In this work, we study the out-of-equilibrium dynamics of the paradigmatic quasi-one-dimensional material (TaSe$_4$)$_2$I, that exhibits a transition into an incommensurate CDW phase when cooled just below room temperature, namely at T$_{\rm{CDW}} $= 263 K. We make use of both optical laser and free-electron laser (FEL) based time-resolved spectroscopies in order to study the effect of a selective excitation on the normal-state and on the CDW phases, by probing the near-infrared/visible optical properties both along and perpendicularly to the direction of the CDW, where the system is metallic and insulating, respectively. Excitation of the core-levels by ultrashort X-ray FEL pulses at 47 eV and 119 eV induces reflectivity transients resembling those recorded when only exciting the valence band of the compound - by near-infrared pulses at 1.55 eV - in the case of the insulating sub-system. Conversely, the metallic sub-system displays a relaxation dynamics which depends on the energy of photo-excitation. Moreover, excitation of the CDW amplitude mode is recorded only for excitation at low-photon-energy. This fact suggests that the coupling of light to ordered states of matter can predominantly be achieved when directly injecting delocalized carriers in the valence band, rather than localized excitations in the core levels. On a complementary side, table-top experiments allow us to prove the quasi-unidirectional nature of the CDW phase in (TaSe$_4$)$_2$I, whose fingerprints are detected along its $c$-axis only. Our results provide new insights on the symmetry of the ordered phase of (TaSe$_4$)$_2$I perturbed by a selective excitation, and suggest a novel approach based on complementary table-top and FEL spectroscopies for the study of complex materials.
Precision-controlled ultrafast electron microscope platforms. A case study: Multiple-order coherent phonon dynamics in 1T-TaSe$_2$ probed at 50 femtosecond - 10 femtometer scales
Orientation-resolved ultrafast spin reorientation dynamics in ferrimagnetic DyCo$_5$
Max-Born-Institut für Nichtlineare Optik und Kurzzeitspektroskopie（马克斯·玻恩非线性光学和超快光谱学研究所）；Leibniz-Institut für Kristallzüchtung（莱布尼茨晶体生长研究所）；European XFEL（欧洲X射线自由电子激光设施）；Technische Universität Berlin, Institut für Optik und Atomare Physik（柏林工业大学光学与原子物理研究所）
Dynamic competition between phason and amplitudon observed by ultrafast multimodal scanning tunneling microscopy
Department of Physics, Grainger College of Engineering, University of Illinois Urbana-Champaign（伊利诺伊大学厄巴纳-香槟分校格雷恩格工程学院物理系）；Department of Physics and Chemistry, Daegu Gyeongbuk Institute of Science and Technology (DGIST)（大邱庆北科学技术院物理化学系）；Department of Materials Science and Engineering, Grainger College of Engineering, University of Illinois Urbana-Champaign（伊利诺伊大学厄巴纳-香槟分校格雷恩格工程学院材料科学与工程系）；Department of Physics, Boston College（波士顿学院物理系）
