2023
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| 5. | S. Ito, M. Schüler, M. Meierhofer, S. Schlauderer, J. Freudenstein, R. Reimann, D. Afanasiev, K. A. Kokh, O. E. Tereshchenko, J. Güdde, M. A. Sentef, U. Höfer, R. Huber
Build-up and dephasing of Floquet-Bloch bands on subcycle timescales Journal Article In: Nature, vol. 616, pp. pages 696–701, 2023. @article{Ito2023,
title = {Build-up and dephasing of Floquet-Bloch bands on subcycle timescales},
author = {S. Ito and M. Schüler and M. Meierhofer and S. Schlauderer and J. Freudenstein and R. Reimann and D. Afanasiev and K. A. Kokh and O. E. Tereshchenko and J. Güdde and M. A. Sentef and U. Höfer and R. Huber
},
doi = {10.1038/s41586-023-05850-x},
year = {2023},
date = {2023-04-12},
urldate = {2023-04-12},
journal = {Nature},
volume = {616},
pages = {pages 696–701},
abstract = {Strong light fields have created opportunities to tailor novel functionalities of solids. Floquet–Bloch states can form under periodic driving of electrons and enable exotic quantum phases. On subcycle timescales, lightwaves can simultaneously drive intraband currents and interband transition, which enable high-harmonic generation and pave the way towards ultrafast electronics. Yet, the interplay of intraband and interband excitations and their relation to Floquet physics have been key open questions as dynamical aspects of Floquet states have remained elusive. Here we provide this link by visualizing the ultrafast build-up of Floquet–Bloch bands with time-resolved and angle-resolved photoemission spectroscopy. We drive surface states on a topological insulator with mid-infrared fields—strong enough for high-harmonic generation—and directly monitor the transient band structure with subcycle time resolution. Starting with strong intraband currents, we observe how Floquet sidebands emerge within a single optical cycle; intraband acceleration simultaneously proceeds in multiple sidebands until high-energy electrons scatter into bulk states and dissipation destroys the Floquet bands. Quantum non-equilibrium calculations explain the simultaneous occurrence of Floquet states with intraband and interband dynamics. Our joint experiment and theory study provides a direct time-domain view of Floquet physics and explores the fundamental frontiers of ultrafast band-structure engineering.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Strong light fields have created opportunities to tailor novel functionalities of solids. Floquet–Bloch states can form under periodic driving of electrons and enable exotic quantum phases. On subcycle timescales, lightwaves can simultaneously drive intraband currents and interband transition, which enable high-harmonic generation and pave the way towards ultrafast electronics. Yet, the interplay of intraband and interband excitations and their relation to Floquet physics have been key open questions as dynamical aspects of Floquet states have remained elusive. Here we provide this link by visualizing the ultrafast build-up of Floquet–Bloch bands with time-resolved and angle-resolved photoemission spectroscopy. We drive surface states on a topological insulator with mid-infrared fields—strong enough for high-harmonic generation—and directly monitor the transient band structure with subcycle time resolution. Starting with strong intraband currents, we observe how Floquet sidebands emerge within a single optical cycle; intraband acceleration simultaneously proceeds in multiple sidebands until high-energy electrons scatter into bulk states and dissipation destroys the Floquet bands. Quantum non-equilibrium calculations explain the simultaneous occurrence of Floquet states with intraband and interband dynamics. Our joint experiment and theory study provides a direct time-domain view of Floquet physics and explores the fundamental frontiers of ultrafast band-structure engineering. |
2022
|
| 4. | J. Freudenstein, M. Borsch, M. Meierhofer, D. Afanasiev, C. P. Schmid, F. Sandner, M. Liebich, A. Girnghuber, M. Knorr, M. Kira, R. Huber Attosecond clocking of correlations between Bloch electrons Journal Article In: Nature, vol. 610, pp. 290–295, 2022. @article{Freudenstein2022,
title = {Attosecond clocking of correlations between Bloch electrons},
author = {J. Freudenstein and M. Borsch and M. Meierhofer and D. Afanasiev and C. P. Schmid and F. Sandner and M. Liebich and A. Girnghuber and M. Knorr and M. Kira and R. Huber},
doi = {10.1038/s41586-022-05190-2},
year = {2022},
date = {2022-10-12},
urldate = {2022-10-12},
journal = {Nature},
volume = {610},
pages = {290--295},
abstract = {Delocalized Bloch electrons and the low-energy correlations between them determine key optical, electronic and entanglement functionalities of solids, all the way through to phase transitions. To directly capture how many-body correlations affect the actual motion of Bloch electrons, subfemtosecond (1 fs = 10−15 s) temporal precision is desirable. Yet, probing with attosecond (1 as = 10−18 s) high-energy photons has not been energy-selective enough to resolve the relevant millielectronvolt-scale interactions of electrons near the Fermi energy. Here, we use multi-terahertz light fields to force electron–hole pairs in crystalline semiconductors onto closed trajectories, and clock the delay between separation and recollision with 300 as precision, corresponding to 0.7% of the driving field’s oscillation period. We detect that strong Coulomb correlations emergent in atomically thin WSe2 shift the optimal timing of recollisions by up to 1.2 ± 0.3 fs compared to the bulk material. A quantitative analysis with quantum-dynamic many-body computations in a Wigner-function representation yields a direct and intuitive view on how the Coulomb interaction, non-classical aspects, the strength of the driving field and the valley polarization influence the dynamics. The resulting attosecond chronoscopy of delocalized electrons could revolutionize the understanding of unexpected phase transitions and emergent quantum-dynamic phenomena for future electronic, optoelectronic and quantum-information technologies.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Delocalized Bloch electrons and the low-energy correlations between them determine key optical, electronic and entanglement functionalities of solids, all the way through to phase transitions. To directly capture how many-body correlations affect the actual motion of Bloch electrons, subfemtosecond (1 fs = 10−15 s) temporal precision is desirable. Yet, probing with attosecond (1 as = 10−18 s) high-energy photons has not been energy-selective enough to resolve the relevant millielectronvolt-scale interactions of electrons near the Fermi energy. Here, we use multi-terahertz light fields to force electron–hole pairs in crystalline semiconductors onto closed trajectories, and clock the delay between separation and recollision with 300 as precision, corresponding to 0.7% of the driving field’s oscillation period. We detect that strong Coulomb correlations emergent in atomically thin WSe2 shift the optimal timing of recollisions by up to 1.2 ± 0.3 fs compared to the bulk material. A quantitative analysis with quantum-dynamic many-body computations in a Wigner-function representation yields a direct and intuitive view on how the Coulomb interaction, non-classical aspects, the strength of the driving field and the valley polarization influence the dynamics. The resulting attosecond chronoscopy of delocalized electrons could revolutionize the understanding of unexpected phase transitions and emergent quantum-dynamic phenomena for future electronic, optoelectronic and quantum-information technologies. |
2021
|
| 3. | R. Wallauer, R. Perea-Causin, L. Münster, S. Zajusch, S. Brem, J. Güdde, K. Tanimura, K. -Q. Lin, R. Huber, E. Malic, U. Höfer Momentum-Resolved Observation of Exciton Formation Dynamics in Monolayer WS2 Journal Article In: Nano Lett., vol. 21, pp. 5867–5873, 2021. @article{Wallauer2021,
title = {Momentum-Resolved Observation of Exciton Formation Dynamics in Monolayer WS2},
author = {R. Wallauer and R. Perea-Causin and L. Münster and S. Zajusch and S. Brem and J. Güdde and K. Tanimura and K. -Q. Lin and R. Huber and E. Malic and U. Höfer},
doi = {10.1021/acs.nanolett.1c01839},
year = {2021},
date = {2021-01-01},
journal = {Nano Lett.},
volume = {21},
pages = {5867--5873},
abstract = {The dynamics of momentum-dark exciton formation in transition metal dichalcogenides is difficult to measure experimentally, as many momentum-indirect exciton states are not accessible to optical interband spectroscopy. Here, we combine a tunable pump, high-harmonic probe laser source with a 3D momentum imaging technique to map photoemitted electrons from monolayer WS2. This provides momentum-, energy- and time-resolved access to excited states on an ultrafast time scale. The high temporal resolution of the setup allows us to trace the early-stage exciton dynamics on its intrinsic time scale and observe the formation of a momentum-forbidden dark KΣ exciton a few tens of femtoseconds after optical excitation. By tuning the excitation energy, we manipulate the temporal evolution of the coherent excitonic polarization and observe its influence on the dark exciton formation. The experimental results are in excellent agreement with a fully microscopic theory, resolving the temporal and spectral dynamics of bright and dark excitons in WS2.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
The dynamics of momentum-dark exciton formation in transition metal dichalcogenides is difficult to measure experimentally, as many momentum-indirect exciton states are not accessible to optical interband spectroscopy. Here, we combine a tunable pump, high-harmonic probe laser source with a 3D momentum imaging technique to map photoemitted electrons from monolayer WS2. This provides momentum-, energy- and time-resolved access to excited states on an ultrafast time scale. The high temporal resolution of the setup allows us to trace the early-stage exciton dynamics on its intrinsic time scale and observe the formation of a momentum-forbidden dark KΣ exciton a few tens of femtoseconds after optical excitation. By tuning the excitation energy, we manipulate the temporal evolution of the coherent excitonic polarization and observe its influence on the dark exciton formation. The experimental results are in excellent agreement with a fully microscopic theory, resolving the temporal and spectral dynamics of bright and dark excitons in WS2. |
2018
|
| 2. | J. Reimann, S. Schlauderer, C. P. Schmid, F. Langer, S. Baierl, K. A. Kokh, O. E. Tereshchenko, A. Kimura, C. Lange, J. Güdde, U. Höfer, R. Huber Subcycle observation of lightwave-driven Dirac currents in a topological surface band Journal Article In: Nature, vol. 562, pp. 396–400, 2018. @article{Reimann2018,
title = {Subcycle observation of lightwave-driven Dirac currents in a topological surface band},
author = {J. Reimann and S. Schlauderer and C. P. Schmid and F. Langer and S. Baierl and K. A. Kokh and O. E. Tereshchenko and A. Kimura and C. Lange and J. Güdde and U. Höfer and R. Huber},
doi = {10.1038/s41586-018-0544-x},
year = {2018},
date = {2018-01-01},
journal = {Nature},
volume = {562},
pages = {396--400},
abstract = {Harnessing the carrier wave of light as an alternating-current bias may enable electronics at optical clock rates1. Lightwave-driven currents have been assumed to be essential for high-harmonic generation in solids2–6, charge transport in nanostructures7,8, attosecond-streaking experiments9–16 and atomic-resolution ultrafast microscopy17,18. However, in conventional semiconductors and dielectrics, the finite effective mass and ultrafast scattering of electrons limit their ballistic excursion and velocity. The Dirac-like, quasi-relativistic band structure of topological insulators19–29 may allow these constraints to be lifted and may thus open a new era of lightwave electronics. To understand the associated, complex motion of electrons, comprehensive experimental access to carrier-wave-driven currents is crucial. Here we report angle-resolved photoemission spectroscopy with subcycle time resolution that enables us to observe directly how the carrier wave of a terahertz light pulse accelerates Dirac fermions in the band structure of the topological surface state of Bi2Te3. While terahertz streaking of photoemitted electrons traces the electromagnetic field at the surface, the acceleration of Dirac states leads to a strong redistribution of electrons in momentum space. The inertia-free surface currents are protected by spin–momentum locking and reach peak densities as large as two amps per centimetre, with ballistic mean free paths of several hundreds of nanometres, opening up a realistic parameter space for all-coherent lightwave-driven electronic devices. Furthermore, our subcycle-resolution analysis of the band structure may greatly improve our understanding of electron dynamics and strong-field interaction in solids.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Harnessing the carrier wave of light as an alternating-current bias may enable electronics at optical clock rates1. Lightwave-driven currents have been assumed to be essential for high-harmonic generation in solids2–6, charge transport in nanostructures7,8, attosecond-streaking experiments9–16 and atomic-resolution ultrafast microscopy17,18. However, in conventional semiconductors and dielectrics, the finite effective mass and ultrafast scattering of electrons limit their ballistic excursion and velocity. The Dirac-like, quasi-relativistic band structure of topological insulators19–29 may allow these constraints to be lifted and may thus open a new era of lightwave electronics. To understand the associated, complex motion of electrons, comprehensive experimental access to carrier-wave-driven currents is crucial. Here we report angle-resolved photoemission spectroscopy with subcycle time resolution that enables us to observe directly how the carrier wave of a terahertz light pulse accelerates Dirac fermions in the band structure of the topological surface state of Bi2Te3. While terahertz streaking of photoemitted electrons traces the electromagnetic field at the surface, the acceleration of Dirac states leads to a strong redistribution of electrons in momentum space. The inertia-free surface currents are protected by spin–momentum locking and reach peak densities as large as two amps per centimetre, with ballistic mean free paths of several hundreds of nanometres, opening up a realistic parameter space for all-coherent lightwave-driven electronic devices. Furthermore, our subcycle-resolution analysis of the band structure may greatly improve our understanding of electron dynamics and strong-field interaction in solids. |
2016
|
| 1. | T. L. Cocker, D. Peller, P. Yu, J. Repp, R. Huber Tracking the ultrafast motion of a single molecule by femtosecond orbital imaging Journal Article In: Nature, vol. 539, pp. 263–267, 2016. @article{Cocker2016,
title = {Tracking the ultrafast motion of a single molecule by femtosecond orbital imaging},
author = {T. L. Cocker and D. Peller and P. Yu and J. Repp and R. Huber},
doi = {10.1038/nature19816},
year = {2016},
date = {2016-01-01},
urldate = {2020-06-09},
journal = {Nature},
volume = {539},
pages = {263--267},
abstract = {Watching a single molecule move calls for measurements that combine ultrafast temporal resolution with atomic spatial resolution; this is now shown to be possible by combining scanning tunnelling microscopy with lightwave electronics, through a technique that involves removing a single electron from the highest occupied orbital of a single pentacene molecule in a time window shorter than an oscillation cycle of light.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Watching a single molecule move calls for measurements that combine ultrafast temporal resolution with atomic spatial resolution; this is now shown to be possible by combining scanning tunnelling microscopy with lightwave electronics, through a technique that involves removing a single electron from the highest occupied orbital of a single pentacene molecule in a time window shorter than an oscillation cycle of light. |
