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

 

Z. Chen, C. B. Curry, R. Zhang, F. Treffert, N. Stojanovic, S. Toleikis, R. Pan, M. Gauthier, E. Zapolnova, L. E. Seipp, A. Weinmann, M. Z. Mo, J. B. Kim, B. B. L. Witte, S. Bajt, S. Usenko, R. Soufli, T. Pardini, S. Hau-Riege, C. Burcklen, J. Schein, R. Redmer, Y. Y. Tsui, B. K. Ofori-Okai, S. H. Glenzer 

The experimental setup to measure terahertz (THz) transmission through free-standing gold foils excited by extreme ultraviolet (XUV) pulses.

https://www.nature.com/articles/s41467-021-21756-6

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.

Abstract-Observation of Terahertz Radiation via the Two-Color Laser Scheme with Uncommon Frequency Ratios

L.-L. Zhang, W.-M. Wang, D. Wu, R. Zhang, S.-J. Zhang, C.-L. Zhang, Y. Zhang, Z.-M. Sheng, X.-C. Zhang

(Submitted on 19 Oct 2017)

https://arxiv.org/abs/1710.06998

In the widely-studied two-color laser scheme for terahertz (THz) radiation from a gas, the frequency ratio of the two lasers is usually fixed at ω2/ω1=1:2. We investigate THz generation with uncommon frequency ratios. Our experiments show, for the first time, efficient THz generation with new ratios of ω2/ω1=1:4 and 2:3. We observe that the THz polarization can be adjusted by rotating the longer-wavelength laser polarization and the polarization adjustment becomes inefficient by rotating the other laser polarization; the THz energy shows similar scaling laws with different frequency ratios. These observations are inconsistent with multi-wave mixing theory, but support the gas-ionization model. This study pushes the development of the two-color scheme and provides a new dimension to explore the long-standing problem of the THz generation mechanism.