This paper presents a method for generating bright, coherent ultrahigh harmonics (UHGH) in the keV X-ray regime using mid-infrared femtosecond lasers. The research team successfully generated ultrahigh harmonics up to orders greater than 5000, producing a bright supercontinuum that spans the entire electromagnetic spectrum from the ultraviolet to over 1.6 keV. This allows for the generation of pulses as short as 2.5 attoseconds. The multi-atmosphere gas pressures required for bright, phase-matched emission also support laser beam self-confinement, enhancing the X-ray yield. The X-ray beam exhibits high spatial coherence, even though at high gas density, the recolliding electrons responsible for high harmonic generation encounter other atoms during the emission process. The unique ability of X-rays to capture structure and dynamics at the nanoscale has spurred the development of large-scale X-ray free-electron lasers based on accelerator physics, as well as high harmonic generation (HHG) techniques in the X-ray region that employ tabletop femtosecond lasers. The HHG process represents nonlinear optics at an extreme, enabling unprecedented femtosecond-to-attosecond duration pulses with full spatial coherence, which make it possible to track the dynamics of electrons in atoms, molecules and materials. X-rays can probe the oxidation or spin state in molecules and materials with element-specificity, because the position of the characteristic X-ray absorption edges of individual elements is sensitive to the local environment and structure. Ultrashort X-ray pulses can capture the coupled motions of charges, spins, atoms and phonons by monitoring changes in absorption or reflection that occur near these edges as a material or molecule changes state or shape. However, many inner-shell absorption edges in advanced correlated-electron, magnetic and catalytic materials (Fe, Co, Ni, Cu) lie at photon energies nearing 1 keV. In contrast, most applications that use HHG light have been limited to the extreme ultraviolet (EUV) region of the spectrum (< 150 eV), where efficient frequency upconversion is possible using widely available Ti:Sapphire lasers operating at 0.8 μm wavelength. The researchers sought to extend bright HHG to a higher energy soft X-ray region. High harmonic generation is a universal response of atoms and molecules in strong femtosecond laser fields. In a simple analogy, HHG represents the coherent version of the Röntgen X-ray tube: instead of boiling electrons off a hot filament, accelerating them in an electric field, and generating incoherent X-rays when the high-energy electrons strike a target, HHG begins with tunnel ionization of an atom in a strong laser field. The portion of the electron wavefunction that escapes the atom is accelerated by the laser electric field, and when driven back to its parent ion by the laser, can coherently convert its kinetic energy into a high harmonic photonThis paper presents a method for generating bright, coherent ultrahigh harmonics (UHGH) in the keV X-ray regime using mid-infrared femtosecond lasers. The research team successfully generated ultrahigh harmonics up to orders greater than 5000, producing a bright supercontinuum that spans the entire electromagnetic spectrum from the ultraviolet to over 1.6 keV. This allows for the generation of pulses as short as 2.5 attoseconds. The multi-atmosphere gas pressures required for bright, phase-matched emission also support laser beam self-confinement, enhancing the X-ray yield. The X-ray beam exhibits high spatial coherence, even though at high gas density, the recolliding electrons responsible for high harmonic generation encounter other atoms during the emission process. The unique ability of X-rays to capture structure and dynamics at the nanoscale has spurred the development of large-scale X-ray free-electron lasers based on accelerator physics, as well as high harmonic generation (HHG) techniques in the X-ray region that employ tabletop femtosecond lasers. The HHG process represents nonlinear optics at an extreme, enabling unprecedented femtosecond-to-attosecond duration pulses with full spatial coherence, which make it possible to track the dynamics of electrons in atoms, molecules and materials. X-rays can probe the oxidation or spin state in molecules and materials with element-specificity, because the position of the characteristic X-ray absorption edges of individual elements is sensitive to the local environment and structure. Ultrashort X-ray pulses can capture the coupled motions of charges, spins, atoms and phonons by monitoring changes in absorption or reflection that occur near these edges as a material or molecule changes state or shape. However, many inner-shell absorption edges in advanced correlated-electron, magnetic and catalytic materials (Fe, Co, Ni, Cu) lie at photon energies nearing 1 keV. In contrast, most applications that use HHG light have been limited to the extreme ultraviolet (EUV) region of the spectrum (< 150 eV), where efficient frequency upconversion is possible using widely available Ti:Sapphire lasers operating at 0.8 μm wavelength. The researchers sought to extend bright HHG to a higher energy soft X-ray region. High harmonic generation is a universal response of atoms and molecules in strong femtosecond laser fields. In a simple analogy, HHG represents the coherent version of the Röntgen X-ray tube: instead of boiling electrons off a hot filament, accelerating them in an electric field, and generating incoherent X-rays when the high-energy electrons strike a target, HHG begins with tunnel ionization of an atom in a strong laser field. The portion of the electron wavefunction that escapes the atom is accelerated by the laser electric field, and when driven back to its parent ion by the laser, can coherently convert its kinetic energy into a high harmonic photon