Jens Herrmann

Attosecond dynamical Franz-Keldysh effect in polycrystalline diamond

Shining a fast light on diamonds Conceptually, the electronic structure of matter is a fixed scaffold of energy levels, which electrons climb with the help of light absorption. In reality, the lights electromagnetic field distorts the scaffold, a phenomenon that becomes increasingly evident with rising field intensity. Lucchini et al. studied a manifestation of this phenomenon, termed the dynamical Franz Keldysh effect, in diamond substrates exposed to sudden, moderately intense infrared fields. Using attosecond probe pulses and accompanying theoretical simulations, they resolved and accounted for the extremely rapid ensuing electron dynamics. Science, this issue p. 916 Attosecond spectroscopy probes shifts in the electronic state structure of diamond induced by an intense infrared field. Short, intense laser pulses can be used to access the transition regime between classical and quantum optical responses in dielectrics. In this regime, the relative roles of inter- and intraband light-driven electronic transitions remain uncertain. We applied attosecond transient absorption spectroscopy to investigate the interaction between polycrystalline diamond and a few-femtosecond infrared pulse with intensity below the critical intensity of optical breakdown. Ab initio time-dependent density functional theory calculations, in tandem with a two-band parabolic model, accounted for the experimental results in the framework of the dynamical Franz-Keldysh effect and identified infrared induction of intraband currents as the main physical mechanism responsible for the observations.

Resolving intra-atomic electron dynamics with attosecond transient absorption spectroscopy

Attosecond transient absorption spectroscopy is a recent addition to the experimental tool set of attosecond science. This all-optical method measures different observables than the previously existing techniques based on electron and ion detection and overcomes several of their limitations. We review the present state-of-the-art of attosecond transient absorption experiments and theory. Applications cover the exploration of ultrafast electron dynamics in atoms as well as in solid-state systems. As an example we discuss the observation of transiently bound electron wavepacket dynamics in helium in more detail. This example illustrates how transient absorption spectroscopy can provide information that is fundamentally inaccessible to the techniques based on ionisation, namely dynamics occurring well below the ionisation threshold. Furthermore, we show that a model based on wavepacket interference and originally developed to explain modulations in the ion yield is not sufficient to explain the observed optical response of the system. The optical response on extremely short timescales and in systems exposed to strong laser fields is still not fully understood. This makes the method also attractive for fundamental studies. Furthermore, it is expected that the technique will also find future applications for studying molecular systems in gas phase, in solution, or as solids and will greatly benefit from the advances of ultrafast lasers with multi-100-W average power improving signal-to-noise ratio by many orders of magnitude in the near future.

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

Attosecond control of the optical response of helium atoms to extreme ultraviolet radiation in the presence of moderately strong infrared laser light has been recently demonstrated both by employing attosecond pulse trains (APTs) and single attosecond pulses. In the case of APTs the interference between different transiently bound electron wavepackets excited by consecutive attosecond light bursts in the train was indicated as the predominant mechanism leading to the control. We studied the same physical system with transient absorption spectroscopy using elliptically polarized infrared pulses or APTs with a varying number of pulses down to a single pulse. Our new results are not consistent with this kind of wavepacket interference being the dominant mechanism and show that its role in the control over the photoabsorption probability has to be rediscussed.

Virtual single-photon transition interrupted : Time-gated optical gain and loss

This work was supported by the National Center of Competence in Research Molecular Ultrafast Science and Technology (NCCR MUST), research instrument of the Swiss National Science Foundation. P.R. acknowledges a Juan de la Cierva Contract Grant from MICINN, and the COST Action CM0702. We thank H. R. Reiss and M. Lucchini for fruitful discussions

Ultrafast Relaxation Dynamics of the Ethylene Cation C2H4

We present a combined experimental and computational study of the relaxation dynamics of the ethylene cation. In the experiment, we apply an extreme-ultraviolet-pump/infrared-probe scheme that permits us to resolve time scales on the order of 10 fs. The photoionization of ethylene followed by an infrared (IR) probe pulse leads to a rich structure in the fragment ion yields reflecting the fast response of the molecule and its nuclei. The temporal resolution of our setup enables us to pinpoint an upper bound of the previously defined ethylene-ethylidene isomerization time to 30 ± 3 fs. Time-dependent density functional based trajectory surface hopping simulations show that internal relaxation between the first excited states and the ground state occurs via three different conical intersections. This relaxation unfolds on femtosecond time scales and can be probed by ultrashort IR pulses. Through this probe mechanism, we demonstrate a route to optical control of the important dissociation pathways leading to separation of H or H2.

Optical response of electron wave-packet interference revisited

This work reports on the use of a pump-probe scheme, but change the time duration of the APT ranging from a pulse train envelope of - 25 fs down to a SAP of - 300 as.

Theoretical analysis of attosecond quantum beat spectroscopy of helium excited states

Photoelectron spectra measured in the attosecond quantum beat spectroscopy experiment on helium were computed solving the single-electron time-dependent Schrdinger equation. Final momenta of photoelectrons were obtained from the probability flux at the boundary of the simulation region using the time-dependent surface flux (t-SURFF) method [1]. Reaching a very good agreement with the experimental results, simulations allow for deeper understanding of processes underlying the quantum beating, which-way interference and polarization of the excited state.

Interrupted virtual single-photon transition

The temporal evolution of the dipole response of a system excited by an electromagnetic field usually is not accessible with traditional spectroscopy. Only the time-integrated dipole response (TIDR) is detected. Here, we investigate the case of the off-resonant excitation of a quantum-mechanical two-level system (TLS) where the TIDR is expected to be zero. Our time-frequency representation of the dipole response reveals that even in this case positive and negative contributions are present during its temporal evolution. The zero TIDR is a result of the exact balance of positive and negative contributions, which cancel out. We present a way to create and control optical gain and loss by interrupting the evolution of the dipole in time. In this way, we make these nonzero contributions accessible for spectroscopy.

Probing electron wave-packet interference

We use attosecond transient absorption spectroscopy to investigate electron dynamics around the first ionization threshold of helium. Our results expose that electron wave-packet interference effects are insufficient to explain all the observed phenomena.