Ultrafast High-Field THz beamline at X-ray FEL

THz sources at FLASH utilize spent electron beam from a soft X-ray FEL to generate very intense (up to 150µJ), tunable frequency (1-300THz) and ultrafast narrowband (~10%) THz pulses, which are naturally synchronized to soft X-ray pulses [1]. This unique combination allows for wide range of element specific pump-probe experiments in physics, material science and biology. Here we discuss the unique features of the FLASH THz pulses and the accelerator source that bring along a set of challenges in the diagnostics of their key parameters: pulse energy, spectral, temporal and spatial profiles.VII International School and Conference on Photonics : PHOTONICA2019 : Abstracts of Tutorial, Keynote, Invited Lectures, Progress Reports and Contributed Papers; August 26-30; Belgrad

Tunable High- field THz source at FLASH: Spectral and spatial characterization.

4 th generation X-ray light sources have attracted enormous attention of scientists from various fields, allowing conducting pump-probe experiments, which can reveal the mechanisms of chemical reactions and processes, occurring on the molecular and atomic level and on the time-scale of few femtoseconds. FLASH (Free Electron Laser in Hamburg) has a world-wide unique ability to generate tunable, broadband, high-field THz pulses, which are synchronized with XUV pulses on the order of few femtoseconds [1, 2]. It opens the door to new exiting THz pump X-ray probe experiments, such as THz driven magnetization and ion dynamics, coherent spin control, etc. For such experiments it’s extremely important to know the properties of the radiation, which is delivered to the experiment; therefore suitable tools for characterization are required. We are presenting a custom made ultra-broadband FTIR Spectrometer (Fourier Transform Infrared), optimised for THz source at FLASH and a 2D beam-profiler, both optimized to detect high-field, broadband THz radiation (0.6 µm to 600 um) and which are used as a part of the THz characterization tool, developed at THz beamline at FLASH. THz characterization tool also includes the Electro-optic Sampling (EOS) set up and Spectral Decoding (EOSD) set up, developed together in collaboration with the Institute of Physics and Vinca Institute of Nuclear Sciences in Belgrade, Serbia. We will also present the experiments, which have been conducted in THz beamline during last couple of years to give an overview on the exiting opportunities for light-matter interaction experiments with such an intense and broadband THz source.VI International School and Conference on Photonics and COST actions: MP1406 and MP1402 : PHOTONICA2017 : program and the book of abstracts; August 23 - September 1, 2017; Belgrad

En route: single-shot THz characterization technique for THz beamline at FLASH1

High-field THz radiation is a fascinating tool to study the interaction of electromagnetic waves with matter, such as demagnetization dynamics [1], tracking the ultrafast motion of a signal molecule [2], resonant control of states of matter [3] and et al. THz beamline at FLASH1 provides both tunable narrow bandwidth (1-30THz) and broad bandwidth intense THz pulses for user's experiment. Recently we have developed a THz characterization tool, optimized for FLASH's THz spectral range. It is based on THz electro-optic (EO) sampling, in combination of EO spectral decoding for THz pulse arrival timing jitter correction. It enables characterization of THz pulse, as a statistical average used in the experiment, with high temporal and spectral resolution. Because of the highly fluctuating nature of the THz generation process at FLASH, for certain class of experiments, knowledge of the individual THz pulse properties is essential. Thus a fast diagnostic of THz parameters is required and single-shot method offers clear advantages. The EO spectral decoding subsystem in the THz pulse characterization tool can be used as independent setup at performing single-shot measurements. However, due to the frequency interference in a long chirped laser pulse, the measured THz temporal profile is almost always distorted [4], particularly when characterizing ultra-broadband THz pulses with realistic probing laser. In order to study this single-shot technique, the broad bandwidth THz dump radiation is measured by EO spectral decoding setup. The distortion is analyzed in details and the measured profiles show agreement with our calculations. The measurements can help us to study possible retrieval strategies of the original THz electric field from the distorted measured one. This as a final goal has a single shot characterization of THz pulses for FLASH user experiments.VI International School and Conference on Photonics and COST actions: MP1406 and MP1402 : PHOTONICA2017 : program and the book of abstracts; August 23 - September 1, 2017; Belgrad

Electronic decay of core-excited HCl molecules probed by THz streaking

The ultrafast electronic decay of HCl molecules in the time domain after resonant core excitation was measured. Here, a Cl-2p core electron was promoted to the antibonding σ* orbital initiating molecular dissociation, and simultaneously, the electronic excitation relaxes via an Auger decay. For HCl, both processes compete on similar ultrashort femtosecond time scales. In order to measure the lifetime of the core hole excitation, we collinearly superimposed 40 fs soft x-ray pulses with intense terahertz (THz) radiation from the free-electron laser in Hamburg (FLASH). Electrons emitted from the molecules are accelerated (streaked) by the THz electric field where the resulting momentum change depends on the field's phase at the instant of ionization. Evaluation of a time-shift between the delay-dependent streaking spectra of photo- and Auger electrons yields a decay constant of (11 ± 2) fs for LMM Auger electrons. For further validation, the method was also applied to the MNN Auger decay of krypton. Reproduction of the value already published in the literature confirms that a temporal resolution much below the duration of the exciting x-ray pulses can be reached

Ultrafast multi-cycle terahertz measurements of the electrical conductivity in strongly excited solids

Key insights in materials at extreme temperatures and pressures can be gained by accurate measurements that determine the electrical conductivity. Free-electron laser pulses can ionize and excite matter out of equilibrium on femtosecond time scales, modifying the electronic and ionic structures and enhancing electronic scattering properties. The transient evolution of the conductivity manifests the energy coupling from high temperature electrons to low temperature ions. Here we combine accelerator-based, high-brightness multi-cycle terahertz radiation with a single-shot electro-optic sampling technique to probe the evolution of DC electrical conductivity using terahertz transmission measurements on sub-picosecond time scales with a multi-undulator free electron laser. Our results allow the direct determination of the electron-electron and electron-ion scattering frequencies that are the major contributors of the electrical resistivity