Ultrashort electron beams with a narrow energy spread, high charge, and low jitter are essential for resolving phase transitions in metals, semiconductors, and molecular crystals. These accelerated beams, produced by phototriggered electron guns, are also injected into accelerators for x-ray light sources. The achievable resolution of these time-resolved electron diffraction or x-ray experiments has been hindered by surface field and timing jitter limitations in conventional RF guns, which thus far are $<200\mathrm{MV}/\mathrm{m}$ and $>96\mathrm{fs}$, respectively. A gun driven by optically generated single-cycle terahertz (THz) pulses provides a practical solution for enabling not only GV/m surface fields but also absolute timing stability, since the pulses are generated by the same laser as the phototrigger. Here, we demonstrate an all-optical THz gun yielding a peak electron energy approaching 1 keV, accelerated by $>300\mathrm{MV}/\mathrm{m}$ THz fields in a micrometer-scale waveguide structure. We also achieve quasi-monoenergetic, sub-kiloelectron volt bunches with 32 fC of charge, which can already be used for time-resolved low-energy electron diffraction. Such ultracompact, easy-to-implement guns—driven by intrinsically synchronized THz pulses that are pumped by an amplified arm of the already-present photoinjector laser—provide a new tool with the potential to transform accelerator-based science.

X-ray crystallography is one of the main methods to determine atomic-resolution 3D images of the whole spectrum of molecules ranging from small inorganic clusters to large protein complexes consisting of hundred-thousands of atoms that constitute the macromolecular machinery of life. Life is not static, and unravelling the structure and dynamics of the most important reactions in chemistry and biology is essential to uncover their mechanism. Many of these reactions, including photosynthesis which drives our biosphere, are light induced and occur on ultrafast timescales. These have been studied with high time resolution primarily by optical spectroscopy, enabled by ultrafast laser technology, but they reduce the vast complexity of the process to a few reaction coordinates. In the AXSIS project at CFEL in Hamburg, funded by the European Research Council, we develop the new method of attosecond serial X-ray crystallography and spectroscopy, to give a full description of ultrafast processes atomically resolved in real space and on the electronic energy landscape, from co-measurement of X-ray and optical spectra, and X-ray diffraction. This technique will revolutionize our understanding of structure and function at the atomic and molecular level and thereby unravel fundamental processes in chemistry and biology like energy conversion processes. For that purpose, we develop a compact, fully coherent, THz-driven atto-second X-ray source based on coherent inverse Compton scattering off a free-electron crystal, to outrun radiation damage effects due to the necessary high X-ray irradiance required to acquire diffraction signals. This highly synergistic project starts from a completely clean slate rather than conforming to the specifications of a large free-electron laser (FEL) user facility, to optimize the entire instrumentation towards fundamental measurements of the mechanism of light absorption and excitation energy transfer. A multidisciplinary team formed by laser-, accelerator,- X-ray scientists as well as spectroscopists and biochemists optimizes X-ray pulse parameters, in tandem with sample delivery, crystal size, and advanced X-ray detectors. Ultimately, the new capability, attosecond serial X-ray crystallography and spectroscopy, will be applied to one of the most important problems in structural biology, which is to elucidate the dynamics of light reactions, electron transfer and protein structure in photosynthesis.

We describe a cw Kerr-lens mode-locked ${\text{Cr}}^{2+}$:ZnSe laser pumped by a 1800 nm thulium fiber laser. The astigmatically compensated asymmetric x cavity contained a 2.4-mm-long ${\text{Cr}}^{2+}$:ZnSe sample with a pump absorption coefficient of $11.6{\text{cm}}^{-1}$ and was operated with a 1% output coupler. The dispersion compensation was achieved by using a ${\text{MgF}}_2$ prism pair. During Kerr-lens mode-locked operation, we could generate 95 fs pulses at a pulse repetition rate of 94.3 MHz and with 40 mW of output power. The center wavelength of the pulses was 2420 nm. The pulses had a spectral width of 69 nm and a time–bandwidth product of 0.335, which is close to the transform limit for hyperbolic secant pulses.

Terahertz (THz)-based electron acceleration and manipulation has recently been shown to be feasible and to hold tremendous promise as a technology for the development of next-generation, compact electron sources. Previous work has concentrated on structures powered transversely by short, single-cycle THz pulses, with millimeter-scale, segmented interaction regions that are ideal for acceleration of electrons in the sub- to few-MeV range, where electron velocities vary significantly. However, in order to extend this technology to the multi-MeV range, an investigation of approaches supporting longer interaction lengths is needed. Here, we demonstrate first steps in electron acceleration and manipulation using dielectrically lined waveguides powered by temporally long, narrow-band, multicycle THz pulses that copropagate with the electrons. This geometry offers centimeter-scale single-stage interaction lengths and offers the opportunity to further increase interaction lengths by cascading acceleration stages that recycle the THz energy and rephase the interaction. We prove the feasibility of THz-energy recycling for the first time by demonstrating acceleration, compression, and focusing in two sequential ${\mathrm{Al}}_2{\mathrm{O}}_3$-based dielectric capillary stages powered by the same multicycle THz pulse. Since the multicycle THz energy achievable using laser-based sources is currently a limiting factor for the maximum electron acceleration, recycling the THz pulses provides a key factor for reaching relativistic energies with existing sources and paves the way for applications in future ultrafast electron diffraction and free-electron lasers.