Towards direct access to THz-driven transient electronic states with time-resolved ARPES

In this contribution we present the concept for a novel experimental setup at the THz facility TELBE at Helmholtz-Zentrum Dresden-Rossendorf [1], which combines THz excitation with time- and angleresolved photoelectron spectroscopy (tr-ARPES). This challenging combination of two state-of-the-art techniques promises new insights into highly relevant non-equilibrium processes in matter, since THz excitation provides resonant access to a multitude of fundamental modes, e.g., lattice vibrations, molecular rotations, spin precession and the motion of free electrons. Until now, the corresponding changes to the electronic structure of a material can be probed only indirectly, e.g., by measuring the material's optical properties. The obvious method – tr-ARPES – which gives direct access to the electronic states on a femtosecond timescale (cf. Fig. 1) has not been implemented due to the previously unaccomplishable high duty cycle of the THz source (>> 10 kHz repetition rate quasi-cw) to provide sufficient statistics for tr-ARPES. This limitation is overcome at the TELBE which provides tunable and CEP-stable THz pulses at a repetition rate of 100 kHz based on superradiant emission, and timing stability of < 30 fs due to a novel pulse-to-pulse diagnostics scheme [3]. Over the next three years a tr-ARPES setup shall hence be implemented at the THz facility TELBE which aims at establishing feasibility and dynamic range despite obvious obstacles such as residual streaking of the photoelectrons by the high THz excitation fields (see, e.g., [2]). This contribution is discussing the challenges and the opportunities of tr-ARPES in THz control experiments and will outline the current design of the planned tr-ARPES endstation.
[1] B. Green et al., Sci. Rep. 6 (2016), 22256.
[2] U. Fruehling et al., Nat. Phot.. 3 (2009), 523.
[3] see poster presentation of S. Kovalev