Interaction of Ultrashort Pulses with Matter
Ultrashort pulses as first characterized at TU Wien  are the ultimate tool to investigate the fundamentals of light-matter interactions and the meaning of time and time differences in quantum mechanics .
Snapshots of the formation of wavepackets
We have studied, e.g., the full two-electron dynamics of helium atoms irradiated by strong and short laser pulses to calculate the apparent time delay of photoemission from different initial states of the parent atom (together with MPQ Garching, TU Munich). To this end, we have solved the two-electron Schrödinger equation for helium and have successfully simulated so-called attosecond streaking and RABBITT experiments both of which measure the relative time delay of different electron groups reaching the detector. Our simulation results are used to determine time delays on an absolute scale and have allowed for a detailed look into photon-induced excitation processes in atoms .
Our simulation results were also used to work out the time evolution of the temporal build-up of interference structures in photoemission (Fano resonances, MPI Heidelberg). These interferences are caused by two alternative pathways to the same final state, e.g., direct emission vs. delayed emission via a metastable excited state. Pump-probe experiments with variable delay between the two pulses interrupt the build-up of the resonance and allow for taking snapshots of the process thereby showing the temporal evolution of the multipath interference (see figure).
Temporal build-up of a two-path interference in the photoionization of helium. Results of our simulations are compared to experimental data. Asymptotically, the energy spectrum converges to the well-known Fano shape .
Another interesting application of attosecond science is the non-linear upconversion of photons of a strong incident laser pulse to high-order multiples of the original photon energy in a process called high-harmonics generation. Recently, upconversion factors of more than 1000 could be reached at TU Wien (Institute for Photonics) in this process. Our work helps to interpret the experimental results and to optimize this process aimed at generating XUV pulses of sub-as duration. Lately, attosecond experiments have also been performed on extended systems such as dielectrics, where the high target-atom density raises hopes to generate harmonics with larger intensity. Due to the inherent multi-particle nature of such systems, simplified models have been invoked that have given first qualitative insight in the light-driven electronic processes in dielectrics. However, we could show that such simplifications often fail to reproduce even the qualitative behavior of realistic systems let alone provide a quantitative prediction for any observable in experiment. We have taken the first steps to a multi-scale description of laser-solid interactions combining the microscopic electronic motion described by the Schrödinger equation with the mesoscopic world of light propagation (Maxwell's equations) .
On the path to opto-electronics
Working together with experimental groups in Munich, Zurich, and Graz we have elucidated the interplay of different processes involved in light-solid and light-liquid interactions and have helped interpret fundamental experiments in the field possibly paving the way to light-driven electronics on the PHz scale .
 M. Hentschel, R. Kienberger, Ch. Spielmann, G. A. Reider, N. Milosevic, T. Brabec, P. Corkum, U. Heinzmann, M. Drescher, and F. Krausz, Attosecond metrology, Nature 414, 509 (2001), opens an external URL in a new window.
 R. Pazourek, S. Nagele, and J. Burgdörfer, Attosecond chronoscopy of photoemission, Rev. Mod. Phys. 87, 765 (2015), opens an external URL in a new window.
 A. Kaldun, A. Blättermann, V. Stooß, S. Donsa, H. Wei, R. Pazourek, S. Nagele, C. Ott, C. D. Lin, J. Burgdörfer, T. Pfeifer, Observing the ultrafast buildup of a Fano resonance in the time domain, Science 354, 738 (2016), opens an external URL in a new window.
 I. Floss, C. Lemell, G. Wachter, V. Smejkal, S. Sato, X.-M. Tong, K. Yabana, and J. Burgdörfer, Ab initio multiscale simulation of high-order harmonic generation in solids, Phys. Rev. A 97, 011401 (2018), opens an external URL in a new window.
 M. Ossiander, K. Golyari, K. Scharl, L. Lehnert, F. Siegrist, J. P. Bürger, D. Zimin, J. A. Gessner, M. Weidman, I. Floss, V. Smejkal, S. Donsa, C. Lemell, F. Libisch, N. Karpowicz, J. Burgdörfer, F. Krausz, and M. Schultze, The speed limit of optoelectronics, Nature Communications 13, 1620 (2022), opens an external URL in a new window.