A.R. Khorsand

Spot size characterization of focused non-Gaussian X-ray laser beams

We present a new technique for the characterization of non-Gaussian laser beams which cannot be described by an analytical formula. As a generalization of the beam spot area we apply and refine the definition of so called effective area (A(eff)) [1] in order to avoid using the full-width at half maximum (FWHM) parameter which is inappropriate for non-Gaussian beams. Furthermore, we demonstrate a practical utilization of our technique for a femtosecond soft X-ray free-electron laser. The ablative imprints in poly(methyl methacrylate) - PMMA and amorphous carbon (a-C) are used to characterize the spatial beam profile and to determine the effective area. Two procedures of the effective area determination are presented in this work. An F-scan method, newly developed in this paper, appears to be a good candidate for the spatial beam diagnostics applicable to lasers of various kinds.

Soft x-ray free electron laser microfocus for exploring matter under extreme conditions

We have focused a beam (BL3) of FLASH (Free-electron LASer in Hamburg: lambda = 13.5 nm, pulse length 15 fs, pulse energy 10-40 microJ, 5 Hz) using a fine polished off-axis parabola having a focal length of 270 mm and coated with a Mo/Si multilayer with an initial reflectivity of 67% at 13.5 nm. The OAP was mounted and aligned with a picomotor controlled six-axis gimbal. Beam imprints on poly(methyl methacrylate) - PMMA were used to measure focus and the focused beam was used to create isochoric heating of various slab targets. Results show the focal spot has a diameter of < or =1 microm. Observations were correlated with simulations of best focus to provide further relevant information.

Optical excitation of thin magnetic layers in multilayer structures

To the Editor — Ultrafast laser-induced demagnetization is often ascribed to the absorption of light by electrons in the magnetic medium, with subsequent redistribution of energy and angular momentum1. Recently, an alternative mechanism was introduced, namely, a superdiffusive spin transport between adjacent layers2. Eschenlohr et al. claim to validate this mechanism1. In an optically excited Au/Ni layered structure, they found that the demagnetization of Ni can be up to 80%. It was calculated that light absorption in the magnetic layer is negligible compared with the absorption in the gold capping layer. Thus they conclude: “In strong contrast to existing knowledge we find that direct optical excitation is not a precondition for ultrafast demagnetization”. Knowledge of the exact absorption profile of light in multilayered thin films is crucial to disentangle thermal and non-thermal phenomena, such as in ref. 1. In this Correspondence, a comprehensive approach is given for absorption profile calculations for multilayer structures. In particular, we show that a crucial error was made in the calculations in ref. 1, which led to a misinterpretation of the experimental results, and a conclusion that cannot be substantiated with the presented experiment. The light intensity I(z) at position z in a material is defined by the Poynting vector (equation 24 in ref. 3), and is given by

Optical emission spectroscopy of various materials irradiated by soft x-ray free-electron laser

The beam of Free-Electron Laser in Hamburg (FLASH) tuned at either 32.5 nm or 13.7 nm was focused by a grazing incidence elliptical mirror and an off-axis parabolic mirror coated by Si/Mo multilayer on 20-micron and 1-micron spot, respectively. The grazing incidence and normal incidence focusing of ~10-fs pulses carrying an energy of 10 μJ lead at the surface of various solids (Si, Al, Ti, Ta, Si3N4, BN, a-C/Si, Ni/Si, Cr/Si, Rh/Si, Ce:YAG, poly(methyl methacrylate) - PMMA, stainless steel, etc.) to an irradiance of 1013 W/cm2 and 1016 W/cm2, respectively. The optical emission of the plasmas produced under these conditions was registered by grating (1200 lines/mm and/or 150 lines/mm) spectrometer MS257 (Oriel) equipped with iCCD head (iStar 720, Andor). Surprisingly, only lines belonging to the neutral atoms were observed at intensities around 1013 W/cm2. No lines of atomic ions have been identified in UV-vis spectra emitted from the plasmas formed by the FLASH beam focused in a 20-micron spot. At intensities around 1016 W/cm2, the OE spectra are again dominated by the atomic lines. However, a weak emission of Al+ and Al2+ was registered as well. The abundance ratio of Al/Al+ should be at least 100. The plasma is really cold, an excitation temperature equivalent to 0.8 eV was found by a computer simulation of the aluminum plasma OE spectrum. A broadband emission was also registered, both from the plasmas (typical is for carbon; there were no spectral lines) and the scintillators (on Ce:YAG crystal, both the luminescence bands and the line plasma emission were recorded by the spectrometer).

Non-thermal damage to lead tungstate induced by intense short-wavelength laser radiation (Conference Presentation)

Interaction of short-wavelength free-electron laser (FEL) beams with matter is undoubtedly a subject to extensive investigation in last decade. During the interaction various exotic states of matter, such as warm dense matter, may exist for a split second. Prior to irreversible damage or ablative removal of the target material, complicated electronic processes at the atomic level occur. As energetic photons impact the target, electrons from inner atomic shells are almost instantly photo-ionized, which may, in some special cases, cause bond weakening, even breaking of the covalent bonds, subsequently result to so-called non-thermal melting. The subject of our research is ablative damage to lead tungstate (PbWO4) induced by focused short-wavelength FEL pulses at different photon energies. Post-mortem analysis of complex damage patterns using the Raman spectroscopy, atomic-force (AFM) and Nomarski (DIC) microscopy confirms an existence of non-thermal melting induced by high-energy photons in the ionic monocrystalline target. Results obtained at Linac Coherent Light Source (LCLS), Free-electron in Hamburg (FLASH), and SPring-8 Compact SASE Source (SCSS) are presented in this Paper.

Laser-Induced Giant Skyrmions and Skyrmion-Compounds in a Thin Magnetic Film of TbFeCo

Here we report about the formation of topologically protected Skyrmions through illumination of a thin metallic film with short laser pulses (about 150 fs). By tuning the laser fluence it is possible to change the topological properties of the created structure.

Element-Specific Probing of Ultrafast Magnetization Dynamics in the Visible Spectral Range

Femtosecond laser excitation of thin magnetic films consisting of multiple magnetic sublattices triggers ultrafast spin dynamics and even magnetization reversal driven by the exchange interaction between the sublattices. To explore these intriguing phenomena requires element specific studies.We demonstrate that element specific probing of ultrafast spin dynamics can also be realized with visible light and does not require sophisticated X-ray facilities.

Ultrafast generation of nanostructures with tunable topological properties by single laser pulse illumination

Skyrmions, which have originally been introduced to explain how baryons could topologically emerge from a continuous meson field, have found many exciting applications in condensed-matter physics, where they describe topological states of matter in a wide range of systems. In magnetic materials they emerge as excitations corresponding to a spin arrangement in which the spins point in all the directions wrapping a sphere. Skyrmions have indeed been observed in chiral magnets, where they form regular lattices and are stabilized under an external magnetic field thanks to the presence of the Dzyaloshinskii-Moriya interaction (DMI). More recently, a new mechanism of Skyrmion materialization has been proposed, in which the frustration introduced in a thin ferromagnetic film by the magnetic dipole-dipole interaction leads to the stabilization of Skyrmions larger than those stabilized by DMI, consisting of magnetic domains at the center of which the magnetization points out of the film plane in the opposite direction with respect to the magnetization of the surrounding material. We report about the real-space observation by means of near-field optical Faraday microscopy of such stable Skyrmions in a thin TbFeCo film. The Skyrmions are generated after local excitation of the magnetic system by means of an intense laser pulse and do not need an external magnetic field for stabilization. The unique combination of ultrashort laser-induced magnetic excitation with subdiffraction near-field optical microscopy allows us not only to produce and observe Skyrmions as individual entities, but to also create and characterize bound Skyrmion-antiSkyrmion pairs, forming a topologically neutral entity.

Perspective for high energy density studies using x-ray free electron lasers

A general overview of the potential for both warm and hot dense matter research for the future will be presented. First, a discussion of the regime defined as relevant to warm dense matter will be attempted in terms of the underlying physical phenomena that define the field. Next a categorization of the facilities to be included in the perspective will be given. With this as background a series of schematic experiments will be discussed with respect to the facilities where they will be pursued. Comments on the interaction amongst the various experiments and between the various facilities will be outlined.

Perspective for high energy density studies on x-ray FELs

We report on the x-ray absorption of Warm Dense Matter experiment at the FLASH Free Electron Laser (FEL) facility at DESY. The FEL beam is used to produce Warm Dense Matter with soft x-ray absorption as the probe of electronic structure. A multilayer-coated parabolic mirror focuses the FEL radiation, to spot sizes as small as 0.3μm in a ~15fs pulse of containing >1012 photons at 13.5 nm wavelength, onto a thin sample. Silicon photodiodes measure the transmitted and reflected beams, while spectroscopy provides detailed measurement of the temperature of the sample. The goal is to measure over a range of intensities approaching 1018 W/cm2. Experimental results will be presented along with theoretical calculations. A brief report on future FEL efforts will be given.