Karsten Holldack

Ultrafast spin transport as key to femtosecond demagnetization

Irradiating a ferromagnet with a femtosecond laser pulse is known to induce an ultrafast demagnetization within a few hundred femtoseconds. Here we demonstrate that direct laser irradiation is in fact not essential for ultrafast demagnetization, and that electron cascades caused by hot electron currents accomplish it very efficiently. We optically excite a Au/Ni layered structure in which the 30 nm Au capping layer absorbs the incident laser pump pulse and subsequently use the X-ray magnetic circular dichroism technique to probe the femtosecond demagnetization of the adjacent 15 nm Ni layer. A demagnetization effect corresponding to the scenario in which the laser directly excites the Ni film is observed, but with a slight temporal delay. We explain this unexpected observation by means of the demagnetizing effect of a superdiffusive current of non-equilibrium, non-spin-polarized electrons generated in the Au layer.

THz near-field imaging employing synchrotron radiation

Terahertz scanning near-field infrared microscopy below 1 THz is demonstrated at an electron storage ring using coherent synchrotron radiation. Spatial resolution below the diffraction limit down to about λ/40 at 2 cm−1 is derived from the transmittance spectra of a conical aperture probe. The potential of the technique is exemplified by imaging wet biological samples. Strongly absorbing living leaves have been imaged in transmittance with a spatial resolution of 130 μm at about 12 cm−1. The THz near-field images reveal distinct structural differences in the mesophytic and xerophytic leaves investigated.

A novel monochromator for experiments with ultrashort X‐ray pulses

Aiming at advancing storage-ring-based ultrafast X-ray science, over the past few years many upgrades have been undertaken to continue improving beamline performance and photon flux at the Femtoslicing facility at BESSY II. In this article the particular design upgrade of one of the key optical components, the zone-plate monochromator (ZPM) beamline, is reported. The beamline is devoted to optical pump/soft X-ray probe applications with 100 fs (FWHM) X-ray pulses in the soft X-ray range at variable polarization. A novel approach consisting of an array of nine off-axis reflection zone plates is used for a gapless coverage of the spectral range between 410 and 1333 eV at a designed resolution of E/ΔE = 500 and a pulse elongation of only 30 fs. With the upgrade of the ZPM the following was achieved: a smaller focus, an improved spectral resolution and bandwidth as well as excellent long-term stability. The beamline will enable a new class of ultrafast applications with variable optical excitation wavelength and variable polarization.

Zero-field splittings in metHb and metMb with aquo and fluoro ligands: a FD-FT THz-EPR study

A combined X-band and frequency-domain Fourier-transform THz electron paramagnetic resonance (FD-FT THz-EPR) approach has been employed to determine heme Fe(III) S = 5/2 zero-field splitting (ZFS) parameters of frozen metHb and metMb solutions, both with fluoro and aquo ligands. Frequency-domain EPR measurements have been carried out by an improved synchrotron-based FD-FT THz-EPR spectrometer. ZFS has been determined by field dependence of spin transitions within the mS = ±1/2 manifold, for all four protein systems, and by zero-field spin transitions between mS = ±1/2 and mS = ±3/2 levels, for metHb and metMb flouro-states. FD-FT THz-EPR data were simulated with a novel numerical routine based on Easyspin, which allows now for direct comparison of EPR spectra in field and frequency domain. We found purely axial ZFSs of D = 5.0(1) cm−1 (flouro-metMb), D = 9.2(4) cm−1 (aquo-metMb), D = 5.1(1) cm−1 (flouro-metHB) and D = 10.4(2) cm−1 (aquo-metHb).

Ultrafast dynamics of antiferromagnetic order studied by femtosecond resonant soft x-ray diffraction

Femtosecond (fs) soft x-ray diffraction at the Eu-M5 resonance was employed to study the dynamics of antiferromagnetic (AFM) order in a thin film of the magnetic semiconductor EuTe after fs laser excitation. The AFM Bragg peak intensity displays an ultrafast decay with an upper limit for the time constant of (700±200) fs followed by fast recovery with (47±10) ps. Reciprocal space scans across the diffraction peak could be recorded with picosecond and fs time resolution, demonstrating that even at synchrotron slicing sources ultrafast studies of spatial magnetic correlations are feasible. The results show that fs dynamics of materials with complex order parameters can be addressed.

THz near-field imaging of biological tissues employing synchrotron radiation

Terahertz scanning near-field infrared microscopy (SNIM) below 1 THz is demonstrated. The near-field technique benefits from the broadband and highly brilliant coherent synchrotron radiation (CSR) from an electron storage ring and from a detection method based on locking on to the intrinsic time structure of the synchrotron radiation. The scanning microscope utilizes conical waveguides as near-field probes with apertures smaller than the wavelength. Different cone approaches have been investigated to obtain maximum transmittance. Together with a Martin-Puplett spectrometer the set-up enables spectroscopic mapping of the transmittance of samples well below the diffraction limit. Spatial resolution down to about λ/40 at 2 wavenumbers (0.06 THz) is derived from the transmittance spectra of the near-field probes. The potential of the technique is exemplified by imaging biological samples. Strongly absorbing living leaves have been imaged in transmittance with a spatial resolution of 130 μm at about 12 wavenumbers (0.36 THz). The THz near-field images reveal distinct structural differences of leaves from different plants investigated. The technique presented also allows spectral imaging of bulky organic tissues. Human teeth samples of various thicknesses have been imaged between 2 and 20 wavenumbers (between 0.06 and 0.6 THz). Regions of enamel and dentin within tooth samples are spatially and spectrally resolved, and buried caries lesions are imaged through both the outer enamel and into the underlying dentin.

Simulating Frequency-Domain Electron Paramagnetic Resonance: Bridging the Gap between Experiment and Magnetic Parameters for High-Spin Transition-Metal Ion Complexes.

We present a comparison of experimental and simulated frequency- and field-domain electron paramagnetic resonance (EPR) spectra of integer and half-integer high-spin transition-metal ion complexes. For the simulation of EPR spectra a new tool within the EPR simulation software EasySpin is introduced, which allows for field- and frequency-domain EPR simulations with the same theoretical model and the same set of spin Hamiltonian parameters. The utility of this approach is demonstrated on the integer-spin complexes NiBr2(PPh3)2 and [Tp2Mn]SbF6 (both S = 1) and the half-integer-spin Fe(III) porphyrins, hemin (Fe(PPIX)Cl) and Fe(TPP)Cl (both S = 5/2). We demonstrate that the combination of field- and frequency-domain EPR techniques allows the determination of spin Hamiltonian parameters, in particular large zero-field splittings, with high accuracy.

Multifaceted magnetization dynamics in the mononuclear complex [ReIVCl4(CN)2]2−

The mononuclear complex (Bu4N)2[ReIVCl4(CN)2]·2DMA (DMA = N,N-dimethylacetamide) displays intricate magnetization dynamics, implying Orbach, direct, and Raman-type relaxation processes. The Orbach relaxation process is characterized by an energy barrier of 39 K (27 cm-1) that is discussed based on high-field electron paramagnetic resonance (EPR), inelastic neutron scattering and frequency-domain THz EPR investigations.

Phase-locked MHz pulse selector for x-ray sources

Picosecond x-ray pulses are extracted with a phase-locked x-ray pulse selector at 1.25 MHz repetition rate from the pulse trains of the accelerator-driven multiuser x-ray source BESSY II preserving the peak brilliance at high pulse purity. The system consists of a specially designed in-vacuum chopper wheel rotating with ≈1  kHz angular frequency. The wheel is driven in an ultrahigh vacuum and is levitated on magnetic bearings being capable of withstanding high centrifugal forces. Pulses are picked by 1252 high-precision slits of 70 μm width on the outer rim of the wheel corresponding to a temporal opening window of the chopper of 70 ns. We demonstrate how the electronic phase stabilization of ±2  ns together with an arrival time jitter of the individual slits of the same order of magnitude allows us to pick short single bunch x-ray pulses out of a 200 ns ion clearing gap in a multibunch pulse train as emitted from a synchrotron facility at 1.25 MHz repetition rate with a pulse purity below the shot noise detection limit. The approach is applicable to any high-repetition pulsed radiation source, in particular in the x-ray spectral range up to 10 keV. The opening window in a real x-ray beamline, its stability, as well as the limits of mechanical pulse picking techniques in the MHz range are discussed.

New Developments in Femtosecond Soft X‐ray Spectroscopy

Recent instrumentation developments in X‐ray spectroscopy for ultra‐fast time‐resolved measurements with soft X‐rays done in HZB Berlin during the last years are described. The significant performance improvements achieved this way are based on Fresnel diffraction from structures being fabricated on a surface of a total externally reflecting mirror. The first type of this spectrometer, an off‐axis reflection zone plate, has been implemented at the BESSY Femtoslicing setup and shows on the order of 20 times higher flux in the focal plane compared to the classical grating monochromator beamline. It has proven to serve very precise experiments with a time resolution down to 100 fs on magnetic materials after optical laser pulse excitation.