FTh2B.3.pdf
Frontiers in Optics 2017 © OSA 2017
(Angle Resolved) Photoemission Spectroscopy Utilizing Attosecond Pulse Trains in Argon and Tungsten S. Heinrich1,2, A. Guggenmos1,2, F. Apfelbeck1, M. Stanislawski1, J. Schmidt1 and U. Kleineberg1,2 1
Fakultät für Physik, Ludwig-Maximilians-Universität München, Am Coulombwall 1, 85748 Garching, Germany 2 Max-Planck-Institut für Quantenoptik, Hans-Kopfermann-Str. 1, 85748 Garching, Germany
[email protected]
Abstract: We report on experiments with attosecond pulse trains that are focused on gas and solid state targets. Using the RABBITT-technique the emitted photoelectrons reveal information about the pulse train itself and the system under study. OCIS codes: (320.0320) Ultrafast optics; (340.7480) X-rays, soft x-rays, extreme ultraviolet (EUV) ; (320.5550) Pulses ; (240.6675) Surface photoemission and photoelectron spectroscopy
1. Introduction High harmonic generation (HHG) has enabled experiments with attosecond time resolution which allowed for significant insights into the photoemission process and electron dynamics in matter. So far, this has almost exclusively been investigated using isolated attosecond pulses (IAP) [1]. However, it has been demonstrated recently that by reconstruction of attosecond beating by two-photon transitions (RABBITT) it is possible to resolve phenomena on the attosecond timescale using attosecond pulse trains (APT) instead of IAPs [2]. Without the need for single-cycle-laser pulse generation RABBITT benefits in comparison to streaking with IAPs from lower probe intensities and measures simultaneously at different pump photon energies. Moreover, the excitation of a sample with the broad spectral bandwidth of IAPs doesn’t allow for photoemission measurements with sub-eV resolution [3]. In contrast to that, APTs feature spectrally very narrow harmonics that permit experiments with high energy resolution that can resolve typical band structures in solids and therefore offer the opportunity to combine attosecond time resolution with angle resolved photoemission spectroscopy (ARPES) [4]. 2. Experimental methods and results Our laser system consists of a Ti:Sa oscillator and a Ti:Sa multipass amplifier. It yields 29 fs pulses centered around 780 nm with a pulse energy of 400 μJ. The laser pulses are focused into an argon jet from which well separated high harmonics with cut-off energies of up to 60 eV are emitted. The FWHM of each harmonic is around 400 meV. The XUV pump and the NIR probe pulses are spatially separated and spectrally filtered around 44 eV photon energy at a double multilayer mirror. They are focused alternatively onto an argon gas target or tungsten crystal (W(110)). The emitted photoelectrons are detected with a time-of-flight spectrometer with angular resolution. The complete setup is depicted in Figure 1.
Fig. 1. The incoming 29 fs pulse is focused into an argon gas jet from which high harmonics are emitted. IR and XUV are then spatially separated by a 200 nm Al filter and delayed with respect to each other by a multilayer focusing double mirror. The photoelectrons from the target (argon or tungsten) are captured by an angle resolving electron time-of-flight spectrometer (Themis 1000, SPECS GmbH).
FTh2B.3.pdf
Frontiers in Optics 2017 © OSA 2017
Photoelectron spectra from an argon target are displayed in Figure 2a). With a 27 fs delay between XUV and IR there is no temporal overlap between pump and probe pulse and the individual high harmonics translate directly into corresponding peaks in the photoelectron spectrum shifted by the Ar-3p binding energy of ~15.9 eV (red line). If pump and probe pulse do overlap temporally (and spatially), two-photon transitions give rise to sidebands between the harmonic peaks (blue line). From the modulation of these sidebands as a function of the pump-probe delay the spectral phase of the harmonics and the temporal shape of the APT can be reconstructed [5]. Unlike photoelectrons from argon, the photoelectron spectrum from a tungsten target have a staircase-shaped form, which can be seen in Figure 2b) (red line). This can be explained if one takes into account the relatively broad tungsten valence band, which is where the electrons stem from. A convolution of the density of states of tungsten with the XUV spectrum (blue line) is in good agreement with the measured data. (a)
(b)
Fig. 2. a) Photoelectron spectra from an argon target. If there is a 27 fs delay between IR and XUV (red line) the visible peaks correspond to the high harmonics in the XUV spectrum. With overlap, two-photon transitions give rise to sidebands in between the harmonic peaks (blue line). b) Photoelectron spectra from a tungsten target. The measured curve (red line) agrees well with the simulated curve (blue line) that is obtained by convoluting the XUV spectrum with the electron density of states of tungsten.
3. Conclusion We have investigated photoelectron spectra from gas and solid state targets using attosecond pulse trains. In gas we were able to observe sidebands that can be used to characterize the APTs by means of the RABBITT-technique. The spectra that we obtain from tungsten targets have been well explained by accounting for the electron density of states of tungsten. In a next step, RABBITT and ARPES measurements on single crystal surfaces with attosecond time resolution are to be combined in order to gain insights about the dependance of electron dynamics on the band structure of solids.
4. References [1] S. Neppl, R. Ernstorfer, A. L. Cavalieri, C. Lemell, G. Wachter, E. Magerl, E. M. Bothschafter, M. Jobst, M. Hofstetter, U. Kleineberg, J. V. Barth, D. Menzel, J. Burgdörfer, P. Feulner, F. Krausz, and R. Kienberger, “Direct observation of electron propagation and dielectric screening on the atomic length scale,” Nature 517, 342–346 (2015). [2] Reto Locher, Luca Castiglioni, Matteo Lucchini, Michael Greif, Lukas Gallmann, Jürg Osterwalder, Matthias Hengsberger, and Ursula Keller, "Energy-dependent photoemission delays from noble metal surfaces by attosecond interferometry," Optica 2, 405-410 (2015). [3] Alexander Guggenmos, Ayman Akil, Marcus Ossiander, Martin Schäffer, Abdallah Mohammed Azzeer, Gerhard Boehm, Markus-Christian Amann, Reinhard Kienberger, Martin Schultze, and Ulf Kleineberg, "Attosecond photoelectron streaking with enhanced energy resolution for small-bandgap materials," Opt. Lett. 41, 3714-3717 (2016). [4] Z. Tao, C. Chen, T. Szilvási, M. Keller, M. Mavrikakis, H. Kapteyn, and M. Murnane, “Direct time-domain observation of attosecond finalstate lifetimes in photoemission from solids,” Science 353 (6294), 62–67 (2016). [5] P. M. Paul, E. S. Toma, P. Breger, G. Mullot, F. Augé, P. Balcou, H. G. Muller, and P. Agostini, “Observation of a train of attosecond pulses from high harmonic generation,” Science 292, 1689–1692 (2001).
