Macroscopic effects in generation of attosecond XUV pulses via high-order frequency mixing in gases and plasma

TL;DR

This paper investigates high-order frequency mixing (HFM) in gases and plasma as a method for generating intense, narrow-linewidth attosecond XUV pulses, demonstrating its advantages over traditional high-order harmonic generation (HHG).

Contribution

It provides a comparative analysis of HFM and HHG, including analytical and numerical results showing HFM's higher macroscopic signal and controllable attosecond pulse trains.

Findings

HFM yields several times higher XUV intensity than HHG in gases.

HFM can produce up to three orders of magnitude higher intensity in plasma.

HFM generates very narrow XUV spectral lines with controllable attosecond pulse trains.

Abstract

We study the generation of attosecond XUV pulses via high-order frequency mixing (HFM) of two intense generating fields, and compare this process with the more common high-order harmonic generation (HHG) process. We calculate the macroscopic XUV signal by numerically integrating the 1D propagation equation coupled with the 3D time-dependent Schr\"odinger equation. We analytically find the length scales which limit the quadratic growth of the HFM macroscopic signal with propagation length. Compared to HHG these length scales are much longer for a group of HFM components, with orders defined by the frequencies of the generating fields. This results in a higher HFM macroscopic signal despite the microscopic response being lower than for HHG. In our numerical simulations, the intensity of the HFM signal is several times higher than that for HHG in a gas, and it is up to three orders of magnitude higher for generation in plasma; it is also higher for longer generating pulses. The HFM provides very narrow XUV lines ($\delta \omega / \omega = 4.6 \times 10^{-4}$) with well-defined frequencies, thus allowing for a simple extension of optical frequency standards to the XUV range. Finally, we show that the group of HFM components effectively generated due to macroscopic effects provides a train of attosecond pulses such that the carrier-envelope phase of an individual attosecond pulse can be easily controlled by tuning the phase of one of the generating fields.