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Direct Visualization of Light-Driven Atomic-Scale Carrier Dynamics in Space and Time

Periodic Reporting for period 4 - DIVI (Direct Visualization of Light-Driven Atomic-Scale Carrier Dynamics in Space and Time)

Période du rapport: 2019-11-01 au 2020-07-31

Electronics is rapidly speeding up. Ultimately, miniaturization will reach atomic dimensions and the switching speed will reach optical frequencies. This ultimate regime of lightwave electronics, where atomic-scale charges are controlled by few-cycle laser fields, holds promise to advance information processing technology from today’s microwave frequencies to the thousand times faster regime of optical light fields. All materials, including dielectrics, semiconductors and molecular crystals, react to such field oscillations with an intricate interplay between atomicscale charge displacements (polarizations) and collective carrier motion on the nanometer scale (currents). This entanglement provides a rich set of potential mechanisms for switching and control. However, our ability to eventually realize lightwave electronics, or even to make first steps, will critically depend on our ability to actually measure electronic motion in the relevant environment: within/around atoms. The most fundamental approach would be a direct visualization in space and time. This project, if realized, will offer that: a spatiotemporal recording of electronic motion with sub-atomic spatial resolution and sub-optical-cycle time resolution, i.e. picometers and few-femtoseconds/attoseconds. Drawing on our unique combination of expertise covering electron diffraction and few-cycle laser optics likewise, we will replace the photon pulses of conventional attosecond spectroscopy with freely propagating single-electron pulses at picometer de Broglie wavelength, compressed in time by sculpted laser fields. Stroboscopic diffraction/microscopy will provide, after playback of the image sequence, a direct visualization of fundamental electronic activity in space and time. Profound study of atomic-scale light-matter interaction in simple and complex materials will provide a comprehensive picture of the fundamental physics allowing or limiting the high-speed electronics of the future.
(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.
The realization of THz-compressed electron pulses and the concepts behind it are transformative for other electron-beam-based experiments as well and is already being adopted by several other research teams worldwide. Attosecond electron microscopy advances ultrafast space-time imaging form the regime of atomic motions into the domain of purely charge-density motions and optical responses in materials and nanostructures. Our progress with optical-pulse manipulation and shortening in the THz and mid-infrared regimes is also advancing the state-of-the-art and helpful for other researchers. The concepts and proof-of-principle experiments on waveform electron microscopy advance the state-of-the-art in electron microscopy and potentially provide novel contrast mechanisms in imaging. The results that have been achieved are simply the project goals, namely realizing and demonstrating the capability of ultrafast electron-based science to image fundamental light-driven electronic motion in space and time at the fundamental resolutions. We therefore regard our project an almost full success and thank the ERC in the strongest possible way for their awesome support and patronage.