Ultrafast Dynamics of Magnetic Multilayer Films: Magneto-Optical Spectroscopy and Resonant Scattering in the Extreme Ultraviolet and Soft X-Ray Spectral Regions

This thesis focuses on studying ferromagnetic thin films with high temporal and spatial resolution using tabletop extreme ultraviolet (EUV) light sources based on high harmonic generation (HHG) and ultrafast soft X-rays from a free-electron laser. In Chapter 4, a new magneto-optical technique is developed. It allows a direct measurement of the full resonant complex EUV magneto-optical permittivity on a tabletop and thus, through a comparison with first principles calculations, is capable of capturing the microscopic mechanisms of ultrafast laser-induced demagnetization. It is found that, in Co, the demagnetization response is dominated by magnon excitations with possible smaller contributions from other mechanisms. Chapter 5 discusses the development of an efficient approach for resonant magnetic scattering (RMS) with a tabletop HHG source. This approach is used to study magnetic textures with spatial resolution. In an external magnetic field, a transition from a disordered network of stripe domains to an ordered lattice of magnetic vortices is observed in an Fe-Gd thin film. Chapter 6 presents the results of a dynamic soft X-ray RMS experiment on a disordered domain network performed at the Linear Coherent Light Source (LCLS). By directly applying the experimental data to a carefully simulated domain pattern, laser-induced transient changes in the domains are captured in real space, and strong non-uniformities in the demagnetization across the sample are observed. These non-uniformities are attributed to a combined effect of ultrafast spin-polarized currents and a gradient in the pump absorption throughout the thickness of the sample. Chapter 7 provides an outlook towards time-resolved lensless magnetic spectro-microscopy with HHG sources

Generation of bright phase-matched circularly-polarized extreme ultraviolet high harmonics

Circularly-polarized extreme ultraviolet and X-ray radiation is useful for analysing the structural, electronic and magnetic properties of materials. To date, such radiation has only been available at large-scale X-ray facilities such as synchrotrons. Here, we demonstrate the first bright, phase-matched, extreme ultraviolet circularly-polarized high harmonics source. The harmonics are emitted when bi-chromatic counter-rotating circularly-polarized laser pulses field-ionize a gas in a hollow-core waveguide. We use this new light source for magnetic circular dichroism measurements at the M-shell absorption edges of Co. We show that phase-matching of circularly-polarized harmonics is unique and robust, producing a photon flux comparable to linearly polarized high harmonic sources. This work represents a critical advance towards the development of table-top systems for element-specific imaging and spectroscopy of multiple elements simultaneously in magnetic and other chiral media with very high spatial and temporal resolution. Circularly-polarized radiation in the extreme ultraviolet (EUV)and soft X-ray spectral regions has proven to be extremelyuseful for investigating chirality-sensitive light–matter inter-actions. It enables studies of chiral molecules using photoelectron circular dichroism1, ultrafast molecular decay dynamics2, the direct measurement of quantum phases (for example, Berry’s phase and pseudo-spin) in graphene and topological insulators3,4 and reconstruction of band structure and modal phases in solids5

Influence of microscopic and macroscopic effects on attosecond pulse generation using two-color laser fields

Attosecond pulses and pulse trains generated by high-order harmonic generation are finding broad applications in advanced spectroscopies and imaging, enabling sub-femtosecond electron dynamics to be probed in atomic, molecular and material systems. To date, isolated attosecond pulses have been generated either by using very short few-cycle driving pulses, or by using temporal and polarization gating, or by taking advantage of phase-matching gating. Here we show that by driving high harmonics with a two-color linearly polarized laser field, the temporal window for time-gated phase matching is shorter than for the equivalent singe-color driving laser. As a result, we can generate quasi-isolated attosecond pulses with a peak width of ∼ 450 as using relatively long 26 femtosecond laser pulses. Our experimental data are in good agreement with theoretical simulations, and show that the phase matching window decreases by a factor of 4 - from four optical cycles in the case of a single-color fundamental driving laser, to one optical cycle in the case of two-color (ω-2ω) laser drivers. Finally, we also demonstrate that by changing the relative delay between the two-color laser fields, we can control the duration of the attosecond bursts from 450 as to 1.2 fs.National Science Foundation (NSF) (1125844); Air Force Office of Scientific Research (FA9550-16-1-0121); REA (328334); Junta de Castilla y León (SA046U16); MINECO (FIS2013-44174-P, FIS2016-75652-P)

Eye-Safe Solid-State Quasi-CW Raman Laser with Millisecond Pulse Duration

We demonstrate the first quasi-CW (ms-long pulses, pump duty cycle of 10%) end-diode pumped solid state laser generating eye-safe radiation via intracavity Raman conversion. The output power at the first Stokes wavelength (1524 nm) was 250 mW. A theoretical model was applied to analyze the laser system and provide routes for optimization. The possibility of true CW operation was discussed.Comment: Preprint accepted for publication in Optics Communications on Feb 6, 201

Bright, single helicity, high harmonics driven by mid-infrared bicircular laser fields

[EN]High-harmonic generation (HHG) is a unique tabletop light source with femtosecond-to-attosecond pulse duration and tailorable polarization and beam shape. Here, we use counter-rotating femtosecond laser pulses of 0.8 µm and 2.0 μm to extend the photon energy range of circularly polarized high-harmonics and also generate single-helicity HHG spectra. By driving HHG in helium, we produce circularly polarized soft x-ray harmonics beyond 170 eV—the highest photon energy of circularly polarized HHG achieved to date. In an Ar medium, dense spectra at photon energies well beyond the Cooper minimum are generated, with regions composed of a single helicity—consistent with the generation of a train of circularly polarized attosecond pulses. Finally, we show theoretically that circularly polarized HHG photon energies can extend beyond the carbon K edge, extending the range of molecular and materials systems that can be accessed using dynamic HHG chiral spectro-microscopiesDepartment of Energy BES (DE-FG02-99ER14982); Air Force Office of Scientific Research (FA9550-16-1-0121); National Science Foundation (DGE-1144083, DGE-1650115); European Research Council (8511201); Ministerio de Ciencia, Innovación y Universidades (PID2019-106910GB-100); Junta de Castilla y León (SA287P18); Ramón y Cajal contract (RYC-2017-22745)

Helicity-Selective Enhancement and Polarization Control of Attosecond High Harmonic Waveforms Driven by Bichromatic Circularly Polarized Laser Fields

source of bright, circularly polarized, extreme ultraviolet, and soft x-ray beams, where the individual harmonics themselves are completely circularly polarized. Here, we demonstrate the ability to preferentially select either the right or left circularly polarized harmonics simply by adjusting the relative intensity ratio of the bichromatic circularly polarized driving laser field. In the frequency domain, this significantly enhances the harmonic orders that rotate in the same direction as the higher-intensity driving laser. In the time domain, this helicity-dependent enhancement corresponds to control over the polarization of the resulting attosecond waveforms. This helicity control enables the generation of circularly polarized high harmonics with a user-defined polarization of the underlying attosecond bursts. In the future, this technique should allow for the production of bright highly elliptical harmonic supercontinua as well as the generation of isolated elliptically polarized attosecond pulses.H. K. and M. M. graciously acknowledge support from the Department of Energy BES Award No. DE-FG02- 99ER14982 for the experimental implementation, as well as a MURI grant from the Air Force Office of Scientific Research under Award No. FA9550-16-1-0121 for the theory. J. E. and C. M. acknowledge support from National Science Foundation Graduate Research Fellowships (Grant No. DGE-1144083). C. H.-G. acknowl- edges support from the Marie Curie International Outgoing Fellowship within the EU Seventh Framework Programme for Research and Technological Development (2007-2013), under REA Grant No. 328334, from Junta de Castilla y León (Project No. SA046U16) and Spanish Ministerio de Economía y Competitividad, MINECO (Projects No. FIS2013-44174-P and No. FIS2016-75652-P). Part of this work utilized the Janus supercomputer, which is sup- ported by the U.S. National Science Foundation (Grant No. CNS-0821794) and the University of Colorado Boulder

Phase matching of noncollinear sum and difference frequency high harmonic generation above and below the critical ionization level

We investigate the macroscopic physics of noncollinear high harmonic generation (HHG) at high pressures. We make the first experimental demonstration of phase matching of noncollinear high-order-difference-frequency generation at ionization fractions above the critical ionization level, which normally sets an upper limit on the achievable cutoff photon energies. Additionally, we show that noncollinear high-order-sum-frequency generation requires much higher pressures for phase matching than single-beam HHG does, which mitigates the short interaction region in this geometry. We also dramatically increase the experimentally realized cutoff energy of noncollinear circularly polarized HHG, reaching photon energies of 90 eV. Finally, we achieve complete angular separation of high harmonic orders without the use of a spectrometer.Department of Energy BES Award DE-FG02-99ER14982. MURI grant from the Air Force Office of Scientific Research under Award Number FA9550-16-1-0121. National Science Foundation Graduate Research Fellowship (DGE-1144083). Junta de Castilla y León (Project SA046U16) and Spanish MINECO (FIS2013-44174-P, FIS2016-75652-P)