Phase matching of high harmonic generation in the soft and hard X-ray regions of the spectrum

TLDR

The authors demonstrate that bright, fully coherent hard‑x‑ray beams can be produced by nonlinear up‑conversion of femtosecond laser light. They use long‑wavelength mid‑infrared lasers at moderate intensity to achieve full phase matching of high‑harmonic generation, experimentally confirming the dominant phase‑matching mechanism by generating phase‑matched soft‑x‑ray emission near 330 eV in a high‑pressure, weakly ionized gas medium. Their results show that conversion efficiency improves with longer driving wavelengths, making fully coherent multi‑keV x‑ray sources feasible, as the reduced single‑atom yield is offset by higher optimal pressures and reduced reabsorption at higher photon energies.

Abstract

We show how bright, fully coherent, hard x-ray beams can be generated through nonlinear upconversion of femtosecond laser light. By using longer-wavelength mid-infrared driving lasers of moderate peak intensity, full phase matching of the high harmonic generation process can extend, in theory, into the hard x-ray region of the spectrum. We identify the dominant phase matching mechanism for long wavelength driving lasers, and verify our predictions experimentally by demonstrating phase-matched up-conversion into the soft x-ray region of the spectrum around 330 eV using an extended, high-pressure, gas medium that is weakly ionized by the laser. Scaling of the overall conversion efficiency is surprisingly favorable as the wavelength of the driving laser is increased, making useful, fully coherent, multi-keV x-ray sources feasible. Finally, we show that the rapidly decreasing microscopic single-atom yield at longer driving wavelengths is compensated macroscopically by an increasing optimal pressure for phase matching and a rapidly decreasing reabsorption of the generated light at higher photon energies.