Development of Advanced VUV/XUV sources
Ultrafast photoelectron spectroscopies with narrowband high-harmonics probe at the T-ReX and SPRINT facilities
Elettra Highlights 2018-2019; p. 144
Original Paper: R. Cucini et al., Structural Dynamics 7, 014303 (2020); DOI: 10.1063/1.5131216
Non-equilibrium photoelectron spectroscopies can reveal fundamental insights in the properties of materials. We developed a source of ultrafast photon pulses in the 15-35 eV range, obtained by HHG (High-Harmonic-Generation) of a turn-key laser source at 1030 nm. This source seeds two beamlines for advanced photoemision spectroscopy, at the T-ReX and SPRINT facilities at FERMI. Here, the photoelectron final state can be fully characterized, by measuring its energy, momentum and spin-polarization. Test experiments demonstrated <35 meV energy resolution, sub-100fs pulse duration and operation at 200 kHz repetition rate.
The possibility to produce ultrashort light pulses in the EUV (Extreme UltraViolet) range for spectroscopic applications, as produced by HHG (High Harmonic Generation) of near-infrared pulses from table-top laser sources, has emerged as a fundamental breakthrough in the field of time-resolved spectroscopies. Thanks to the recent availability of high-power, turn-key laser sources, it is nowadays possible to push HHG towards high-repetition rate operation. Hence, it became possible to perform equilibrium and out-of-equilibrium photoemission experiments in a regime in which the space-charge related issues are strongly mitigated and a high photon flux is available. Our development makes use of a Yb:KGW laser amplifier delivering 300 fs pulses at 1030 nm, with an energy-per-pulse of 100 μJ (at 200 kHz). The second harmonics at 515 nm is used to seed the HHG process. The HHG beamline we developed can provide light alternatively to two endstations, the T-ReX TR-ARPES endstation (a FERMI facility) and the SPRINT SPIN-ARPES endstation (a CNR – NFFA facility). Fig. 1a shows a sketch of the beamline. Fig. 1b shows a typical HHG spectrum, as recorded with a 400 gr/mm grating. Four harmonics are available, in the range 16.8 eV (7th) to 31.2 eV (13th). Fig. 1c shows the result of a photoemission measurement on a polycrystalline Au sample, with hν=21.6 eV. From this measurement, we retrieved the harmonics bandwidth, which equals to <35 meV. This source is matched to two state-of-the-art endstations devoted to Spin, Time and Angle-Resolved Photoelectron Spectroscopy experiments, open to users through the FERMI and NFFA call-for-users.
Figure 1: a) Spectrum of the high harmonics, obtained with 50 μJ/pulse energy at 515 nm, focused in Ar gas; b) Photoemission measurements of polycrystalline gold; from the Fermi edge width, we retrieve the harmonics bandwidth, of the order of 35 meV for harmonic 9.
Time-Resolved VUV ARPES at 10.8 eV photon energy and MHz repetition rate
Elettra Highlights 2019-2020; p. 70
Original Paper: S. Peli et al., J. Electron Spectrosc. Relat. Phenomena 243, 146978 (2020); DOI: 10.1016/j.elspec.2020.146978
The possibility to access the ultrafast electron dynamics over the full Brillouin zone of materials is one of the major open issues in Time-Resolved and Angle-Resolved Photoemission Spectroscopy (TR-ARPES). The two standard techniques employed at this scope, harmonics generation in non- linear crystals and high harmonics generation by ionization of inert gases (HHG), suffer either from the limited photon energy attainable or from the low energy resolution and photoemission count rates, respectively. Here, we describe a novel approach to produce light pulses at a photon energy of 10.8 eV, by employing third-order nonlinearities in Xe gas to generate the harmonics. This scheme allows operating at few MHz of repetition rate while preserving high energy resolution. The generation scheme developed to achieve photon pulses at 10.8 eV is shown in Fig. 1(a). An Yb-fiber based laser source provides pulses at 1035 nm (1.19 eV) with a duration of ~300 fs. The third harmonics of the fundamental beam is obtained through two up-conversion stages in BBO crystals. The resulting beam is focused in a cell filled with Xe gas, where a four-wave mixing process generates its third harmonics at 115 nm (10.8 eV); the two beams are finally separated by means of a MgF2 prism. Example measurements performed on the topological insulator Bi2Se3 show the high data quality obtainable with the new source. The repetition rate was set to 1 MHz in order to mitigate sample heating in the out-of-equilibrium experiment. We found that an energy- per-pulse of 10 μJ for the fundamental probe beam suffices to provide a suitable number of photoelectrons-per-second, while displaying no space charge effects. The quality of the equilibrium spectrum, shown in Fig. 1(b), allows to distinguish clearly, in addition to the topological surface state (TSS), the population of the bulk conduction band (BCB) and a narrower parabolic band, signaling the presence of a two-dimensional electron gas (EG) due to Se vacancies at the surface. The overall energy resolution of the experiment (~26 meV) is comparable with the analyzer resolution, hence is not limited by the probe bandwidth. The false color plot shown in Fig. 1(c) is obtained by subtracting the equilibrium spectrum shown in Fig. 1(b) from the one acquired 1 ps after photoexcitation at 1.19 eV. The transient population located above the Fermi level, indicated in red in Fig. 1(c), highlights the possibility to investigate the electron distribution and dynamics of the unoccupied states after photoexcitation. This experimental setup, operational at the T-ReX facility at FERMI and coupled to a state-of-the-art ARPES endstation, opens the possibility of performing ultrafast TR-ARPES measurements over the full first Brillouin zone of most complex materials, offering an unprecedented combination of energy-momentum resolutions and count rate.
Figure 1: (a) Generation scheme for the 10.8 eV photons: second harmonic generation of the fundamental (SHG), sum-frequency generation (SFG) between the second harmonic and the fundamental, and third harmonic generation (THG) in a cell filled with Xenon gas. (b) Equilibrium ARPES spectrum of Bi2Se3. TSS, topological surface state; BCB, bulk conduction band; EG, two-dimensional electron gas. (c) False color difference map between spectrum acquired 1 ps after the arrival of the pump and the one shown in (b).
