arXiv subjects

Anne Weber

Publications and source records attributed to Anne Weber.

5 recordsLinked to original sources

arXiv2026

Caustics and catastrophes in strong-field physics -- Picard--Lefschetz theory as a universal approach to saddle-point methods in attosecond science

Ultrashort laser pulses on the attosecond timescale are typically achieved via high-order harmonic generation (HHG), a nonlinear process in which atoms interact with intense light fields to emit a broad spectrum of harmonics. HHG is commonly described in terms of a `quantum orbits' model based on several interfering electron trajectories, thereby incorporating both quantum-mechanical effects and an intuitive picture of classical dynamics. By tuning the parameters of the driving laser field, the interplay between these trajectories can be controlled, shaping the emitted light. Mathematically, this model expresses the harmonic response as a highly oscillatory integral. Applying saddle-point methods to this integral allows it to be decomposed into contributions from distinct saddle points of the semi-classical action, thereby linking quantum dynamics to classical trajectories. However, a general framework for applying these methods across arbitrary parameters and laser configurations has been missing. In this thesis, we introduce Picard--Lefschetz theory and develop practical numerical methods for its application. These enable the evaluation of oscillatory integrals and identification of contributions from individual critical points. We apply these techniques to strong-field ionisation and HHG, focusing on caustics -- enhancement features where trajectories coalesce and standard approximations fail. Our methods remain valid in these regions, allowing systematic analysis of parameter regimes and revealing previously inaccessible features. This work improves the understanding and control of ultrafast light--matter interactions.

quant-ph↗

arXiv2026

2D quantum-path interference in high-harmonic generation driven by highly-bichromatic fields

We experimentally observe a new type of quantum-path interference, in two-dimensions (2D-QPI), in high-harmonic generation (HHG) driven by an orthogonally-polarised highly-bichromatic field. This regime is marked by comparable intensities of the two orthogonal colours. In this highly-bichromatic regime, we demonstrate that 2D-QPI is encoded in the measured harmonic intensity modulations with respect to the relative phase of the two-colour field. The modulations of the odd-order harmonics show a monomodal behaviour, whereas the even harmonics are modulated in a bimodal structure. Our calculations using the strong-field approximation and saddle-point method disentangle contributions from multiple quantum orbits in this HHG regime, revealing that the dipole response for both odd and even harmonics inherits the dynamic symmetry of the orthogonally-polarised driving field. This new type of 2D-QPI offers a novel route to HHG spectroscopy of attosecond electron dynamics by lifting up the dimensionality of the quantum paths involved in the interference.

quant-ph↗

arXiv2025

A universal approach to saddle-point methods in attosecond science

Light-matter interactions within the strong-field regime, where intense laser fields can ionise a target via tunnelling, give rise to fascinating phenomena such as the generation of high-order harmonic radiation (HHG). On the atomic scale, these strong-field processes are described in terms of highly-oscillatory time integrals which are often approximated using saddle-point methods. These methods simultaneously simplify the calculations and let us understand the physical processes in terms of semi-classical electron trajectories, or quantum orbits. However, applying saddle-point methods for HHG driven by polychromatic laser fields without clear dynamical symmetries has remained challenging. Here we introduce Picard-Lefschetz theory as a universal and robust link between the time integrals and the semi-classical trajectories. The continuous deformation of the integration contour towards so-called Lefschetz thimbles allows an exact evaluation of the integral, as well as the identification of relevant quantum orbits, for arbitrary driving fields. The latter is realised via the ``necklace algorithm'', a novel solution to the open problem of determining the relevance of saddle points for a two-dimensional integral, which we introduce here. We demonstrate the versatility and rigour of Picard-Lefschetz methods by studying Stokes transitions and spectral caustics arising in HHG driven by two-colour laser fields. For example, we showcase a quantum-orbit analysis of the colour switchover, which links the regime of perturbative two-colour fields with that of fully bichromatic driving fields. With this work, we set the foundation for a rigorous application of quantum-orbit based approaches in attosecond science that enables the interpretation of state-of-the-art experimental setups, and guides the design of future ones.

quant-ph↗

arXiv2023

Quantum tunnelling without a barrier

Tunneling is an iconic concept that captures the peculiarity of quantum dynamics but, despite its ubiquity, questions remain. We focus on strong-field tunneling, which is vital to all attosecond science. We find an unexpected optical tunneling event that happens when the instantaneous electric field vanishes and there is no barrier. This event arises from a color switchover in a strongly polychromatic field. The tunneling without a barrier reveals the disconnect between the standard intuition built on the picture of a quasistatic barrier and the nonadiabatic nature of the process.

quant-ph↗

arXiv2021

Torus Knot Angular Momentum in Twisted Attosecond Pulses from High Harmonic Generation

Bicircular twisted Laguerre-Gaussian beams possess a definite torus knot angular momentum (TKAM) as a new form of angular momentum. TKAM is conserved in nonlinear atomic processes such as high harmonic generation and can be classified by a time delay parameter $\tau$ and a coordination parameter $\gamma$. These parameters are defined by the respective projected orbital angular momentum and the energy of the two superimposed Laguerre-Gaussian beams. We derive a consistent geometric method to determine $\tau$ and $\gamma$ from the driving beam as well as from the high harmonic radiation. This method relates both invariance parameters ($\tau$ and $\gamma$) to the emitted high harmonic radiation. Therefore, $\tau$ and $\gamma$ can be read off of two different torus knots. These knots can be constructed from the spatio-temporal evolution of the electric field of the respective high harmonic radiation or the driving beam. We demonstrate the classification of the invariance parameters for a planar atomic gas target irradiated by bicircular Laguerre-Gaussian beams explicitly. Furthermore, we demonstrate that the respective torus knots determined by $\tau$ and $\gamma$ can be mapped onto each other within minor modifications. This geometric method yields a different way to interpret the invariance parameters $\tau$ and $\gamma$ as well as their underlying relation compared to a purely formal derivation. The investigations presented in this work are in good agreement with previous findings and provide insight into the dynamical symmetry of TKAM in the context of high harmonic generation induced by bicircular twisted Laguerre-Gaussian beams.

physics.optics↗