High Harmonic Generation via Continuum Wave-Packet Interference

TL;DR

This paper presents a theoretical study of high-order harmonic generation in the over-the-barrier ionization regime, highlighting interference effects between free electron wave packets that produce coherent radiation at specific photon energies.

Contribution

It introduces a new perspective on HHG involving continuum wave-packet interference, expanding beyond the classical three-step model and demonstrating how this process dominates after ground state depletion.

Findings

Interference between ionized electron wave packets leads to specific harmonic emissions.

The process dominates HHG after atomic ground state depletion.

Continuum wave-packet interference can be isolated via phase-matching techniques.

Abstract

High-order harmonic generation (HHG) is investigated theoretically in the over-the-barrier ionization (OBI) regime revealing the strong signature of interference between two separately ionized and separately propagating free wave packets of a single electron. The interference leads to the emission of coherent light at a photon energy corresponding to the kinetic-energy difference of the two recolliding electron quantum paths, thus complementary to the well-known classical three-step picture of HHG. As will be shown by time-frequency analysis of the emitted radiation, the process entirely dominates the coherent HHG emission after the atomic ground state has been depleted by a strong field. Moreover, it can be isolated from the continuum--bound harmonics via phase-matching.