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.

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Phase-matched harmonic conversion of visible laser light into soft x-rays was demonstrated. The recently developed technique of guided-wave frequency conversion was used to upshift light from 800 nanometers to the range from 17 to 32 nanometers. This process increased the coherent x-ray output by factors of 10(2) to 10(3) compared to the non-phase-matched case. This source uses a small-scale (sub-millijoule) high repetition-rate laser and will enable a wide variety of new experimental investigations in linear and nonlinear x-ray science.

Distinguishing insulators by the dominant type of interaction is a central problem in condensed matter physics. Basic models include the Bloch-Wilson and the Peierls insulator due to electron-lattice interactions, the mott and the excitonic insulator caused by electron-electron interactions, and the Anderson insulator arising from electron-impurity interactions. In real materials, however, all the interactions are simultaneously present so that classification is often not straightforward. Here, we show that time-and angle-resolved photoemission spectroscopy can directly measure the melting times of electronic order parameters and thus identify-via systematic temporal discrimination of elementary electronic and structural processes-the dominant interaction. specifically, we resolve the debates about the nature of two peculiar charge-density-wave states in the family of transition-metal dichalcogenides, and show that Rb intercalated 1T-Tas 2 is a Peierls insulator and that the ultrafast response of 1T-Tise 2 is highly suggestive of an excitonic insulator.