FemtoSpeX: a versatile optical pump–soft X-ray probe facility with 100 fs X-ray pulses of variable polarization

Abstract

Here the major upgrades of the femtoslicing facility at BESSY II are reviewed, giving a tutorial on how elliptical-polarized ultrashort soft X-ray pulses from electron storage rings are generated at high repetition rates. Employing a 6 kHz femtosecond-laser system consisting of two amplifiers that are seeded by one Ti:Sa oscillator, the total average flux of photons of 100 fs duration (FWHM) has been increased by a factor of 120 to up to 10 6 photons s À1 (0.1% bandwidth) À1 on the sample in the range from 250 to 1400 eV. Thanks to a new beamline design, a factor of 20 enhanced flux and improvements of the stability together with the top-up mode of the accelerator have been achieved. The previously unavoidable problem of increased picosecond-background at higher repetition rates, caused by 'halo' photons, has also been solved by hopping between different 'camshaft' bunches in a dedicated fill pattern ('3 + 1 camshaft fill') of the storage ring. In addition to an increased X-ray performance at variable (linear and elliptical) polarization, the sample excitation in pumpprobe experiments has been considerably extended using an optical parametric amplifier that supports the range from the near-UV to the far-IR regime. Dedicated endstations covering ultrafast magnetism experiments based on timeresolved X-ray circular dichroism have been either upgraded or, in the case of time-resolved resonant soft X-ray diffraction and reflection, newly constructed and adapted to femtoslicing requirements. Experiments at low temperatures down to 6 K and magnetic fields up to 0.5 T are supported. The FemtoSpeX facility is now operated as a 24 h user facility enabling a new class of experiments in ultrafast magnetism and in the field of transient phenomena and phase transitions in solids.

Photoinduced Demagnetization and Insulator-to-Metal Transition in Ferromagnetic InsulatingBaFeO3Thin Films

“…Thus, BaFeO 3 thin films are appropriate samples to carry out time-resolved magnetic circular dichroism experiments. Furthermore, the investigation of the demagnetization dynamics of insulators allows one to relate electronic structure to magnetic dynamics.In order to investigate the magnetic dynamics of ferromagnetic insulating BaFeO 3 thin films, we performed time-resolved reflectivity studies at the Femtospex slicing facility at the synchrotron radiation source BESSY II [18], using circularly polarized x-ray pulses. Our experimental method has the advantage that, in one reflectivity experiment, we can probe electronic structure as well as magnetism.…”

“…In order to investigate the magnetic dynamics of ferromagnetic insulating BaFeO 3 thin films, we performed time-resolved reflectivity studies at the Femtospex slicing facility at the synchrotron radiation source BESSY II [18], using circularly polarized x-ray pulses. Our experimental method has the advantage that, in one reflectivity experiment, we can probe electronic structure as well as magnetism.…”

Design Study of Time-Preserving Grating Monochromators for Ultrashort Pulses in the Extreme-Ultraviolet and Soft X-Rays

“…Also, zone plates may be used for the monochromatization of ultrashort pulses, especially for high energies. Zone-plates monochromators are used in femtoslicing facilities with residual pulse elongation of a few tens of femtoseconds [17][18][19].…”

Capturing ultrafast magnetic dynamics by time-resolved soft x-ray magnetic circular dichroism

“…[15][16][17][18][19][20][21] The time-resolved x-ray magnetic circular dichroism (Tr-XMCD) and resonant soft x-ray scattering (Tr-RSXS) measurements for magnetic and electronic materials have been conducted in LCLS, 13,14 ALS, 9 and BESSY II slicing facilities. 10,11,[15][16][17][18][19] The timeresolved x-ray magneto optical Kerr effect (XMOKE) measurements have also been conducted in FERMI. 12 Tr-XMCD measurements at the M absorption below hv <100 eV have also been performed by using high harmonics generation from tabletop lasers in recent years.…”

“…The x-ray reflectivity and XMOKE measurements have also been performed for non-transparent thin films or bulk samples but with rather grazed settings for their magnetized axis. 11,12,19 Here, we presents a setting for Tr-XAS and Tr-XMCD in the partial electron yield (PEY) and total fluorescence yield (TFY) modes to measure non-transmissive as-grown samples at nearly normal incidence. The PEY mode, in which the emitted photoelectrons are measured, is rather surface sensitive but these spectra are known to usually be similar to those obtained in the TEY.…”