Robert Boge

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).

Resonance Effects in Photoemission Time Delays

We present measurements of single-photon ionization time delays between the outermost valence electrons of argon and neon using a coincidence detection technique that allows for the simultaneous measurement of both species under identical conditions. The analysis of the measured traces reveals energy-dependent time delays of a few tens of attoseconds with high energy resolution. In contrast to photoelectrons ejected through tunneling, single-photon ionization can be well described in the framework of Wigner time delays. Accordingly, the overall trend of our data is reproduced by recent Wigner time delay calculations. However, besides the general trend we observe resonance features occurring at specific photon energies. These features have been qualitatively reproduced and identified by a calculation using the multiconfigurational Hartree-Fock method, including the influence of doubly excited states and ionization thresholds.

Combining attosecond XUV pulses with coincidence spectroscopy

Here we present a successful combination of an attosecond beamline with a COLTRIMS apparatus, which we refer to as AttoCOLTRIMS. The setup provides either single attosecond pulses or attosecond pulse trains for extreme ultraviolet-infrared pump-probe experiments. We achieve full attosecond stability by using an active interferometer stabilization. The capability of the setup is demonstrated by means of two measurements, which lie at the heart of the COLTRIMS detector: firstly, we resolve the rotating electric field vector of an elliptically polarized few-cycle infrared laser field by attosecond streaking exploiting the access to the 3D momentum space of the charged particles. Secondly, we show streaking measurements on different atomic species obtained simultaneously in a single measurement making use of the advantage of measuring ions and electrons in coincidence. Both of these studies demonstrate the potential of the AttoCOLTRIMS for attosecond science.

Thin disk amplifier-based 40 mJ, 1 kHz, picosecond laser at 515 nm.

We report on a frequency-doubled picosecond Yb:YAG thin disk regenerative amplifier, developed as a pump laser for a kilohertz repetition rate OPCPA. At a repetition rate of 1 kHz, the compressed output of the regenerative amplifier has a pulse duration of 1.2 ps and pulse energy of 90 mJ with energy stability of σ < 0.8% and M2 < 1.2. The pulses are frequency doubled in an LBO crystal yielding 42 mJ at 515 nm.

Broadband OPCPA system with 11 mJ output at 1 kHz, compressible to 12 fs

We report on a broadband OPCPA system, pumped at 515 nm by frequency doubled Yb:YAG thin disk lasers. The system delivers 11.3 mJ pulses at a central wavelength of 800 nm with a spatial beam quality of M2 = 1.25 and > 25% pump-to-signal conversion efficiency. The broadband pulses were demonstrated to be compressible to 12 fs using a chirped mirror compressor.

Energy-Dependent Photoemission Time Delays of Noble Gas Atoms Using Coincidence Attosecond Streaking

We present photoemission time-delay measurements between electrons originating from the valence shells of neon and argon obtained by attosecond streaking. After giving a brief review of the different techniques, we focus on more detailed analysis using the attosecond streaking technique. We show that the temporal structure of the ionizing single attosecond pulse may significantly affect the obtained time delays, and we propose a procedure how to take this contribution properly into account. Our analysis reveals a delay of a few tens of attoseconds in a photon energy range between 28 and 40 eV in the emission of electrons ionized from argon with respect to those liberated from neon.

Revealing the time-dependent polarization of ultrashort pulses with sub-cycle resolution.

We report on the first experiments characterizing the complete time-dependent 2D vector potential of a few-cycle laser pulse. The instantaneous amplitude and orientation of the electric field is determined with sub-cycle resolution, directly giving access to the polarization state of the pulse at any instant in time. This is achieved by performing an attosecond streaking experiment using a reaction microscope, where the full pulse characterization is performed directly in the target region.

Multi-channel, fiber-based seed pulse distribution system for femtosecond-level synchronized chirped pulse amplifiers

We report on the design and performance of a fiber-based, multi-channel laser amplifier seed pulse distribution system. The device is designed to condition and distribute low energy laser pulses from a mode-locked oscillator to multiple, highly synchronized, high energy amplifiers integrated into a laser beamline. Critical functions such as temporal pulse stretching well beyond 100 ps/nm, pulse picking, and fine control over the pulse delay up to 300 ps are all performed in fiber eliminating the need for bulky and expensive grating stretchers, Pockels cells, and delay lines. These functions are characterized and the system as a whole is demonstrated by seeding two high energy amplifiers in the laser beamline. The design of this system allows for complete computer control of all functions, including tuning of dispersion, and is entirely hands-free. The performance of this device and its subsystems will be relevant to those developing lasers where reliability, size, and cost are key concerns in addition to performance; this includes those developing large-scale laser systems similar to ours and also those developing table-top experiments and commercial systems.

Time delay anisotropy in photoelectron emission from isotropic helium

In contrast to expectations, we observe that the photoionization time delay from the 1s2 spherically symmetric ground state of He depend on the electron emission direction with respect to an external reference. We attribute the observed anisotropy to the interplay between different final quantum states, which become accessible once two photons are involved in the photoionization process. This is a universal effect, which needs to be taken into account for any study dealing with photoionization dynamics.

Development of high energy, sub-15 fs OPCPA system operating at 1 kHz repetition rate for ELI-Beamlines facility

We report on the status of the high repetition rate, high energy, L1 laser beamline at the ELI-Beamlines facility. The beamline is based on picosecond optical parametric chirped pulse amplification (OPCPA) of pulses from a mode-locked Ti:Sapphire oscillator and has a target energy/repetition rate of 100 mJ/1 kHz with < 15fs pulse duration. The OPCPA pump lasers use thin disk technology to achieve the high energy and average power required to pump such a high energy, high repetition rate broadband amplifier. Here we report on the progress in beamline development and discuss the technical challenges involved in producing such a system and their solutions. A major focus of the laser development is reliable, robust operation and long term stability; mechanical, optical, and control system architecture design considerations to achieve our goals of long term stability are discussed.