Generation of broad XUV continuous high harmonic spectra and isolated attosecond pulses with intense mid-infrared lasers

TL;DR

This paper demonstrates that intense mid-infrared lasers can generate broad, continuous high harmonic spectra and isolated attosecond pulses through strong temporal and spatial reshaping of the driving field, enabling new ultrafast light sources.

Contribution

It introduces a method to produce broad XUV spectra and IAPs using mid-IR lasers without carrier-envelope phase stabilization, supported by experimental and simulation results.

Findings

Near-continuum HHG spectra appear at high intensities.

Proper spatial filtering enables isolated attosecond pulse generation.

Spectral broadening results from field reshaping effects.

Abstract

We present experimental results showing the appearance of a near-continuum in the high-order harmonic generation (HHG) spectra of atomic and molecular species as the driving laser intensity of an infrared pulse increases. Detailed macroscopic simulations reveal that these near-continuum spectra are capable of producing IAPs in the far field if a proper spatial filter is applied. Further, our simulations show that the near-continuum spectra and the IAPs are a product of strong temporal and spatial reshaping (blue shift and defocusing) of the driving field. This offers a possibility of producing IAPs with a broad range of photon energy, including plateau harmonics, by mid-IR laser pulses even without carrier-envelope phase stabilization.