Phase-matched coherent hard x-rays from relativistic high-order harmonic generation

TL;DR

This paper explores phase-matched high-order harmonic generation using relativistic laser pulses and counterpropagating attosecond pulse trains, enabling coherent x-ray production at multi-keV energies.

Contribution

It introduces a method to compensate phase mismatch in relativistic HHG, allowing efficient generation of high-energy harmonics in plasma.

Findings

Phase mismatch can be compensated by an additional phase depending on pulse delay.

Feasible phase-matched emission of multi-keV harmonics from underdense plasma.

Scalability of HHG into the multi-keV regime demonstrated.

Abstract

High-order harmonic generation (HHG) with relativistically strong laser pulses is considered employing electron ionization-recollisions from multiply charged ions in counterpropagating, linearly polarized attosecond pulse trains. The propagation of the harmonics through the medium and the scaling of HHG into the multi-kilo-electronvolt regime are investigated. We show that the phase mismatch caused by the free electron background can be compensated by an additional phase of the emitted harmonics specific to the considered setup which depends on the delay time between the pulse trains. This renders feasible the phase-matched emission of harmonics with photon energies of several tens of kilo-electronvolt from an underdense plasma.