We investigate the absorption line shapes of laser-dressed atoms beyond the single-atom response, by using extreme ultraviolet (XUV) attosecond pulse trains to probe an optically thick helium target under the influence of a strong infrared (IR) field. We study the interplay between the IR-induced phase shift of the microscopic time-dependent dipole moment and the resonant-propagation-induced reshaping of the macroscopic XUV pulse. Our experimental and theoretical results show that as the optical depth increases, this interplay leads initially to a broadening of the IR-modified line shape, and subsequently to the appearance of new, narrow features in the absorption line.
We propose a theoretical approach for the interpretation of pump-probe measurements where an attosecond pulse is absorbed in the presence of an intense laser pulse. This approach is based on abstractly defined dressed bound states, which capture the essential aspects of the interaction with the laser pulse and facilitate a perturbative description of transitions induced by the attosecond pulse. Necessary properties of dressed bound states are defined and various choices are discussed and compared to accurate numerical solutions of the time-dependent Schroedinger equation.
Using extreme-ultraviolet attosecond-pulse-trains, we investigate the photoionization dynamics of a Helium atom in the presence of moderately-strong (~10^12 W/cm^2) femtosecond laser pulses. The electronic structure of a laser-dressed atom is traced in real-time through precision measurements of ion-yields and photo-electron angular distributions. Quantum interferences between photo-excitation paths are interpreted using the Floquet formalism. As the laser pulse intensity ramps on femtosecond timescales, we observe transitions between ionization channels mediated by different atomic resonances…
We calculate the transient absorption of an isolated attosecond pulse by helium atoms subject to a delayed infrared (\ir) laser pulse. With the central frequency of the broad attosecond spectrum near the ionization threshold, the absorption spectrum is strongly modulated at the sub-\ir-cycle level. Given that the absorption spectrum results from a time-integrated measurement, we investigate the extent to which the delay-dependence of the absorption yields information about the attosecond dynamics of the atom-field energy exchange. We find two configurations in which this is possible. The first…
We investigate the evolution of extreme ultraviolet (XUV) spectral lineshapes in an optically-thick helium gas under near-infrared (IR) perturbation. In our experimental and theoretical work, we systematically vary the IR intensity, time-delay, gas density and IR polarization parameters to study lineshape modifications induced by collective interactions, in a regime beyond the single atom response of a thin, dilute gas. In both experiment and theory, we find that specific features in the frequency-domain absorption profile, and their evolution with propagation distance, can be attributed to th…
An extreme ultraviolet (EUV) single attosecond pulse passing through a laser-dressed dense gas is studied theoretically. The weak EUV pulse pumps the helium gas from the ground state to the 2s2p(1P) autoionizing state, which is coupled to the 2s2(1S) autoionizing state by a femtosecond infrared laser with the intensity in the order of 10^{12} W/cm2. The simulation shows how the transient absorption and emission of the EUV are modified by the coupling laser. A simple analytical expression for the atomic response derived for delta-function pulses reveals the strong modification of the Fano lines…
Noncollinear wave-mixing spectroscopies with attosecond extreme ultraviolet (XUV) pulses provide unprecedented insight into electronic dynamics. In infrared and visible regimes, heterodyne detection techniques utilize a reference field to amplify wave-mixing signals while simultaneously allowing for phase-sensitive measurements. Here, we implement a self-heterodyned detection scheme in noncollinear wave-mixing measurements with a short attosecond XUV pulse train and two few-cycle near infrared (NIR) pulses. The initial spatiotemporally overlapped XUV and NIR pulses generate a coherence of both…
Light-induced states are commonly observed in the photoionization spectra of laser-dressed atoms. The properties of autoionizing polaritons, entangled states of light and Auger resonances, however, are largely unexplored. We employ attosecond transient-absorption spectroscopy to study the evolution of autoionizing states in argon, dressed by a tunable femtosecond laser pulse. The avoided crossings between the $3s^{-1}4p$ and several light-induced states indicates the formation of polariton multiplets. We measure a controllable stabilization of the polaritons against ionization, in excellent ag…
We simulate the transient absorption of attosecond pulses of infrared-laser-dressed atoms by considering a three-level system with the adiabatic approximation. The delay-dependent interference features are investigated from the perspective of the coherent interaction processes between the attosecond pulse and the quasi-harmonics. We find that many features of the interference fringes in the absorption spectra of the attosecond pulse can be attributed to the coherence phase difference. However, the modulation signals of laser-induced sidebands of the dark state is found related to the populatio…
An optical coherent control scheme has been proposed and theoretically investigated where an extreme ultraviolet single attosecond pulse (SAP) propagates through a dense helium gas dressed by a time-delayed femtosecond laser pulse. The laser pulse couples the 2s2p(^1P) and 2s^2(^1S) autoionizing states when the SAP excites the 2s2p state. After going through the gas, the spectral and temporal profiles of the SAP are strongly distorted. A narrowed but enhanced spike in the spectrum shows up for specific intensities and time delays of the laser, which exemplifies the control of a broadband photo…
The dynamics of autoionizing Rydberg states of oxygen are studied using attosecond transient absorption technique, where extreme ultraviolet (XUV) initiates molecular polarization and near infrared (NIR) pulse perturbs its evolution. Transient absorption spectra show positive optical density (OD) change in the case of $nsσ_g$ and $ndπ_g$ autoionizing states of oxygen and negative OD change for $ndσ_g$ states. Multiconfiguration time-dependent Hartree-Fock (MCTDHF) calculation are used to simulate the transient absorption spectra and their results agree with experimental observations. The time …
We present a general description of the interaction between multi-color laser pulses and atoms and molecules, focusing on the experimentally relevant example of infrared (IR) pulses overlapped with attosecond pulse trains (APTs). This formulation reveals explicitly and analytically the role of the delay between the IR pulse and APT as a coherent control parameter. Our formulation also shows the nearly equivalent roles of the delay and the carrier-envelope phase in controlling the interference between different multiphoton pathways. We illustrate these points by investigating the single ionizat…
Strong-field manipulation of autoionizing states is a crucial aspect of electronic quantum control. Recent measurements of the attosecond transient absorption spectrum of helium dressed by a few-cycle visible pulse [Ott et al., arXiv:1205.0519[physics.atom-ph] ] provide evidence of novel ultrafast resonant phenomena, namely, two-photon Rabi oscillations between doubly-excited states and the inversion of Fano profiles. Here we present the results of accurate \emph{ab-initio} calculations that agree with these observations and in addition predict that (i) inversion of Fano profiles is actually p…
The blooming of attosecond science (1 as = $10^{-18}$ s) has raised the need to exquisitely control the delay between two ultrashort light pulses, one of them being intense and in the visible spectral range, while the second is weak and in the Extreme Ultra-Violet spectral range. Here we introduce a robust technique, named LIZARD (Laser-dressed IoniZation for the Adjustment of the pump-pRobe Delay), allowing an active stabilization of this pump-probe delay. The originality of the method lies in an error signal calculated from a two-photon photoelectron signal obtained by photoionizing a gas ta…
The derivation of approximate wave functions for an electron submitted to both a coulomb and a time-dependent laser electric fields, the so-called Coulomb-Volkov (CV) state, is addressed. Despite its derivation for continuum states does not exhibit any particular problem within the framework of the standard theory of quantum mechanics (QM), difficulties arise when considering an initially bound atomic state. Indeed the natural way of translating the unperturbed momentum by the laser vector potential is no longer possible since a bound state does not exhibit a plane wave form including explicit…
We devise an ab initio formalism for the quantum dynamics of Auger decay by laser-dressed atoms which are inner-shell ionized by extreme ultraviolet (XUV) light. The optical dressing laser is assumed to be sufficiently weak such that ground-state electrons are neither excited nor ionized by it. However, the laser has a strong effect on continuum electrons which we describe in strong-field approximation with Volkov waves. The XUV light pulse has a low peak intensity and its interaction is treated as a one-photon process. The quantum dynamics of the inner-shell hole creation with subsequent Auge…
Ubiquitous to most molecular scattering methods is the challenge to retrieve bond distance and angle from the scattering signals since this requires convergence of pattern matching algorithms or fitting methods. This problem is typically exacerbated when imaging larger molecules or for dynamic systems with little a priori knowledge. Here, we employ laser-induced electron diffraction (LIED) which is a powerful means to determine the precise atomic configuration of an isolated gas-phase molecule with picometre spatial and attosecond temporal precision. We introduce a simple molecular retrieval m…
We present transient absorption spectra of an extreme ultraviolet attosecond pulse train in helium dressed by an 800 nm laser field with intensity ranging from $2\times10^{12}$ W/cm$^2$ to $2\times10^{14}$ W/cm$^2$. The energy range probed spans 16-42 eV, straddling the first ionisation energy of helium (24.59 eV). By changing the relative polarisation of the dressing field with respect to the attosecond pulse train polarisation we observe a large change in the modulation of the absorption reflecting the vectorial response to the dressing field. With parallel polarized dressing and probing fie…
Ott et al. (Science (340, 716 (2013)) successfully transferred Fano profile into Lorentzian lineshape using an intense infrared laser, after excitation of autoionizing states in helium by attosecond XUV pulse. This is a very important step forward of quantum phase control. However, here we show experimentally that an autoionizing state can have both Fano and Lorentzian behavior naturally, depending on the process involved. This study utilized the inverse process of photon absorption ionization, i.e. electron ion recombination with photon emission, making sure the resonant autoionizing state is…
The possibility of accelerating molecular reactions by lasers has attracted considerable theoretical and experimental interest. A particular example of laser-modified reaction dynamics is laser catalysis, a process in which the tunneling through a potential barrier is enhanced by transient excitation to a bound electronic state. We have performed detailed calculations of pulsed laser catalysis on one- and two-dimensional potentials, as a function of the reactants' collision energy and the laser's central frequency. In agreement with previous CW results, the reactive lineshapes are Fano-type cu…
