberg states approaching the first ionization threshold of Ar.There are two major effects of the NIR pulse in ATA experiments. First, it shifts the energy of the individual states, generally known as the ac Stark shift. Secondly, it transfers population between different bright states by a two photon process. How these two effects manifest themselves in an ATA measurement is the essence of understanding the observables in such an experiment. The ac Stark shift corresponds to the energy difference between a dressed energy level and its field-free counterpart. In the time domain, it originates from the extra phase modulation, introduced by the NIR field, on the states of interest. The ac Stark shift manifests itself as reshaping of the absorption line profile in an ATA experiment [14,15]. On the other hand, population transfer between different b...

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“…For a further reduction of the magnetic flux a second shielding (closer to the actual interaction area) can be inserted. In the effort to make the transition between attosecond transient absorption measurements [34][35][36] and photoelectron/photoion spectroscopy as straightforward and convenient as possible, we designed a movable double target (Fig. 4).…”

Section: First Interaction Chambermentioning

confidence: 99%

“…We have used this APT control for transient absorption measurements to influence the optical response of helium atoms to XUV radiation using APTs with reducing numbers of attosecond pulses down to the SAP limit. 35 The attosecond streaking technique 14 was applied to determine the temporal characteristics of our SAPs. For this purpose argon gas was injected into the first interaction chamber using the needle target.…”

Section: A Attosecond Streaking Measurementmentioning

confidence: 99%

Versatile attosecond beamline in a two-foci configuration for simultaneous time-resolved measurements

Locher

Lucchini

Herrmann

et al. 2014

Review of Scientific Instruments

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We present our attoline which is a versatile attosecond beamline at the Ultrafast Laser Physics Group at ETH Zurich for attosecond spectroscopy in a variety of targets. High-harmonic generation (HHG) in noble gases with an infrared (IR) driving field is employed to generate pulses in the extreme ultraviolet (XUV) spectral regime for XUV-IR cross-correlation measurements. The IR pulse driving the HHG and the pulse involved in the measurements are used in a non-collinear set-up that gives independent access to the different beams. Single attosecond pulses are generated with the polarization gating technique and temporally characterized with attosecond streaking. This attoline contains two target chambers that can be operated simultaneously. A toroidal mirror relay-images the focus from the first chamber into the second one. In the first interaction region a dedicated double-target allows for a simple change between photoelectron/photoion measurements with a time-of-flight spectrometer and transient absorption experiments. Any end station can occupy the second interaction chamber. A surface analysis chamber containing a hemispherical electron analyzer was employed to demonstrate successful operation. Simultaneous RABBITT measurements in two argon jets were recorded for this purpose.

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Role of electron wavepacket interference in the optical response of helium atoms

Cited by 24 publications

References 27 publications

Polarisation response of delay dependent absorption modulation in strong field dressed helium atoms probed near threshold

Polarisation response of delay dependent absorption modulation in strong field dressed helium atoms probed near threshold

Attosecond transient absorption of argon atoms in the vacuum ultraviolet region: line energy shifts versus coherent population transfer

Versatile attosecond beamline in a two-foci configuration for simultaneous time-resolved measurements

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