HELIOS
The Department of Physics and Astronomy operates a central facility intended to provide femtosecond optical and vacuum ultraviolet pulses to Uppsala University groups.
Description
HELIOS is an HHG source optimized for very short pulses (<40 fs) at comparably high photon energies of 60-80 eV. Accessing shallow core levels at 60-80 eV is ideally suited for ultrafast element-specific studies in magnetization and chemical dynamics through access to M absorption edges of 3d transition metals and M/N edges of Se, Br, I. Pulse lengths of <40 fs are essential for state-of-the-art chemical and magnetization dynamics studies (see, e.g., the molecular excited state dynamics time scale of 100 fs).
Future upgrades
We aim to expand the series of complementary ultrafast X-ray facilities with new capabilities for time-resolved X-ray spectroscopy that will enable a wide range of scientific applications at the institution. We plan to develop and use lab-based high-harmonic generation (HHG) to produce overtone harmonics for femtosecond optical lasers over a broad extended ultraviolet (XUV) and soft X-ray spectral range. We aim to realize two facilities: One optimized for ultrafast element-specific short-pulse X-ray spectroscopy at the existing HELIOS laboratory and one optimized for time- and angle-resolved photoemission spectroscopy with small spectral bandwidth at the FREIA laboratory. This requires the installation of two new femtosecond optical laser systems (laser systems I and II) which we are applying for here and we will provide matching funds to add installations to realize the described facilities.
Spectral and temporal resolution as well as pulse energy and repetition rate of an HHG X-ray source are determined by the properties of the driving femtosecond optical laser system. High spectral and temporal resolutions cannot be combined in one source due to the limitations imposed by the Fourier transform limit. Current laser technology limits the average power (pulse energy times repetition rate) of ultrafast lasers to 10-100 W. This prevents having both high peak power (high pulse energy) and high average power. The two areas of application ultrafast element-specific studies and time-resolved ARPES therefore require very different driving laser systems. Element-specific studies require high peak power at moderate repetition rates and short pulses (laser system I with pulse energies of several mJ and repetition rates of a few kHz). ARPES requires high average power (low peak power) at high repetition rates with longer pulses (laser system II with multiple 0.1 mJ pulses at multiple 100 kHz repetition rates).
At HELIOS, a new laser system will be used to generate XUV photon energies above the current 60 eV up to 200-300 eV, matching the core level energies of a range of elements relevant in ultrafast chemistry, photocatalysis and energy research. This will enable new time-resolved studies at the core level of hitherto inaccessible classes of materials such as perovskite solar cells or organic systems in solution. A second laser system will be the core of a new facility optimized for time-resolved ARPES at FREIA with an XUV spectral bandwidth of <20 meV, matching the excitation energies of elementary particles (charges, spins, phonons and magnons). This will enable state-of-the-art ARPES with exceptional spectral resolution and femtosecond time resolution for ultrafast materials science.