It is all about phases: ultrafast holographic photoelectron imaging

Abstract

Photoelectron holography constitutes a powerful tool for the ultrafast imaging of matter, as it combines high electron currents with subfemtosecond resolution, and gives information about transition amplitudes and phase shifts. Similarly to light holography, it uses the phase difference between the probe and the reference waves associated with qualitatively different ionization events for the reconstruction of the target and for ascertaining any changes that may occur. These are major advantages over other attosecond imaging techniques, which require elaborate interferometric schemes in order to extract phase differences. For that reason, ultrafast photoelectron holography has experienced a huge growth in activity, which has led to a vast, but fragmented landscape. The present review is an organizational effort towards unifying this landscape. This includes a historic account in which a connection with laserinduced electron diffraction is established, a summary of the main holographic structures encountered and their underlying physical mechanisms, a broad discussion of the theoretical methods employed, and of the key challenges and future possibilities. We delve deeper in our own work, and place a strong emphasis on quantum interference, and on the residual Coulomb potential.

Spiral-like holographic structures: Unwinding interference carpets of Coulomb-distorted orbits in strong-field ionization

“…In order to reconstruct targets, one must measure both amplitudes and phase differences. This requirement has caused the development of ultrafast photoelectron holography [10][11][12][13], exploiting the quantum interference of different paths that an electron can take during strong-field ionization to produce a holographic image, which includes the phase information. Typically, there is a direct (reference) pathway and one that reinteracts with the target (probe).…”

“…SFA-like approaches are Born-type series which establish artificial boundaries between "direct" and "rescattered" orbits and exclude those in between [33]. There are, however, orbit-based methods that incorporate the residual potential and the driving field on an equal footing [13]. One such method, used in this work, is the Coulomb quantum orbit strong-field approximation (CQSFA).…”

Holographic detection of parity in atomic and molecular orbitals

“…According to the Coulomb quantum-orbit strong-field approximation (CQSFA) theory [14], the phase difference between the involved trajectories plays a crucial role in determining the interference structures. The phase for each trajectory mainly includes two components: the phase obtained along the continuum propagation Re[S s ] and the phase of the prefactor 0,s relating mainly to the phase of the initial state, which includes the parity of the atomic or molecular orbital.…”

“…Ultrafast photoelectron holography [13,14] is a very powerful imaging technique based on the physical picture of laserdriven recollision [15] which combines high electron currents with subfemtosecond time resolution. Thereby, a probe and a reference wave are employed to reconstruct a target by recording phase differences between them.…”

“…Both probe and reference terms stem from qualitatively different ionization pathways, which can be associated with interfering electron wave packets. This phase encoding makes it an ideal tool to probe the parity of the atomic and molecular orbitals (for a review see [14]).…”