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

Abstract

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.

Nonequilibrium Band Occupation and Optical Response of Gold After Ultrafast XUV Excitation

“…The increasing lattice temperature exceeds melting temperature long before it equilibrates with the electron temperature, again in rough agreement with the experimental data of Ref. [12].…”

“…We demonstrate the capabilities of the three-band model (1) by a direct comparison with a recent experiment 12 . To that end, we consider excitation of gold with a XUV pump pulse of 150 fs duration (full width at half maximum), a wavelength of 13.6 nm and a total excitation energy of 0.89 MJkg −1 .…”

“…In turn, models for the important parameters ) data from Ref. [12] (a) n eq sp with g ei from Ref. [35] with g ei from Ref.…”

“…Theoretical models are needed here in two different ways: they provide estimates for the properties of highly excited matter that is created, and they also support the diagnostics by linking the microscopic states to the global behaviour measured. Interpreting transmission or reflectivity data [9][10][11][12] , X-ray or electron diffraction patterns [13][14][15] , or spectra from X-ray Thomson scattering [16][17][18] and X-ray near-edge spectroscopy 19,20 are examples where such models are required, in particular, for nonequilibrium conditions.…”

“…Moreover, FELs can provide radiation in the XUV and X-ray regimes that predominantly couples to core states and, thus, initiate a different excitation and relaxation pathway than visible light. Up to now, experiments employ pulse lengths that can resolve all relaxation stages mentioned above, except the fully kinetic stage, i.e., before the electrons have established a Fermi distribution 4,12 . Thus, a comprehensive theoretical model that allows a one-to-one comparison with the experiment needs to include the upper bands of the target material, at least one core state and all processes that are driven by the new, elevated electron temperature.…”