(WP1) We constructed a femtosecond Yb:YAG amplifier at 50-500 kHz repetition rate with 20 W output power. The output pulses were further shortened to 30 fs via cascaded nonlinear-optical broadening and hollow-fiber compression followed by chirped mirrors. (WP2) Generation of mid-infrared, single-cycle for electron pulse compression, metrology and sample excitation was achieved by a two-stage non-collinear optical parametric amplifier based on white-light generation in yttrium iron garnet and subsequent parametric amplification in beta barium borate and lithium gallium sulfide crystals. These pulses at 8-11 μm wavelength have >1μJ pulse energy and ~1e9 V/m field strength in a focus. They are therefore useful for strong-field specimen excitation. We further stabilized the carrier-envelope phase (Chen et al., Opt. Express 2019). We also developed the necessary tools to guide these few-cycle pulse into our vacuum chamber for sample excitation and electron-beam control (Morimoto and Baum, Phys. Rev. Lett. 2020, in print). (WP3) All-optical compression of electron pulses has been achieved (see WP5). Numerical and analytical theories of light-electron interaction at metallic, dielectric and absorbing materials, confirmed later by experiments, have produced several unexpected discoveries, in particular a close relation between velocity-matching and zero deflection. This theoretical result made it possible to compress electron pulses of almost arbitrarily large beam diameter by terahertz radiation at membrane elements. (WP4) We succeeded in producing single-cycle THz pulses with help of a Cherenkov-type bulk emission scheme with lithium niobate crystals covered by silicon output prisms. The resulting intense single-cycle THz radiation at 0.3 THz central frequency was found to be well sufficient for terahertz electron-pulse compression, streaking metrology and specimen excitation. (WP5) We demonstrated experimentally an all-optical electron pulse compression by terahertz radiation. Here, a major breakthrough was achieved: successful proof-of-principle and every-day compression of electron pulses from picoseconds to femtoseconds by using terahertz radiation, followed by streaking characterization. Control of electron pulses by mid-infrared radiation and optical radiation has also been achieved. A new idea has also emerged: In contrast to pursuing attosecond electron pulse generation in form of a pulse trains (burst) as initially envisioned, we now discovered that it is also feasible to use a continuous-wave laser for the continuous-wave control of single-electron pulses from a standard continuous electron source. The resulting attosecond electron microscopy is an unexpected discovery of our project. (WP6). A successful proof-of-principle experiment that electromagnetic field vectors can be imaged in space and time and vectorial direction is one of the key results of this project. As of nanostructures, we succeeded in measuring time-dependent small-angle diffraction data from a nanostructured waveguide array. We could see symmetry-breaking Bragg spot dynamics that indicate the role of time-frozen but delay-dependent quantum phases to the coherent electron wave packets according to the Aharonov-Bohm effect. (WP7) Atomic-resolution diffraction results from a single crystal of silicon showed that it takes a finite time to scatter electrons into Bragg spots. These results are a proof-of-principle experiment that attosecond electron imaging can reveal atomic-scale charge density modulation in light fields with atomic resolution in space and time. This data shows clearly the emerging possibilities, but a final unambiguous demonstration is still under way with help of the continuous-wave attosecond electron microscopy discovered in the project. Follow-up funding for this remaining research has been secured. (WP8) The above mentioned advances have provided the basis for investigation on even more complex materials than before. Complex nanophotonic waveguide arrays, metamaterial elements (split ring resonators and bow-tie field-enhancers) and nonlinear-optical crystals (beta barium borate) are currently under investigation. Overall, we stayed in all work packages within the planned schedule, and our grand goals have been almost completely achieved.
