Search Results
Robust transverse structures in rescattered photoelectron wavepackets and their consequences
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.
Element-specific magnetization dynamics of complex magnetic systems probed by ultrafast magneto-optical spectroscopy
2020, Korff Schmising, Clemens von, Willems, Felix, Sharma, Sangeeta, Yao, Kelvin, Borchert, Martin, Hennecke, Martin, Schick, Daniel, Radu, Ilie, Strüber, Christian, Engel, Dieter W., Shokeen, Vishal, Buck, Jens, Bagschik, Kai, Viefhaus, Jens, Hartmann, Gregor, Manschwetus, Bastian, Grunewald, Soeren, Düsterer, Stefan, Jal, Emmanuelle, Vodungbo, Boris, Lüning, Jan, Eisebitt, Stefan
The vision to manipulate and control magnetism with light is driven on the one hand by fundamental questions of direct and indirect photon-spin interactions, and on the other hand by the necessity to cope with ever growing data volumes, requiring radically new approaches on how to write, read and process information. Here, we present two complementary experimental geometries to access the element-specific magnetization dynamics of complex magnetic systems via ultrafast magneto-optical spectroscopy in the extreme ultraviolet spectral range. First, we employ linearly polarized radiation of a free electron laser facility to demonstrate decoupled dynamics of the two sublattices of an FeGd alloy, a prerequisite for all-optical magnetization switching. Second, we use circularly polarized radiation generated in a laboratory-based high harmonic generation setup to show optical inter-site spin transfer in a CoPt alloy, a mechanism which only very recently has been predicted to mediate ultrafast metamagnetic phase transitions. © 2020 by the authors. Licensee MDPI, Basel, Switzerland.
Application of the Phase-Space Path Integral to Strong-Laser-Field-Assisted Electron-Ion Radiative Recombination: A Gauge-Covariant Formulation
2020, Esquembre Kučukalić, Ali, Becker, Wilhelm, Milošević, Dejan B.
We consider the problem of the choice of gauge in nonrelativistic strong-laser-field physics. For this purpose, we use the phase-space path-integral formalism to obtain the momentum-space matrix element of the exact time-evolution operator. With the assumption that the physical transition amplitude corresponds to transitions between eigenstates of the physical energy operator rather than the unperturbed Hamiltonian H0=(−i∂/∂r)2/2+V(r), we prove that the aforementioned momentum-space matrix elements obtained in velocity gauge and length gauge are equal. These results are applied to laser-assisted electron-ion radiative recombination (LAR). The transition amplitude comes out identical in length gauge and velocity gauge, and the expression agrees with the one conventionally obtained in length gauge. In addition to the strong-field approximation (SFA), which is the zeroth-order term of our expansion, we present explicit results for the first-order and the second-order terms, which correspond to LAR preceded by single and double scattering, respectively. Our general conclusion is that in applications to atomic processes in strong-field physics the length-gauge version of the SFA (and its higher-order corrections) should be used. Using the energy operator as the basis-defining Hamiltonian, we have shown that the resulting transition amplitude is gauge invariant and agrees with the form commonly derived in length gauge.
Phonon anharmonicities and ultrafast dynamics in epitaxial Sb2Te3
2020, Bragaglia, V., Ramsteiner, M., Schick, D., Boschker, J.E., Mitzner, R., Calarco, R., Holldack, K.
In this study we report on the investigation of epitaxially grown Sb2Te3 by employing Fourier-Transform transmission Spectroscopy (FTS) with laser-induced Coherent Synchrotron Radiation (CSR) in the Terahertz (THz) spectral range. Static spectra in the range between 20 and 120 cm−1 highlight a peculiar softening of an in-plane IR-active phonon mode upon temperature decrease, as opposed to all Raman active modes which instead show a hardening upon temperature decrease in the same energy range. The phonon mode softening is found to be accompanied by an increase of free carrier concentration. A strong coupling of the two systems (free carriers and phonons) is observed and further evidenced by exciting the same phonon mode at 62 cm−1 within an ultrafast pump-probe scheme employing a femtosecond laser as pump and a CSR single cycle THz pulse as probe. Separation of the free carrier contribution and the phonon resonance in the investigated THz range reveals that, both damping of the phonon mode and relaxation of hot carriers in the time domain happen on the same time scale of 5 ps. This relaxation is about a factor of 10 slower than expected from the Lorentz time-bandwidth limit. The results are discussed in the framework of phonon scattering at thermal and laser induced transient free carriers.
Optical inter-site spin transfer probed by energy and spin-resolved transient absorption spectroscopy
2020, Willems, Felix, von Korff Schmising, Clemens, Strüber, Christian, Schick, Daniel, Engel, Dieter W., Dewhurst, J. K., Elliott, Peter, Sharma, Sangeeta, Eisebitt, Stefan
Optically driven spin transport is the fastest and most efficient process to manipulate macroscopic magnetization as it does not rely on secondary mechanisms to dissipate angular momentum. In the present work, we show that such an optical inter-site spin transfer (OISTR) from Pt to Co emerges as a dominant mechanism governing the ultrafast magnetization dynamics of a CoPt alloy. To demonstrate this, we perform a joint theoretical and experimental investigation to determine the transient changes of the helicity dependent absorption in the extreme ultraviolet spectral range. We show that the helicity dependent absorption is directly related to changes of the transient spin-split density of states, allowing us to link the origin of OISTR to the available minority states above the Fermi level. This makes OISTR a general phenomenon in optical manipulation of multi-component magnetic systems.
Ultrashort vortex pulses with controlled spectral gouy rotation
2020, Liebmann, Max, Treffer, Alexander, Bock, Martin, Wallrabe, Ulrike, Grunwald, Ruediger
Recently, the spatio-spectral propagation dynamic of ultrashort-pulsed vortex beams was demonstrated by 2D mapping of spectral moments. The rotation of characteristic anomalies, so-called "spectral eyes", was explained by wavelength-dependent Gouy phase shift. Controlling of this spectral rotation is essential for specific applications, e.g., communication and processing. Here, we report on advanced concepts for spectral rotational control and related first-proof-of-principle experiments. The speed of rotation of spectral eyes during propagation is shown to be essentially determined by angular and spectral parameters. The performance of fixed diffractive optical elements (DOE) and programmable liquid-crystal-on silicon spatial light modulators (LCoS-SLMs) that act as spiral phase gratings (SPG) or spiral phase plates (SPP) is compared. The approach is extended to radially chirped SPGs inducing axially variable angular velocity. The generation of time-dependent orbital angular momentum (self-torque) by superimposing multiple vortex pulses is proposed. © 2020 by the authors.
Transient magnetic gratings on the nanometer scale
2020, Weder, D., von Korff Schmising, C., Günther, C.M., Schneider, M., Engel, D., Hessing, P., Strüber, C., Weigand, M., Vodungbo, B., Jal, E., Liu, X., Merhe, A., Pedersoli, E., Capotondi, F., Lüning, J., Pfau, B., Eisebitt, S.
Laser-driven non-local electron dynamics in ultrathin magnetic samples on a sub-10 nm length scale is a key process in ultrafast magnetism. However, the experimental access has been challenging due to the nanoscopic and femtosecond nature of such transport processes. Here, we present a scattering-based experiment relying on a laser-induced electro- and magneto-optical grating in a Co/Pd ferromagnetic multilayer as a new technique to investigate non-local magnetization dynamics on nanometer length and femtosecond timescales. We induce a spatially modulated excitation pattern using tailored Al near-field masks with varying periodicities on a nanometer length scale and measure the first four diffraction orders in an x-ray scattering experiment with magnetic circular dichroism contrast at the free-electron laser facility FERMI, Trieste. The design of the periodic excitation mask leads to a strongly enhanced and characteristic transient scattering response allowing for sub-wavelength in-plane sensitivity for magnetic structures. In conjunction with scattering simulations, the experiment allows us to infer that a potential ultrafast lateral expansion of the initially excited regions of the magnetic film mediated by hot-electron transport and spin transport remains confined to below three nanometers.
Probing multiphoton light-induced molecular potentials
2020, Kübel, M., Spanner, M., Dube, Z., Naumov, A.Yu., Chelkowski, S., Bandrauk, A.D., Vrakking, M.J.J., Corkum, P.B., Villeneuve, D.M., Staudte, A.
The strong coupling between intense laser fields and valence electrons in molecules causes distortions of the potential energy hypersurfaces which determine the motion of the nuclei and influence possible reaction pathways. The coupling strength varies with the angle between the light electric field and valence orbital, and thereby adds another dimension to the effective molecular potential energy surface, leading to the emergence of light-induced conical intersections. Here, we demonstrate that multiphoton couplings can give rise to complex light-induced potential energy surfaces that govern molecular behavior. In the laser-induced dissociation of H2+, the simplest of molecules, we measure a strongly modulated angular distribution of protons which has escaped prior observation. Using two-color Floquet theory, we show that the modulations result from ultrafast dynamics on light-induced molecular potentials. These potentials are shaped by the amplitude, duration and phase of the dressing fields, allowing for manipulating the dissociation dynamics of small molecules.
Micro Fresnel mirror array with individual mirror control
2020, Poyyathuruthy Bruno, Binal, Schütze, Robert, Grunwald, Ruediger, Wallrabe, Ulrike
We present the design and fabrication of a miniaturized array of piezoelectrically actuated high speed Fresnel mirrors with individual mirror control. These Fresnel mirrors can be used to generate propagation invariant and self-healing interference patterns. The mirrors are actuated using piezobimorph actuators, and the consequent change of the tilting angle of the mirrors changes the fringe spacing of the interference pattern generated. The array consists of four Fresnel mirrors each having an area of 2 × 2 mm2 arranged in a 2x2 configuration. The device, optimized using FEM simulations, is able to achieve maximum mirror deflections of 15 mrad, and has a resonance frequency of 28 kHz.
Imaging plasma formation in isolated nanoparticles with ultrafast resonant scattering
2020, Rupp, Daniela, Flückiger, Leonie, Adolph, Marcus, Colombo, Alessandro, Gorkhover, Tais, Harmand, Marion, Krikunova, Maria, Müller, Jan Philippe, Oelze, Tim, Ovcharenko, Yevheniy, Richter, Maria, Sauppe, Mario, Schorb, Sebastian, Treusch, Rolf, Wolter, David, Bostedt, Christoph, Möller, Thomas
We have recorded the diffraction patterns from individual xenon clusters irradiated with intense extreme ultraviolet pulses to investigate the influence of light-induced electronic changes on the scattering response. The clusters were irradiated with short wavelength pulses in the wavelength regime of different 4d inner-shell resonances of neutral and ionic xenon, resulting in distinctly different optical properties from areas in the clusters with lower or higher charge states. The data show the emergence of a transient structure with a spatial extension of tens of nanometers within the otherwise homogeneous sample. Simulations indicate that ionization and nanoplasma formation result in a light-induced outer shell in the cluster with a strongly altered refractive index. The presented resonant scattering approach enables imaging of ultrafast electron dynamics on their natural timescale.