Reto Locher

Energy-dependent photoemission delays from noble metal surfaces by attosecond interferometry

How quanta of energy and charge are transported on both atomic spatial and ultrafast timescales is at the heart of modern technology. Recent progress in ultrafast spectroscopy has allowed us to directly study the dynamical response of an electronic system to interaction with an electromagnetic field. Here, we present energy-dependent photoemission delays from the noble metal surfaces Ag(111) and Au(111). An interferometric technique based on attosecond pulse trains is applied simultaneously in a gas phase and a solid-state target to derive surface-specific photoemission delays. Experimental delays on the order of 100 as are in the same time range as those obtained from simulations. The strong variation of measured delays with excitation energy in Ag(111), which cannot be consistently explained invoking solely electron transport or initial state localization as supposed in previous work, indicates that final state effects play a key role in photoemission from solids.

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

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.

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

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.

Following attosecond photoemission from solids using interferometry

Implementing RABBITT on solids for the first time, we record energy-resolved absolute photoemission delays from noble metal surfaces. The structure of the observed delays in Ag(111) and Au(111) is inconsistent with previously promoted models.

Accessing Energy-Dependent Photoemission Delays in Solids

Our new detection scheme combines the RABBITT technique in solids with simultaneous measurements in a reference argon target. The experiment resolved attosecond delays in the photoemission from noble metal surfaces beyond simple ballistic transport.

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.

Surface RABBITT for determination of absolute ionization phase: A novel route towards absolute photoemission delays

By extending the RABBITT technique to noble metal surfaces the authors observed, for the first time, sub-cycle resolved dynamics with attosecond pulse trains in condensed matter systems. In contrast to streaking, this method is applicable to low-energetic harmonics and the required IR field strength is significantly lower. A simultaneously recorded RABBITT trace in argon allowed the proponents to evaluate the absolute, surface specific contribution to the transition phase.