Torus-knot angular momentum in twisted attosecond pulses from high-order harmonic generation

Abstract

Bicircular twisted Laguerre-Gaussian beams possess a definite torus knot angular momentum (TKAM) $\stackrel{\ifmmode \hat{}\else \^{}\fi{}}{{J}_{\ensuremath{\gamma}}}=\stackrel{\ifmmode \hat{}\else \^{}\fi{}}{L}+\ensuremath{\gamma}\stackrel{\ifmmode \hat{}\else \^{}\fi{}}{S}$ as an alternative a 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 $\ensuremath{\tau}$ and a coordination parameter $\ensuremath{\gamma}$. These parameters are defined by the respective projected orbital angular momentum ${\ensuremath{\ell}}_{i}$ and the energy $\ensuremath{\hbar}{p}_{i}{\ensuremath{\omega}}_{0}$ of the two superimposed Laguerre-Gaussian beams. We derive a consistent geometric method to determine $\ensuremath{\tau}$ and $\ensuremath{\gamma}$ from the driving beam as well as from the high harmonic radiation. The method relates both invariance parameters ($\ensuremath{\tau}$ and $\ensuremath{\gamma}$) to a torus knot which can be constructed from the emitted high harmonic radiation. These knots are constructed from the spatiotemporal 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. In addition, we demonstrate that the respective torus knots determined by $\ensuremath{\tau}$ and $\ensuremath{\gamma}$ can be mapped onto each other within minor modifications. The numerical calculations are done within the strong-field approximation and the associated quantum orbit approach. Therefore, we also briefly review high harmonic generation by bicircular twisted light beams. This introduced geometric method is a different approach to interpret the invariance parameters $\ensuremath{\tau}$ and $\ensuremath{\gamma}$, as well as their underlying relations, 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.