Measurement station for ultrafast terahertz-driven photoemission spectroscopy

• Subject Area: Condensed Matter Physics
• Term: Funded in 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 465668329

The proposed instrument essentially combines tailored strong-field terahertz (THz) excitation with time-resolved angular-resolved photoemission spectroscopy (tr-ARPES) experiment. It will allow us to observe, with sub-cycle time resolution and fast enough to prevent the surface degradation, the ultrafast nonequilibrium dynamics in a wide class of materials, under unique conditions of powerful “bottom-up” excitation of each material’s fundamental, lowest-energy modes with THz radiation. Physical problems such as energy flow and the development of ultrafast non-equilibrium in Dirac materials (e.g. graphene, 3D Dirac semimetals, topological insulators), magnetic materials (e.g. rare-earth orthoferrites), Peierls semimetals (bismuth or arsenic) and van der Waals heterostructures (e.g. WSe2) will be studied, under the unique conditions of direct excitation of lowest-energy modes in the materials (conduction currents around the Fermi level, infrared-active optical phonons, magnons). “Bottom-up” THz excitation of materials will avoid the parasitic excitations of electronic or lattice subsystems, such as in the case of ultrafast optical excitation, leading to electronic band-to-band transitions and Raman excitation of the lattice. Using ARPES, we will thus be able to directly observe the effect of bottom-up energy deposition into the material on bandstructure and population. Further, the effect of photoemission itself in the presence of external strong THz-fields will be studied, by photoelectron streaking in “slower” (multi-)THz driving fields, as compared to typically used infrared signals. This will provide us unique access to longer timescales in photoemission dynamics. Our setup will be powered by a femtosecond laser delivering the pulses of 5 mJ energy and central wavelength of 1030 nm, at a repetition rate of 100 kHz. The laser will be equipped with light conversion stages allowing emission from EUV to THz. The high repetition rate of the laser enables the acquisition of time- and angle-resolved photoemission data in short enough acquisition times to prevent sample surface degradation. This will allow us to perform our experiments on a wide class of “vulnerable” materials, such as (semi-)metals, semiconductors, magnetic materials, molecular-decorated surfaces etc. As an electron analyzer, the momentum microscope will be used, which, apart from the information on the energy-momentum dependency of the photoemitting electronic state, also allows operation in an imaging mode for sample inspection. A variable temperature sample stage will allow us, in particular, to study the dynamics of materials undergoing phase transitions. The relatively high pulse energy of the driving laser of 5 mJ will enable strong-field THz generation via optimized optical rectification, yielding single- and multi-cycle THz pulses with electric field strength in the rage 0.1 – 1 MV/cm, and with the frequency content 0.1 – 40 THz.

• DFG Programme: Major Research Instrumentation
• Major Instrumentation: Messstation für terahertz-getriebene Photoemissionsspektroskopie mit Ultrakurzzeitauflösung
• Instrumentation Group: 5700 Festkörper-Laser
• Applicant Institution: Universität Bielefeld
• Leader: Professor Dmitry Turchinovich, Ph.D.