Ultrafast dynamics in the charge-density-wave material (TaSe4)2I induced by near-infrared and x-ray excitation

Non-equilibrium spectroscopy is a powerful tool to study properties of complex materials, combining both spectroscopical and temporal information. In the present work we are following a common ansatz, perturbing an ordered state by an intense, ultrashort light pulse and tracing the relaxation of the material back to its equilibrium. However, depending on the energy of the exciting pulse, different excitation mechanisms and pathways are possible: we aim to investigate precisely this issue. In general, while excitation with optical pulses directly addresses the valence band structure of a material, photons in the x-ray range also excite electrons from deeper-lying core levels. These excitations can couple to the valence band structure via secondary processes as x-ray induced fluorescence or Auger effects. X-ray induced optical reflectivity changes have been established as a tool for beam characterization at diverse large-scale facilities, in order to determine femtosecond x-ray/optical cross-correlations. But more than pure technical significance, it also offers the possibility of studying ultrafast many-body responses and opens a new research field, giving access to interesting physics. However, the number of studied materials offering insights in the effect of x-ray excitations on their optical properties is still small.

Figure 1: (a) Time-resolved optical spectroscopy setup for simultaneous broadband probing along two perpendicular crystal axis and pumping at 1.55 eV; (b) Time-resolved optical spectroscopy set-up for x-ray FEL pumping and 1.55 eV probing simultaneously along two crystal axis; (c) Comparison of differential reflectivity transients along the chain direction in (TaSe₄)₂I; (d) Comparison of differential reflectivity transients perpendicular to the chain direction in (TaSe₄)₂I. Adapted from W. Bronsch et al., Faraday Discussions, Advanced Article (2022), DOI: 10.1039/D2FD00019A.

Here, we are presenting a comprehensive study on light-induced optical reflectivity changes in dependence of photon energy and bandstructure. As a platform of this study we chose the paradigmatic quasi one-dimensional material (TaSe₄)₂I, which obeys metallic character along its crystal c-axis and insultating properties along the perpendicular a-axes. By probing light-induced changes in the reflectivity of the material along the different crystal directions we can hence study changes in the dielectric function in an example metal and insulator due to the external stimulus. Using the broadband-probe optical spectroscopy table-top setup available at the T-ReX facility, we are exciting the electronic structure of the sample with a near-infrared beam at 1.55 eV (cf. Fig. 1a). The supercontinuum probe beam, which is reflected at the sample surface, is then split with a Wollaston prism and guided to two detection units in order to disentangle the information along (R_||) and perpendicular to (R_⊥) the metallic chains of (TaSe₄)₂I . The use of a broadband probe, covering the spectral range 1.4-2.6 eV, allows for modelling the photoinduced changes in the dielectric function with a Drude-Lorentz model, hence tracking the temporal evolution of the single paramenters describing intraband and interband excitations. We are comparing our knowledge gained from the analysis of the broadband optical data with data gained after x-ray excitation, in order to learn how the x-ray excitation couples to the optical properties of the sample. To this purpose, a similar setup as used for the broadband optical spectroscopy experiment was used at the TIMER beamline at FERMI (cf. Fig. 1b). Here, the sample is excited by the FEL beam, and its reflectivity is probed at a single-color (800 nm, 1.55 eV) for both directions. In Fig. 1c and 1d we are comparing the results obtained after optical and x-ray excitation along the two directions. Along the crystal a-axis, where the system is insulating, we are observing a similar overall shape of the transient reflectivity, although the x-ray excitation leads to a five times larger signal (cf. Fig. 1c). The dynamics is associated to a bandgap narrowing, due to the redistribution of carriers induced by excitation. In contrast, along the crystal c-axis we are observing different dynamics following optical or x-ray excitation (cf. Fig. 1d). As we were concluding from the analysis of the Drude-Lorentz model, the dynamics induced by excitation at 1.55 eV (red curve) is divided in a fast component, that drafts the effect of a coupling to one of the Lorentz oscillators, and a slow component, associated to a modification of the Drude-term plasma frequency. Strikingly, when pumping at a photon energy of 47 eV (blue curve), the fast component is not observed, and the dynamics follows the time evolution of the Drude-term plasma frequency (grey curve). Indirect excitation of the valence and conduction band structure due to secondary processes after initial core-hole excitations hence does not lead to the excitation of the Lorentz oscillator adressed by IR excitation.
Our results provide new insights on the symmetry of the ordered phase of (TaSe₄)₂I when perturbed by a selective excitation, and suggest a novel approach based on complementary table-top and FEL spectroscopies for the study of complex materials. At present, the study of the influence of photoexcitation via different pathways on the non-equilibrium properties of complex materials is an emerging field, capable of providing new insights about the functional properties of metallic and insulating phases and their possible control.