High-field THz driven dynamics
Recent example experiment: A Terahertz AC field at high repetition rate is used to align the orbital domains in La0.5Sr1.5MnO4 [3,4].
The low quantum energy of ultra-short pulses in the THz regime opens up the fascinating opportunity to excite low energy degrees of freedom in matter selectively and, if done in the right way, in its electronic ground state [1,2]. We are interested in ways to perturb and if possible manipulate matter by the transient electric and/or magnetic field transients of intense THz pulses (one recent success is the control of orbital order [3,4]).
|THz pump probe endstation @ FLASH|
The key technology is here to generate these THz pulses with as much control over its properties (number of cycles, polarization, chirp, field strength, repetition rate) as possible. To this end we operate several laser-based THz sources in our laboratory and operate and develop the High-field High-Repetition-Rate THz facility @ ELBE TELBE and also perform beamtimes at other high-field THz user facilities such as the one at the FLASH FEL [5,6].
TELBE provides world-wide unique repetition rates for high-field THz pulses up to the MHz regime  and thereby will enable to probe the THz driven dynamics with duty-cycle hungry spectroscopic techniqes such as time-resolved spectroscopic ellipsometry  time-resolved ARPES or time-resolved near-field microscopy. A part of our research is aiming at adapting such techniques to high-field THz sources with the goal to get a deeper understanding of the underlying phenomena of THz control in matter. One recent success in this quest is the successful demonstration of time-resolved scanning nearfield microscopy together with our collaborators from the TU Dresden .
Another advantage of the much higher repetition rate available from TELBE is an excellent dynamic range . A technologically important class of low energy excitations In the lower THz frequency range between 0.1 - 3 THz are magnetic resonances. One focus of the groups activity, pursued in collaboration with the FHI, MPI Mainz and the HZDR department FWIN, has meanwhile become the investigation of magnetization dynamics by means of linear and nonlinear THz spectroscopy [7,10,11].
The research is currently funded through the HGF, as well as the BMBF (TiNa - Time-resolved nanoscopy in the deep THz regime) and the European Union (Terahertz RAdio communication using high ANistropy SPIn-torque REsonators - TRANSPIRE)
Selective control of magnetization dynamics: (left) THz driven spin-wave excitations can be followed with high dynamic range utilizing the TELBE undulator source for selective excitation with a unique specral density by means of e.g. transient Faraday rotation [7,10]. This opens up to explore indications for the excitation of higher harmonics in thicker films (middle) and furthermore allows to employ the technique to technologically relevant nanofilms  (right).
 M. Foerst et. al., "THz Control in Correlated Electron Solids: Sources and Applications" in K.-E. Peiponen et. al., (eds.), Terahertz Spectroscopy and Imaging, 1 Springer Series in Optical Sciences 171, DOI: 10.1007/978-3-642-29564-5_23, Springer-Verlag Berlin Heidelberg 2012.
 A. Dienst et al., "Optical excitation of Josephson plasma solitons in a cuprate superconductor", Nat. Mat. 12 (2013), 535.
 T.A. Miller et. al., "Terahertz field control of in-plane orbital order in La0.5Sr1.5MnO4", Nat. Comm. 6 (2015), 8175.
 T. Miller, M. Gensch, S. Wall, "Light control of orbital domains: case of the prototypical manganite La0.5Sr1.5MnO4", Phys. Scrip. 91 (2016), 124002.
 B. Schuette et al., "Evidence for Chirped Auger-Electron Emission ", Phys. Rev. Lett. 108 (2012), 253003.
 U. Fruehling et al., "Single-shot terahertz-field-driven X-ray streak camera", Nat. Phot. 3 (2009), 523.
 B. Green et al., "High-Field High-Repetition-Rate Sources for the Coherent THz Control of Matter", Sci. Rep. 6 (2016), 22256.
 M. Gensch,“Brilliant Infrared Lightsources for Micro-Ellipsometric Studies of Organic Thin Films”, in Ellipsometry of Functional Organic Surfaces and Films, Springer Series in Optical Sciences 52, DOI 10.1007978-3-642-40128-2, © Springer-Verlag Berlin Heidelberg 2014.
 F. Kuschewski et. al., "Optical Nanoscopy of transient states of Matter", Scientific Reports (2015), 12582.
 S. Kovalev et al., "Probing ultra-fast processes with high dynamic range at 4th-generation light sources: Arrival time and intensity binning at unprecedented repetition rates", Struct. Dyn. 4 (2017), 024301.
 N. Awari et al., "Narrow-band tunable terahertz emission from ferrimagnetic Mn3-xGa thin films", App. Phys. Lett. 109 (2016), 032403.