2A shows that the HHG X-ray signal grows quadratically with pressure, even in very dense gas media (with low ionization levels of 0.03%). 1 shows that the phase matched emission extends to >1.6 keV ( 5031th. Using 3.9 μm pulses and 35 atm of He as a nonlinear medium, Fig. X-ray HHG spectra are acquired using the highly-sensitive X-ray CCD camera Newton DO920N-BN, (Andor Technology), cooled down to -50 ☌ to minimize noise. A custom-made grating spectrometer is used for spectral analysis of generated radiation. HHG X-rays are generated by guiding the ultrafast laser beams in a hollow waveguide designed to hold multi-atm phase-matching pressure gases. The subsequent KTA stages of the OPCPA are pumped by a 20 Hz picosecond Nd:YAG laser system and produce uncompressed 30 mJ and compressed 8.5 mJ energy in the signal and the idler beams at 1.46 μm and 3.9 μm, respectively - the highest pulse energy from a femtosecond mid-IR source to date. The front end of the OPCPA is based on a femtosecond Yb:CaF2 chirped pulse amplifier that drives a cascaded femtosecond OPA. In this experiment, 6-cycle FWHM, 3.9 μm, 20 Hz, multi-mJ pulses are generated as the idler from a novel optical parametric chirpedpulse amplification (OPCPA) architecture. In addition, it generates the broadest coherent supercontinuum to date of >1.3 keV, from any light source, large or small scale. The research group demonstrated bright coherent HHG X-rays at photon energies >1.6 keV ( 5031th order) is possible for the first time by using driving laser wavelengths around 3.9 μm. Essentially, the macroscopic full phase matching cutoff energy scales almost as strongly with the wavelength of the driving laser, hνPM cutoff α λL(1.5-1.7), as with the microscopic singleatom cutoff, hνSA cutoff α λL2. In previous research using mid-infrared lasers at wavelengths up to 2 μm to drive HHG, full phase matching was demonstrated in the water window up to photon energies of 0.52 keV. The grand challenge for extending bright HHG to higher energies is the development of phase matching techniques that enable efficient nonlinear upconversion. Extending HHG to photon energies into the keV region would open up a host of important applications in probing thicker samples (since matter is more transparent at higher photon energies), capturing dynamics at the L-edges of magnetic materials, and imaging dynamics with nanometer-scale spatial resolution. However, to date, most applications that use HHG light have been limited to the extreme ultraviolet (EUV) region of the spectrum, ~50 – 100 eV. The unprecedented femtosecond-to-attosecond pulse duration and full spatial coherence of the HHG light make it possible to capture the motions of electrons, atoms, and molecules in real time, to observe element-specific dynamics at the M-shell absorption edges of magnetic materials, to understand heat flow in nanostructures, and to implement table-top microscopes with record spatial resolutions of 20 nm. The unique ability of X-rays for elemental and chemicallyspecific imaging of thick samples at the nanoscale have spurred the development of X-ray free-electron laser sources, as well as ultrafast high harmonic (HHG) X-rays, from tabletop-scale femtosecond lasers. Table-top coherent X-ray source in the keV region
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