Subcycle conservation law in strong-field ionization

The conservation law is a fundamental tool that significantly aids our quest to understand the world, playing a crucial role across various scientific disciplines. Particularly in strong-field physics, these laws enhance our comprehension of atomic and molecular structures as well as the ultrafast dynamics of electrons. For example, when atoms interact with linearly polarized light, the Hamiltonian of the system displays a second-order dynamical symmetry, which stays invariant under the symmetry operation that involves a half-period time translation together with a spatial inversion. This characteristic symmetry is known to result in exclusive odd-order harmonics during high-harmonic generation from rare-gas atoms.

An intriguing phenomenon occurs when atoms interact with circularly polarized light. There, the Hamiltonian exhibits an infinite-order continuous dynamical symmetry, which stays invariant under a symmetry operation that comprises an arbitrary time translation combined with a corresponding rotational operation. The implication of this symmetry for conservation laws presents a compelling topic for exploration.

A team of researchers from the State Key Laboratory of Precision Spectroscopy at East China Normal University has delineated a conservation law between angular momentum and energy at the subcycle level. This was achieved through the analysis of the correlated spectrum of angular momentum and energy (SAME) of photoelectrons, both at the tunnel exit and in the asymptotic region, in the context of strong-field ionization using circularly and elliptically polarized light pulses. The researchers have confirmed that this conservation law stays applicable down to the subcycle level. They have also introduced a protocol utilizing interference-induced electron vortices to directly visualize the conservation law at the subcycle level. Their findings have been published in the journal Ultrafast Science.

Figure 1 illustrates the SAME for photoelectrons at the tunnel exit under circular polarization (ε=1.0) and elliptical polarization (ε=0.7) laser pulses. In the case of circular polarization, while the individual distributions of angular momentum and energy are broad, their correlated distribution forms a distinct straight line. This pattern underscores a rigorously obeyed conservation law between angular momentum  and energy , represented by the equation . The team has further substantiated that this conservation law is consistently applicable throughout an entire optical cycle. For elliptical polarization, the conservation law is naturally extended as  and can be articulated with an effective angular frequency  within the optical cycle.

This work introduces a novel conservation law between angular momentum and energy that operates down to the subcycle level during strong-field ionization. The discovery of this subcycle conservation law is attributed to the infinite-order continuous dynamical symmetry inherent in the interaction between atoms and light pulses with circular or elliptical polarization. This study lays a theoretical groundwork that is instrumental for a profound comprehension of light-matter interactions on the subcycle scale.