Matthias Weger

Ultrafast resolution of tunneling delay time

The question of how long a tunneling particle spends inside the barrier region has remained unresolved since the early days of quantum mechanics. The main theoretical contenders, such as the Buttiker–Landauer, Eisenbud–Wigner, and Larmor time, give contradictory answers. On the other hand, recent attempts at reconstructing valence electron dynamics in atoms and molecules have entered a regime where the tunneling time genuinely matters. Here, we compare the main competing theories of tunneling time against experimental measurements using the attoclock in strong laser field ionization of helium atoms. The attoclock uses a close to circularly polarized femtosecond laser pulse, mapping the angle of rotation of the laser field vector to time similar to the hand of a watch. Refined attoclock measurements reveal a real (not instantaneous) tunneling delay time over a large intensity regime, using two independent experimental apparatus. Only two theoretical predictions are compatible within our experimental error: the Larmor time and the probability distribution of tunneling times constructed using a Feynman Path Integral formulation. The latter better matches the observed qualitative change in tunneling time over a wide intensity range, and predicts a broad tunneling time distribution with a long tail. The implication of such a probability distribution of tunneling times, as opposed to a distinct tunneling time, would imply that one must account for a significant, though bounded and measurable, uncertainty as to when the hole dynamics begin to evolve. We therefore expect our results to impact the reconstruction of attosecond electron dynamics following tunnel ionization.Summary form only given. We present approach and results of an angular streaking experiment with the attoclock method [1] that suggest the existence of a real tunneling time in strong field ionization of Helium. The results are compared with competing theories of tunneling time and show that the only theories that are compatible with the experimental results are the L armor time and a distribution of tunneling times with a long tail constructed using a Feynman Path Integral formulation. We find that the latter matches the experimental data the best. Our results have strong implications on investigations of the electron dynamics in attosecond science since a significant uncertainty must be taken into account about when the electron hole dynamics begins to evolve.The attoclock method is based on the angular streaking of the photoelectron that was released from the atom by tunnel ionization. The angular distribution of the photoelectron momentum distribution contains the timing of the ionization process via an offset of the maximum of the angular distribution from the theoretically predicted value assuming instantaneous tunneling. Our results indicate the existence of a real tunneling time through this angular offset. The attoclock technique was transferred to a velocity map imaging setup (VMIS) in combination with tomographic reconstruction. The gas nozzle was integrated in the repeller plate, a configuration that allows one to achieve target gas densities that are significantly higher compared to setups employing cold atomic beams [2], leading to higher statistics and smaller error bars compared to previous measurements [1, 3]. Helium was leaked into the ultra high vacuum chamber and tunnel ionized by an elliptically polarized sub-10fs few-cycle pulse with a central wavelength of 735 nm and an ellipticity of 0.87. For the tomographic reconstruction, two-dimensional momentum space electron images are recorded in steps of two degrees covering a range of 180 degrees. The three-dimensional momentum distribution and thus the electron momentum distribution in the polarization plane is retrieved by tomographic reconstruction with a filtered backprojection algorithm [4, 5]. The results from the VMIS are confirmed with accurate measurements using a cold target recoil ion momentum spectrometer (COLTRIMS).

Spatial fingerprint of quantum path interferences in high order harmonic generation

We have spatially and spectrally resolved the high order harmonic emission from an argon gas target. Under proper phase matching conditions we were able to observe for the first time the spatial fine structure originating from the interference of the two shortest quantum paths in the harmonic beam. The structure can be explained by the intensity-dependent harmonic phase of the contributions from the two paths. The spatially and spectrally resolved measurements are consistent with previous spatially integrated results. Our measurement method represents a new tool to clearly distinguish between different interference effects and to potentially observe higher order trajectories in the future with improved detection sensitivity. Here, we demonstrate additional experimental evidence that the observed interference pattern is only due to quantum-path interferences and cannot be explained by a phase modulation effect. Our experimental results are fully supported by simulations using the strong field approximation and including propagation.

Transferring the attoclock technique to velocity map imaging.

Attosecond angular streaking measurements have revealed deep insights into the timing of tunnel ionization processes of atoms in intense laser fields. So far experiments of this type have been performed only with a cold-target recoil-ion momentum spectrometer (COLTRIMS). Here, we present a way to apply attosecond angular streaking experiments to a velocity map imaging spectrometer (VMIS) with few-cycle pulses at a repetition rate of 10 kHz and a high ionization yield per pulse. Three-dimensional photoelectron momentum distributions from strong-field ionization of helium with an elliptically polarized, sub-10-fs pulse were retrieved by tomographic reconstruction from the momentum space electron images and used for the analysis in the polarization plane.

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

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.