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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 3 - DIVI (Direct Visualization of Light-Driven Atomic-Scale Carrier Dynamics in Space and Time)

Teaser

Summary

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 todayâ€™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.

Work performed

The ERC consolidator grant project â€œDIVIâ€ aims at imaging light-driven electronic motion in space and time. In the first half of the project (30/60 months), subject to the current intermediate report, we proceeded as follows in the work packages according to the description-of-work. (1a) We successfully designed and set up a femtosecond Yb:YAG thin-disk amplifier working at 50-500 kHz repetition rate with 20 W output power. (1b) We demonstrated the shortening of the pulses to 30fs via cascaded chi(2)-broadening and hollow-fiber compression followed by chirped mirrors. (2a) We generated few-cycle infrared pulses at 30 THz with locked carrier-envelope-phase. (2b) We also succeeded in producing THz pulses at much higher frequency by using a non-collinear optical parametric amplifier (cooperation with Prof. Riedle, as planned) followed by difference-frequency generation. The pulses at 8-11Âµm wavelength have >1ÂµJ energy and <1e9 V/m field strength in a focus. We pulses are currently being characterized and further optimized for stability, including CEP stability. (3a) We designed and simulated an entirely novel electron-pulse compressor based on THz radiation and numerically demonstrated its feasibility. (3b) We made an all-embracing numerical and analytical theory of light-electron interaction at metallic, dielectric and absorbing materials. Several discoveries were made, including an intimate relation between velocity-matching and zero deflection. This theory result proofs that pulses of arbitrarily large beam diameter can indeed be well compressed by THz radiation at membrane elements, as hoped for. (4) We produced intense single-cycle THz radiation at 0.3 THz central frequency that is suitable for THz electron compression. (5a) We demonstrated experimentally some initial steps towards practical electron pulse compression by optical fields. (5b) A major breakthrough was achieved: successful proof-of-principle compression of electron pulses from picoseconds to femtoseconds by using THz radiation (Kealhofer et al, Science 2016). (6). On the way to realizing ultrafast nanoscale imaging, we made a proof-of-principle experiment that proofs that indeed time-frozen electromagnetic field vectors can be recorded in space and time (Ryabov and Baum, Science 2016). Resolution improvements are currently in progress. (7-8) Preliminary progress and first ideas. In all work packages, we stayed within schedule. In some, in particular work package (5) (THz compression), we are even ahead of schedule. Overall, the project runs well, as planned and expected.

Final results

The realization of THz-compressed electron pulses and the concepts behind it are potentially transformative for other electron-beam-based experiments as well. 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 are expected until the end of the project are 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 are very confident to achieve it, as planned, in the second half of the project.