THz ARPES

Probing THz-driven changes to the electronic bandstructure with photoemission spectroscopy

We are currently setting up a new permanent end station for time-resolved terahertz (THz) pump – photoemission probe spectroscopy at TELBE. The goal is to enable groundbreaking experiments on THz-induced phenomena at surfaces and molecular-solid interfaces, thereby expanding the scope of scientific challenges at the TELBE user facility.

The main advantage of THz pumping, as compared to the traditionally-used optical excitation in the near infrared or visible spectral range in tr-ARPES experiments is, that it allows the direct excite the specific low-energy fundamental modes, such as lattice vibrations, magnons or free carriers, while avoiding  parasitic higher energy excitations of the electronic system. The direct access to the electronic band structure of a THz-driven solid E(k), offered by tr-ARPES, enables an unprecedented view on the dynamics of emergent phenomena in complex solids or molecular-solid interfaces. The new endstation at TELBE will allow investigating not yet fully understood phenomena such as charge order and high temperature superconductivity or surface chemistry and catalysis, which are examples of scientific questions with a potentially enormous technological impact on our society.

The TELBE light source is the ideal facility for realizing this project. It offers high-field THz pump pulses at a worldwide-unique high repetition rate of up to 500 kHz with sub-30 fs timing accuracy relative to external laser systems. This repetition rate, by about two orders of magnitude exceeding that of any typical high-field THz laser-based source, results in a superior dynamic range in experiments, leading to an unprecedented reduction of the THz - tr-ARPES measurement time to below 1 hour, which is a typical surface degradation time for most systems. The unique quasi-monochromaticity and frequency tunability of TELBE in the range from 0.1 THz to 2.5 THz is furthermore ideally suited to drive specific low energy excitations in matter selectively, e.g. into an excited spin but electronic ground state. Thereby, it will be possible to e.g. isolate the signatures of THz excited lattice or spin mode nonlinearities (i.e. clearly resolved higher harmonics of the pump THz field) in the response of the ARPES-probed electronic structure of materials. Establishing the proposed THz pump – tr-ARPES end station and eventually providing it to the broad user community requires tackling a number of technological, as well as scientific challenges. Most importantly, the strong interaction of nascent photoelectrons with the THz field (THz streaking) needs to be understood and implemented into the experimental analysis. Furthermore, solving the intricacies of generating sufficiently high THz fields at a precise position on a sample under ultrahigh vacuum (UHV) conditions will be a major step in the first phase of the project.

To ensure the highest performance of the endstation and to provide experiment time, we are operating a high power laser system providing XUV pulses with > 21 eV photon energy. This will enable access to an extensive range of high momentum states in the electronic band structure. In the course of the project, this light source will be expanded to provide THz radiation complementary to the TELBE source. The overall layout of the facility can be seen in the image above.

THz ARPES news

• July 2026: After a long downtime due to issues with the pump laser we have output from the OPCPA again.
• February 2026: We have added a laser table to the setup which gives us much needed space for the table-top THz setup and beam stabilization.