High-harmonic generation (HHG) traditionally combines ~100 near-infrared laser photons to generate bright, phase-matched, extreme ultraviolet beams when the emission from many atoms adds constructively. Here, we show that by guiding a mid-infrared femtosecond laser in a high-pressure gas, ultrahigh harmonics can be generated, up to orders greater than 5000, that emerge as a bright supercontinuum that spans the entire electromagnetic spectrum from the ultraviolet to more than 1.6 kilo-electron volts, allowing, in principle, the generation of pulses as short as 2.5 attoseconds. The multiatmosphere gas pressures required for bright, phase-matched emission also support laser beam self-confinement, further enhancing the x-ray yield. Finally, the x-ray beam exhibits high spatial coherence, even though at high gas density the recolliding electrons responsible for HHG encounter other atoms during the emission process.

We demonstrate a compact 20 Hz repetition-rate mid-IR OPCPA system operating at a central wavelength of 3900 nm with the tail-to-tail spectrum extending over 600 nm and delivering 8 mJ pulses that are compressed to 83 fs (<7 optical cycles). Because of the long optical period (∼13 fs) and a high peak power, the system opens a range of unprecedented opportunities for tabletop ultrafast science and is particularly attractive as a driver for a highly efficient generation of ultrafast coherent x-ray continua for biomolecular and element specific imaging.

Over the past decade intense laser fields with a single-cycle duration and even shorter, subcycle multicolour field transients have been generated and applied to drive attosecond phenomena in strong-field physics. Because of their extensive bandwidth, single-cycle fields cannot be emitted or amplified by laser sources directly and, as a rule, are produced by external pulse compression—a combination of nonlinear optical spectral broadening followed up by dispersion compensation. Here we demonstrate a simple robust driver for high-field applications based on this Kagome fibre approach that ensures pulse self-compression down to the ultimate single-cycle limit and provides phase-controlled pulses with up to a 100 μJ energy level, depending on the filling gas, pressure and the waveguide length.

Recent advances in high-order harmonic generation have made it possible to use a tabletop-scale setup to produce spatially and temporally coherent beams of light with bandwidth spanning 12 octaves, from the ultraviolet up to x-ray photon energies >1.6  keV. Here we demonstrate the use of this light for x-ray-absorption spectroscopy at the K- and L-absorption edges of solids at photon energies near 1 keV. We also report x-ray-absorption spectroscopy in the water window spectral region (284-543 eV) using a high flux high-order harmonic generation x-ray supercontinuum with 10^{9}  photons/s in 1% bandwidth, 3 orders of magnitude larger than has previously been possible using tabletop sources. Since this x-ray radiation emerges as a single attosecond-to-femtosecond pulse with peak brightness exceeding 10^{26}  photons/s/mrad^{2}/mm^{2}/1% bandwidth, these novel coherent x-ray sources are ideal for probing the fastest molecular and materials processes on femtosecond-to-attosecond time scales and picometer length scales.

Intense pulses at low terahertz (THz) frequencies of 0.1-2 THz are an enabling tool for constructing compact particle accelerators and for strong-field control of matter. Optical rectification in lithium niobate provided sub-mJ THz pulse energies, but it is challenging to increase it further. Semiconductor sources suffered from low efficiency. Here, a semiconductor (ZnTe) THz source is demonstrated, collinearly pumped at an infrared wavelength beyond the three-photon absorption edge and utilizing a contact grating for tilting the pump-pulse front. Suppression of free-carrier absorption at THz frequencies in this way resulted in 0.3% THz generation efficiency, two orders of magnitude higher than reported previously from ZnTe. Scaling the THz energy to the mJ level is possible simply by increasing the pumped area. This unique THz source with excellent focusability, pumped by novel, efficient infrared sources, opens up new perspectives for THz high-field applications. Terahertz (THz) pulses with high energy and field strength are enabling novel applications [1-4], including resonant control over ionic motion, bound and free electrons, as well as nonresonant and strong-field interactions [3]. Intense THz pulses hold promise for the development of a new generation of compact particle and x-ray sources [1,2]. Laser-and THz-driven particle accelerators with unprecedented flexibility can be important for free-electron lasers [2,5] and materials science and could revolutionize medical therapy with x-ray, electron, or proton beams [1,2].Single-cycle or nearly single-cycle THz pulses with high energy can be generated by optical rectification of femtosecond laser pulses. The highest so far THz pulse energy reported from such a source, utilizing the novel organic nonlinear material DSTMS, was 0.9 mJ [6]. The spectrum obtained from organic materials is typically centered in the 2 to 10 THz range, well suited for nonlinear spectroscopic studies. THz sources with lower frequencies are optimally fitted to the requirements of particle acceleration [1,2,7]. The frequency range below 2 THz can be better accessed with another nonlinear material, lithium niobate, utilizing pump pulses with a tilted intensity front for non-collinear phase matching [8]. THz pulses with more than 0.4 mJ energy were generated with 0.77% efficiency using this technique [7]. However, increasing the THz energy further turned out to be very challenging because of the large pulse-front tilt angle (63°) and the associated large angular dispersion of the pump [9,10]. The effect of a strong THz field on the pump pulse, owing to their nonlinear interaction, leads to additional difficulties involving the reduction of the THz generation efficiency [11] and the distortion of the THz beam [12].Semiconductor nonlinear materials have been extensively used to access the low-frequency part of the THz spectrum. The most popular material is ZnTe, where collinear phase matching is possible at the commonly used 0.8 μm pump wavelength of Ti:sapphire lasers. The highest THz pu...