Extension of the bright high-harmonic photon energy range via nonadiabatic critical phase matching

TL;DR

This paper introduces a nonadiabatic critical phase matching concept that significantly extends the bright high-harmonic photon energy range into the soft X-ray region, validated through experiments and an analytical model, especially with few-cycle mid-infrared lasers.

Contribution

It reveals a second nonadiabatic critical ionization fraction that enhances phase matching, supported by experimental validation and an analytical predictive model.

Findings

Nonadiabatic critical phase matching extends high-harmonic photon energies.

Strong laser field reshaping enhances spectral extension.

Analytical model predicts achievable spectral ranges.

Abstract

Extending the photon energy range of bright high-harmonic generation to cover the entire soft X-ray region is important for many applications in science and technology. The concept of critical ionization fraction has been essential, because it dictates the maximum driving laser intensity that can be used while preserving bright harmonic emission. In this work, we reveal a second, nonadiabatic critical ionization fraction that substantially extends the maximum phase-matched high-harmonic photon energy, that arises due to strong reshaping of the intense driving laser field. We validate this understanding through a systematic comparison between experiment and theory, for a wide range of pulse durations and driving laser wavelengths. In particular, high harmonics driven by intense few-cycle pulses experience the most pronounced spectral reshaping, significantly extending the bright photon energy range. We also present an analytical model that predicts the spectral extension that can be achieved for different driving lasers. This reveals an increasing role of nonadiabatic critical phase matching when driven by few-cycle mid-infrared lasers. These findings are important for the development of high-brightness soft X-ray sources for applications in spectroscopy and imaging.