Spin-resolved photoelectron spectroscopy using femtosecond extreme ultraviolet light pulses from high-order harmonic generation

Abstract

The fundamental mechanism responsible for optically induced magnetization dynamics in ferromagnetic thin films has been under intense debate since almost two decades. Currently, numerous competing theoretical models are in strong need for a decisive experimental confirmation such as monitoring the triggered changes in the spin-dependent band structure on ultrashort time scales. Our approach explores the possibility of observing femtosecond band structure dynamics by giving access to extended parts of the Brillouin zone in a simultaneously time-, energy-and spin-resolved photoemission experiment. For this purpose, our setup uses a state-of-the-art, highly efficient spin detector and ultrashort, extreme ultraviolet light pulses created by laser-based high-order harmonic generation. In this paper, we present the setup and first spin-resolved spectra obtained with our experiment within an acquisition time short enough to allow pump-probe studies. Further, we characterize the influence of the excitation with femtosecond extreme ultraviolet pulses by comparing the results with data acquired using a continuous wave light source with similar photon energy. In addition, changes in the spectra induced by vacuum space-charge effects due to both the extreme ultraviolet probe-and near-infrared pump-pulses are studied by analyzing the resulting spectral distortions. The combination of energy resolution and electron count rate achieved in our setup confirms its suitability for spin-resolved studies of the band structure on ultrashort time scales. Published by AIP Publishing. [http://dx

Ultrafast demagnetization in iron: Separating effects by their nonlinearity

“…The experimental results indicate that the mechanism for the ultrafast loss of the magnetization is different from the mechanism causing the slow drop on the picosecond time scale 29 . We suggest that novel spin- and time-resolved photoemission experiments will be able to separate and identify the two effects 34,35 and shed further light on the fundamental processes underlying the mechanism for ultrafast demagnetization.…”

New Developments in Spin-Dependent Photoemission

“…The time resolution needs to be in the 100 fs regime to be able to address the time scales of electronic and spin excitations. For this purpose we have implemented a pulsed light source based on the high harmonic generation (HHG) principle into a spin-resolved photoemission experiment [57]. The HHG source is driven by a Ti:Sapphire laser (fundamental energy hν = 1.55) and provides XUV photon pulses at hν = 22.5 eV with less then 40 fs pulse width.…”

Ultrafast extreme ultraviolet photoemission without space charge

“…It is common practice to use the results obtained for simple metals in general considerations of the problem, and reports of the slope parameters m s,b in the literature have varied by a factor of 2 9,10,12,14,18 . Recently, using Cu (001), Plötzing et al 12 have studied these effects for multiple spot sizes and determined m b  = 2.1 × 10 −6  eV mm and m s  = 3.2 × 10 −6  eV mm with an estimated systematic uncertainty of less than 20%. Figure 1 illustrates the constraints on attainable sample current for a given resolution according to Eq.…”

“…(1) using m b from Ref. 12. The dashed lines indicate the space charge limits for sub-ps laser-based systems of different repetition rates assuming a 1 mm spot size—large by ARPES standards.…”

“…For space-charge-limited systems, the ( x , y ) positions represent the case where space charge broadening and the photon bandwidth add equally in quadrature. See Appendix A for detailed explanation on how symbol placement was calculated based on published results 12,14,27–32 …”