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- ItemPhotoelectron holography in strong optical and dc electric fields(Bristol : Institute of Physics Publishing, 2014) Stodolna, A.; Huismans, Y.; Rouzée, A.; Lépine, F.; Vrakking, M.J.J.The application of velocity map imaging for the detection of photoelectrons resulting from atomic or molecular ionization allows the observation of interferometric, and in some cases holographic structures that contain detailed information on the target from which the photoelecrons are extracted. In this contribution we present three recent examples of the use of photoelectron velocity map imaging in experiments where atoms are exposed to strong optical and dc electric fields. We discuss (i) observations of the nodal structure of Stark states of hydrogen measured in a dc electric field, (ii) mid-infrared strong-field ionization of metastable Xe atoms and (iii) the reconstruction of helium electronic wavepackets in an attosecond pump-probe experiment. In each case, the interference between direct and indirect electron pathways, reminiscent of the reference and signal waves in holography, is seen to play an important role.
- ItemPhoton transport in one-dimensional systems coupled to three-level quantum impurities(Bristol : IOP, 2013) Martens, C.; Longo, P.; Busch, K.We discuss the transport properties of a single photon in a one-dimensional waveguide with an embedded three-level atom and utilize both stationary plane-wave solutions and time-dependent transport calculations to investigate the interaction of a photon with driven and undriven V- and Λ-systems. Specifically, for the case of an undriven V-system, we analyze the phenomenon of long-time occupation of the upper atomic levels in conjunction with almost dark states. For the undriven Λ-system, we find non-stationary dark states and we explain how the photon's transmittance can be controlled by an initial phase difference between the energetically lower-lying atomic states. With regard to the driven three-level systems, we discuss electromagnetically induced transparency in terms of the pulse propagation of a single photon through a Λ-type atom. In addition, we demonstrate how a driven V-type atom can be utilized to control the momentum distribution of the scattered photon.
- ItemA compact laboratory transmission X-ray microscope for the water window(Bristol : Institute of Physics Publishing, 2013) Legall, H.; Stiel, H.; Blobel, G.; Seim, C.; Baumann, J.; Yulin, S.; Esser, D.; Hoefer, M.; Wiesemann, U.; Wirtz, M.; Schneider, G.; Rehbein, S.; Hertz, H.M.In the water window (2.2-4.4 nm) the attenuation of radiation in water is significantly smaller than in organic material. Therefore, intact biological specimen (e.g. cells) can be investigated in their natural environment. In order to make this technique accessible to users in a laboratory environment a Full-Field Laboratory Transmission X-ray Microscope (L-TXM) has been developed. The L-TXM is operated with a nitrogen laser plasma source employing an InnoSlab high power laser system for plasma generation. For microscopy the Ly α emission of highly ionized nitrogen at 2.48 nm is used. A laser plasma brightness of 5 × 1011 photons/(s × sr × μm2 in line at 2.48 nm) at a laser power of 70 W is demonstrated. In combination with a state-of-the-art Cr/V multilayer condenser mirror the sample is illuminated with 106 photons/(μm2 × s). Using objective zone plates 35-40 nm lines can be resolved with exposure times < 60 s. The exposure time can be further reduced to 20 s by the use of new multilayer condenser optics and operating the laser at its full power of 130 W. These exposure times enable cryo tomography in a laboratory environment.
- ItemNO2molecular frame photoelectron angular distributions for a range of geometries using the R-matrix method(Bristol : IOP Publ., 2015) Brambila, Danilo S.; Harvey, Alex G.; MaÅ¡Ãn, ZdenÄ›k; Smirnova, OlgaWe present R-matrix calculations of photoionization from NO2, resolved in energy, angle, and both neutral and ionic state, for a range of molecular geometries, including in the vicinity of the 2A1/2B2 conical intersection.
- ItemHigh-order parametric generation of coherent XUV radiation(Washington, DC : Soc., 2021) Hort, O.; Dubrouil, A.; Khokhlova, M.A.; Descamps, D.; Petit, S.; Burgy, F.; Mével, E.; Constant, E.; Strelkov, V.V.Extreme ultraviolet (XUV) radiation finds numerous applications in spectroscopy. When the XUV light is generated via high-order harmonic generation (HHG), it may be produced in the form of attosecond pulses, allowing access to unprecedented ultrafast phenomena. However, the HHG efficiency remains limited. Here we present an observation of a new regime of coherent XUV emission which has a potential to provide higher XUV intensity, vital for applications. We explain the process by high-order parametric generation, involving the combined emission of THz and XUV photons, where the phase matching is very robust against ionization. This introduces a way to use higher-energy driving pulses, thus generating more XUV photons.
- ItemRobust transverse structures in rescattered photoelectron wavepackets and their consequences(Bristol : IOP Publ., 2020) Bredtmann, T.; Patchkovskii, S.Initial-state symmetry has been under-appreciated in strong-field spectroscopies, where laser fields dominate the dynamics. We demonstrate numerically that the transverse photoelectron phase structure, arising from the initial-state symmetry, is robust in strong-field rescattering, and has pronounced effects on strong-field photoelectron spectra. Interpretation of rescattering experiments need to take these symmetry effects into account. In turn, robust transverse photoelectron phase structures may enable attosecond sub-Ångström super-resolution imaging with structured electron beams.
- ItemStrong field ionization of small hydrocarbon chains with full 3D momentum analysis(Bristol : IOP Publ., 2015) Schulz, Claus Peter; Birkner, Sascha; Furch, Federico J.; Anderson, Alexandria; Mikosch, Jochen; Schell, Felix; Vrakking, Marc J. J.Strong field ionization of small hydrocarbon chains is studied in a kinematic complete experiment using a reaction microscope. By coincidence detection of ions and electrons different ionization continua populated during the ionization process are identified. In addition, photoelectron momentum distributions from laser-aligned molecules allow to characterize the electron wavepackets emerging from different Dyson orbitals.
- ItemAttosecond investigation of extreme-ultraviolet multi-photon multi-electron ionization(Washington, DC : OSA, 2022) Kretschmar, M.; Hadjipittas, A.; Major, B.; Tümmler, J.; Will, I.; Nagy, T.; Vrakking, M. J. J.; Emmanouilidou, A.; Schütte, B.Multi-electron dynamics in atoms and molecules very often occur on sub- to few-femtosecond time scales. The available intensities of extreme-ultraviolet (XUV) attosecond pulses have previously allowed the time-resolved investigation of two-photon, two-electron interactions. Here we study double and triple ionization of argon atoms involving the absorption of up to five XUV photons using a pair of intense attosecond pulse trains (APTs). By varying the time delay between the two APTs with attosecond precision and the spatial overlap with nanometer precision, we obtain information on complex nonlinear multi-photon ionization pathways. Our experimental and numerical results show that Ar2+ is predominantly formed by a sequential two-photon process, whereas the delay dependence of the Ar3+ ion yield exhibits clear signatures of the involvement of a simultaneous two-photon absorption process. Our experiment suggests that it is possible to investigate multi-electron dynamics using attosecond pulses for both pumping and probing the dynamics.
- ItemPhotoemission of Bi2Se3 with circularly polarized light: Probe of spin polarization or means for spin manipulation?(College Park : American Institute of Physics Inc., 2014) Sánchez-Barriga, J.; Varykhalov, A.; Braun, J.; Xu, S.-Y.; Alidoust, N.; Kornilov, O.; Minár, J.; Hummer, K.; Springholz, G.; Bauer, G.; Schumann, R.; Yashina, L.V.; Ebert, H.; Hasan, M.Z.; Rader, O.Topological insulators are characterized by Dirac-cone surface states with electron spins locked perpendicular to their linear momenta. Recent theoretical and experimental work implied that this specific spin texture should enable control of photoelectron spins by circularly polarized light. However, these reports questioned the so far accepted interpretation of spin-resolved photoelectron spectroscopy.We solve this puzzle and show that vacuum ultraviolet photons (50-70 eV) with linear or circular polarization indeed probe the initial-state spin texture of Bi2Se3 while circularly polarized 6-eV low-energy photons flip the electron spins out of plane and reverse their spin polarization, with its sign determined by the light helicity. Our photoemission calculations, taking into account the interplay between the varying probing depth, dipole-selection rules, and spin-dependent scattering effects involving initial and final states, explain these findings and reveal proper conditions for light-induced spin manipulation. Our results pave the way for future applications of topological insulators in optospintronic devices.
- ItemTowards shot-noise limited diffraction experiments with table-top femtosecond hard x-ray sources(Melville, NY : AIP Publishing LLC, 2017) Holtz, Marcel; Hauf, Christoph; Weisshaupt, Jannick; Salvador, Antonio-Andres Hernandez; Woerner, Michael; Elsaesser, ThomasTable-top laser-driven hard x-ray sources with kilohertz repetition rates are an attractive alternative to large-scale accelerator-based systems and have found widespread applications in x-ray studies of ultrafast structural dynamics. Hard x-ray pulses of 100 fs duration have been generated at the Cu Kα wavelength with a photon flux of up to 109 photons per pulse into the full solid angle, perfectly synchronized to the sub- 100-fs optical pulses from the driving laser system. Based on spontaneous x-ray emission, such sources display a particular noise behavior which impacts the sensitivity of x-ray diffraction experiments. We present a detailed analysis of the photon statistics and temporal fluctuations of the x-ray flux, together with experimental strategies to optimize the sensitivity of optical pump/x-ray probe experiments. We demonstrate measurements close to the shot-noise limit of the x-ray source.