Low-energy photoelectron structures for arbitrary ellipticity of a strong laser field

TL;DR

This paper investigates low-energy photoelectron structures across all ellipticities and Keldysh parameters in strong laser fields, revealing new regimes and mechanisms behind observed anomalies in electron momentum distributions.

Contribution

It introduces a comprehensive theoretical analysis combining numerical and classical-trajectory methods to explain low-energy structures in strong-field ionization at arbitrary ellipticity and nonadiabatic conditions.

Findings

Low-energy structures form at all ellipticities with large Keldysh parameters.

Three distinct interaction regimes are identified in the ellipticity-Keldysh parameter space.

Modified recollision dynamics explain the origin of low-energy structures and Coulomb bunching.

Abstract

Previous attoclock experiments measuring the photoelectron momentum distribution (PMD) via strong-field ionization in an elliptically polarized laser field have shown anomalously large offset angles in the nonadiabatic regime with large Keldysh parameters ($\gamma$). We investigate the process theoretically in the complete range of ellipticity ($\epsilon$) and large range of $\gamma$, employing numerical solutions of time-dependent Schr\"odinger equation and nonadiabatic classical-trajectory Monte Carlo simulations matched with the under-the-barrier motion via the nonadiabatic strong field approximation. We show the formation of low-energy structures (LES) at any ellipticity value when the Keldysh parameter is sufficiently large. Three regimes of the interaction in the ($\epsilon$-$\gamma$)-space of parameters are identified via the characteristic PMD features. The significant modification of the recollision picture in the nonadiabatic regime, with so-called anomalous and hybrid slow recollisions, is shown to be behind the LES, inducing extreme nonlinear Coulomb bunching in the phase-space in the polarization plane. Our findings elucidate subtle features of the attosecond electron dynamics in strong-field ionization at extreme conditions and indicate limitations on attosecond imaging.