ULAD · Ultrafast Lasers and Attosecond Dynamics

Ultrafast Lasers and Attosecond Dynamics

The following completed research was carried out as part of the ULAD project and involves strong field ionization theory. Most of my results are well-supported by the experimental data from Prof. Keller's group, with which close collaboration was envisioned from the beginning, as described in the ULAD proposal. My recently submitted work on tunnelling time has received a great deal of attention at conferences where it was presented. This work uses a Feynman Path Integral approach (FPI) to calculate a probability distribution of tunnelling times that shows very good agreement with the new experimental data from Prof. Keller group. Below is the description of some of the other related work carried during the ULAD project, as well as a more detailed description of the tunnelling time work, which formed a core part of my original proposal. My coauthors and I have carried out a number of projects of direct relevance to the proposed research on strong field ionization of atoms and molecules. Our work that shows a non-zero longitudinal momentum spread of the electron wave packet at the tunnel exit has appeared in Physical Review Letters. This work raises into question a common assumption in strong field ionization, where the electron is said to appear in the continuum with zero velocity in the direction of tunnelling. A follow-up to this work, where I closely supervised a first author PhD student (C. Hofmann) was published in J. Phys B, and is currently one of the most read publications on IOP select, recently highlighted in Europhysics news due to its high impact. In short, this work introduced a new method of data analysis, which allows for more accurate reconstruction of the shape of the electron wavepacket immediately following strong field ionization. I also published a work that reformulated the previously proposed mechanism of Rydberg state creation. My results showed excellent agreement with experimental data published in a prior PRL by an independent group, and put Rydberg states in the context of high harmonic generation. Specifically, I showed that the transverse velocity of the Rydberg states at the tunnel exit is similar to the electrons involved in HHG, and hence leads to the same probability distribution as a function of ellipticity of laser light. This was the first time that the underlying similarities and differences between the dynamics of the electrons involved in HHG and Rydberg states were investigated. A new work, currently under review, proposes a new connection between Rydberg states, Coulomb asymmetry and Coulomb focusing: all actively studied phenomena in strong field ionization. This work introduces a unified approach for probing long-range interaction between the electron and the parent ion at all ellipticities of laser light, unifying what was previously believed to be distinct phenomena. In addition, it shows that a common approach of neglecting the Coulomb field along the minor axis of polarisation fundamentally breaks down at low ellipticities of laser light. Another new high-profile publication carried out with my collaborators involves probing non-adiabatic effects in strong field tunnel ionization (published in Phys. Rev. Lett., September 2013). This work uses the final angle of the electron momenta distribution as a sensitive probe for non-adiabatic effects, which cannot be otherwise detected using usual methods. Together with a PhD student that I am supervising (C. Hofmann), we just completed a follow-up to this work that investigates the role of non-adiabatic effects in the longitudinal momentum spread. We find that non-adiabatic effects cannot explain the extra longitudinal momentum spread at the tunnel exit, once experimental intensity calibration is taken into account Recently, I completed a work that applies the Feynman path integral approach to the calculation of tunneling time. In this work, the most widely used approaches to tunnelling time were also calculated and compared to the attoclock measurements from the Keller group. 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 (FPI) 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 existence of a probability distribution of tunnelling times has never before been considered in ultrafast science, where even the best techniques for analysis of electron dynamics are based on deterministic time at ionization. The implication of such a probability distribution of tunneling times, as opposed to a distinct tunneling time, challenges how valence electron dynamics are currently reconstructed in ultrafast science. It means that one both can and must account for a significant, though bounded and measurable, uncertainty as to when the hole dynamics begin to evolve. This work has generated a great deal of interest with further experiments planned by the Keller group to directly probe the probability distribution that I found.

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