Main Content
2020
Inhalt ausklappen Inhalt einklappen M. Reutzel, A. Li, Z. Wang, and H. Petek, Coherent multidimensional photoelectron spectroscopy of ultrafast quasiparticle dressing by light, Nature Communications 11, 1-5 (2020).
Depending on the applied strength, electromagnetic fields in electronic materials can induce dipole transitions between eigenstates or distort the Coulomb potentials that define them. Between the two regimes, they can also modify the electronic properties in more subtle ways when electron motion becomes governed by time and space-periodic potentials. The optical field introduces new virtual bands through Floquet engineering that under resonant conditions interacts strongly with the preexisting bands. Under such conditions the virtual bands can become real, and real ones become virtual as the optical fields and electronic band dispersions entangle the electronic response. We reveal optical dressing of electronic bands in a metal by exciting four-photon photoemission from the Cu(111) surface involving a three-photon resonant transition from the Shockley surface band to the first image potential band. Attosecond resolved interferometric scanning between identical pump–probe pulses and its Fourier analysis reveal how the optical field modifies the electronic properties of a solid through combined action of dipole excitation and field dressing.
DOI: 10.1038/s41467-020-16064-4Inhalt ausklappen Inhalt einklappen M. Keunecke, M. Reutzel, D. Schmitt, A. Osterkorn, T. A. Mishra, C. Möller, W. Bennecke, G. S. M. Jansen, D. Steil, S. R. Manmana, S. Steil, S. Kehrein, and S. Mathias, Electromagnetic dressing of the electron energy spectrum of Au(111) at high momenta, Physical Review B 102, 161403 (2020).
Light-engineering of quantum materials via electromagnetic dressing is considered an on-demand approach for tailoring electronic band dispersions and even inducing topological phase transitions. For probing such dressed bands, photoemission spectroscopy is an ideal tool, and we employ here a novel experiment based on ultrafast photoemission momentum microscopy. Using this setup, we measure the in-plane momentum-dependent intensity fingerprints of the electromagnetically-dressed sidebands from a Au(111) surface for 𝑠- and 𝑝-polarized infrared driving. We find that at metal surfaces, due to screening of the driving laser, the contribution from Floquet-Bloch bands is negligible, and the dressed bands are dominated by the laser-assisted photoelectric effect. Also, we find, from calculations, that in contrast to general expectations, 𝑠-polarized light can dress free-electron states at large photoelectron momenta. Our results show that the dielectric response of the material must carefully be taken into account when using photoemission for the identification of light-engineered electronic band structures.
DOI: 10.1103/PhysRevB.102.161403Inhalt ausklappen Inhalt einklappen A. Li, N. A. James, T. Wang, Z. Wang, H. Petek, and M. Reutzel, Towards full surface Brillouin zone mapping by coherent multi-photon photoemission, New Journal of Physics 22, 073035 (2020).
We report a novel approach for coherent multi-photon photoemission in the entire Brillouin zone with infrared light that is readily implemented in a laboratory setting. We excite a solid state material, Ag(110), with intense femtosecond laser pulses to excite higher-order multi-photon photoemission; angle-resolved electron spectroscopic acquisition records photoemission at large in-plane momenta involving optical transitions from the occupied to unoccupied bands of the sample that otherwise might remain hidden by the photoemission horizon. We propose this as a complementary ultrafast method to time- and angle-resolved two-color, e.g. infrared pump and extreme ultraviolet probe, photoemission spectroscopy, with the advantage of being able to measure and control the coherent electron dynamics.
DOI: 10.1088/1367-2630/ab98d6Inhalt ausklappen Inhalt einklappen M. Keunecke, C. Möller, D. Schmitt, H. Nolte, G. S. M. Jansen, M. Reutzel, M. Gutberlet, G. Halasi, D. Steil, S. Steil, and S. Mathias, Time-resolved momentum microscopy with a 1 MHz high-harmonic generation extreme ultraviolet beamline, Review of Scientific Instruments 91, 063905 (2020).
Recent progress in laser-based high-repetition rate extreme ultraviolet (EUV) light sources and multidimensional photoelectron spectroscopy enables the build-up of a new generation of time-resolved photoemission experiments. Here, we present a setup for time-resolved momentum microscopy driven by a 1 MHz fs EUV table-top light source optimized for the generation of 26.5 eV photons. The setup provides simultaneous access to the temporal evolution of the photoelectron’s kinetic energy and in-plane momentum. We discuss opportunities and limitations of our new experiment based on a series of static and time-resolved measurements on graphene.
DOI: 10.1063/5.0006531Inhalt ausklappen Inhalt einklappen T. Bohamud, M. Reutzel, A. Adamkiewicz, U. Höfer, and M. Dürr, Electric-field-induced depolarization of Si-C bond leads to a strongly reduced barrier for alkyl-hopping on Si(001), The Journal of Physical Chemistry C 124, 5270-5274 (2020).
Tip-induced hopping of the ethyl fragment (−C2H5) of diethyl ether molecules reacted on Si(001) was shown to be a field-driven process. Although the hopping rate increases continuously with increasing bias voltage, it remains constant when varying the tunneling current. No hopping events are observed at 50 K. The process is thus concluded to be thermally activated with the respective energy barrier being reduced by the applied electric field. At a positive sample bias, the field in the tunneling gap is strong enough to effectively depolarize and thus weaken the covalent Si–C bond. The effect of this depolarization on the hopping barrier is quantified and compared to the strength of the electric field.
DOI: 10.1021/acs.jpcc.0c00656Inhalt ausklappen Inhalt einklappen M. Reutzel, A. Li, and H. Petek, Above-threshold multiphoton photoemission from noble metal surfaces, Physical Review B 101, 075409 (2020).
Exciting solids with intense femtosecond laser pulses prompts electrons of the interrogated material to respond in a highly nonlinear manner, as is evident in the emission of high-order harmonic radiation and photoelectrons with kinetic energies well above that of the driving photons. Such high-field interactions can be resolved, for example, in above-threshold multiphoton photoemission (ATP) spectroscopy. In this work, we interrogate the nonlinear photoelectric responses of the pristine copper, silver, and gold noble metal surfaces in (111) and (100) crystal orientations in the perturbative regime. Using multiphoton photoemission spectroscopy (mPP) excited by finely tuned optical fields, we characterize enhancement of the mPP and ATP yields from (111) surfaces in selected 𝑘||-momentum ranges when the occupied Shockley surface (SS) states are (near-)resonantly coupled by multiphoton transitions to image potential (IP) intermediate states in the excitation process. The ATP signal from the IP states of (111) surfaces is largely defined by their formation through polarization of SS electrons; this observation is contrasted with ATP experiments from the Ag(100) surface, for which the SS becomes an unoccupied resonance and the IP states can only be excited from bands with significantly more bulk character. In addition, based on the optical power and nonlinear order-dependent mPP spectra, we provide evidence for ATP being a one-step, rather than a sequential process, as previously postulated. DOI: 10.1103/PhysRevB.101.075409