Session G4: Focus Session: Progress in Attosecond Physics

Time-resolved photoemission by attosecond streaking

With the advent of sub-femtosecond ultrashort light pulses novel pathways have opened up for investigating time-resolved electronic processes on the attosecond scale. One of the most fundamental techniques is attosecond streaking which enables time domain studies of photoionization for atoms, molecules, and solids and provides unprecedented information on the release time of photoelectrons. The challenge in interpreting the obtained time delays lies in disentangling the intrinsic time shifts one is interested in and the additional measurement-induced apparent delays caused by the probing infrared (IR) field in the streaking setup. In this talk, these issues will be addressed with the help of a few examples. On the one-electron level we identify effects of the dressing IR field on the extracted streaking delays in the entrance (initial state) and in the exit (continuum) channel for atomic photoemission. On the multi-electron level we study the effect of electron correlations on the time delays and explore their influence in the presence of the probing streaking field. As a prototypical two-electron system we study helium which exhibits rich many-electron effects. We quantify all the contributions to the streaking time shifts with attosecond precision and provide benchmarks for future experiments.   

Time-dependent ac-Stark shift

We probed the AC Stark shift induced by a few-cycle near infrared laser field in helium bound states using isolated attosecond pulses in a transient absorption scheme. The broadband continuous XUV spectrum of the attosecond pulses covering all the excited states simultaneously were generated by the Double Optical Gating method. We uncovered a sub-cycle energy shift in the 1s3p state superimposed on a slower shift that followed the instantaneous laser intensity envelope.   

Generation and characterization of broadband isolated attosecond pulse

A 7.5 fs, 780 nm infrared laser was tightly focused on a Ne gas target to generate an XUV supercontinuum spectrum by applying the Double Optical Gating (DOG) method. This supercontinuum reaches a cutoff at 120 eV photon energy. The XUV pulse was filtered by a Zr filter to compensate the intrinsic chirp from the XUV generation. The spectrum after the filter supports an isolated pulse as short as 60 as centered at 80 eV photon energy. The spectral phase of the XUV pulse was extracted from the measured attosecond streaking trace by the Phase Retrieval by Omega Oscillation Filtering (PROOF).   

Absorption of Attosecond Pulses by Laser-dressed Atoms

We study the transient absorption of attosecond pulses by IR-laser-dressed He atoms using both single-atom and macroscopic methods [1]. In the case of an attosecond pulse train, we report for the first time a quarter-cycle modulation (mixed with the well-known half-cycle modulation [2-4]) in the absorption as a function of time delay, indicating that high-order couplings between the harmonics can be obtained by modifying parameters of laser and gas medium. We also find that the absorption probability is tied to resonant laser-dressed atomic states, and the timing of absorption is sensitive to laser parameters and reshaping of the attosecond pulses. In the case of a single attoseond pulse, we exhibit the attosecond time-scale evolution of the absorption probability as well as that of the AC Stark shift [5]. We find a light-induced state formed by a resonant two-photon absorption process. We also find electron wavepacket interference between two quantum path ways into the continuum (direct and via bound states) [6].\\[4pt] [1] M.B.Gaarde et al Phys.Rev.A 83 013419\\[0pt] [2] P.Johnsson et al Phys.Rev.Lett. 99 233001\\[0pt] [3] P.Ranitovic et al Phys.Rev.Lett. 106 193008\\[0pt] [4] M.Holler et al Phys.Rev.Lett. 106 123601\\[0pt] [5] A.Wirth et al Science 334 195\\[0pt] [6] J.Mauritsson et al Phys. Rev. Lett. 105 053001.   

Nanoplasmonic light field synthesis for isolated attosecond pulse generation

Nanometer-scale metallic structures can lead to a strong concentration of optical fields due to plasmon resonances. This can be used to generate very strong electric fields even with moderate driving laser intensities, enabling high-harmonic generation (HHG) at much lower intensities than usually required. The reduced need for amplifying stages in the driving laser then allows for high repetition rates in the MHz range. However, the sources demonstrated up to now do not produce ultrashort (attosecond) pulses, in part because the temporal response of a plasmon resonance stretches and distorts the incoming laser pulse. We here describe a general approach for generating isolated attosecond pulses using nanoplasmonically enhanced fields. We show that for practically useful structures, pulse shaping of the incoming pulse can compensate for the distortion and temporally confine the plasmon-enhanced field response. Coherent control techniques for pulse shaping, which are applicable at the low required input intensities, can then be used to generate isolated attosecond pulses even if the response of the plasmonic structure is not known a priori.   

Asymmetries in Production of He$^{+}$(n=2) with an Intense Few-Cycle Attosecond Pulse

By solving the two-active-electron time-dependent Schr\"{o}dinger equation (in its full dimensionality) in an intense few-cycle attosecond pulse, we investigate the carrier-envelope-phase (CEP) induced asymmetries in the differential probability for ionization plus excitation of He to the He$^{+}(n=2)$ states. Owing to the broad bandwidth of the intense pulse, substantial asymmetries in the differential probability for ionization of an electron along the positive and negative polarization direction of the pulse are found. Such asymmetry involves prominent interference between direct and indirect ionization pathways seen simultaneously in the partial photoelectron spectra. Electron correlations are probed by comparing projections of the wave packet onto the field-free highly correlated Jacobi matrix wave function [E. Foumouo et al., Phys. Rev. A \textbf{74}, 063409 (2006)] and uncorrelated Coulomb states. The CEP-effect found along the z-axis in the total asymmetry seems to be consistent with perturbation theory [E. A. Pronin et al., Phys. Rev. A \textbf{80}, 063403 (2009)].   

Monitoring Attosecond Electron Motion by High Order Harmonic Generation

Pump-probe schemes are proposed from numerical solutions of Time Dependent Schroedinger Equations' in nonBorn Oppenheimer (nonstatic nuclei) simulations of H2+ to measure and monitor electron motion in molecules.A weak few cycles XUV pump pulse is first used to create a coherent superposition of electron-nuclear wave packets in bound and dissociative electronic states followed by a short intense 800 nm probe pulse which generates harmonics via ionization and recollision of electrons with the initial coherent electron-nuclear wavepacket [1]. We show that by varying the time delay between pump and probe on attosecond time scale induces large suppression of the harmonic signal with an attosecond time periodicity corresponding to the electronic time periodicities in the coherent wavepacket. The three step model of harmonic generation concomitant with the SFA approximation [2] are used to explain the periodic change of harmonic signal and incipient decoherence due to vanishing of nuclear function overlap between different electronic potentials populated in the coherent wave packet. The mechanism is further confirmed by a time-series analysis.\\[4pt] [1] T Bredtmann, S Chelkowski, A D Bandrauk, Phys Rev A 84,021401(2011)\\[0pt] [2] A D Bandrauk, S Chelkowski, Intnl Rev Atom Molec Phys, 2,1-22(2011)