Ultrafast Quasiparticle Dynamics at Complex Interfaces from Different Angles: Opportunities and Challenge

Our group investigates elementary processes in materials and at their interfaces by several different types of ultrafast techniques. In this presentation, I will highlight the strengths and weaknesses of ultrafast optical transient absorption and angle-resolved photoelectron spectroscopy as well as introduce our new approach to time-resolved scanning near field optical microscopy (SNOM):<br/><br/><b>Dynamic screening of quasiparticles in WS₂ monolayers</b>: We unravel the influence of quasiparticle screening in the non-equilibrium exciton dynamics of monolayer WS₂ by femtosecond time-resolved reflectance contrast measurements and a simple, comprehensive model that provides a complete picture of the competing phenomena governing the exciton dynamics in WS₂ upon photoexcitation. Particularly, we unveil the specific impact of excitons and carriers on the renormalization of the quasi-free particle band gap through screening, the exciton binding energy, and the linewidth broadening [1,2].<br/><br/><b>Real photodoping: ultrafast surface metallization of ZnO</b>: The advent of photoinduced phase transitions (PIPT) and the investigation of their non-equilibrium dynamics on ultrafast timescales coined various fashionable terms like hidden phases, new phases of matter, or photodoping. I will discuss these terms using the example of ZnO that undergoes a semiconductor-to-metal transition upon real photodoping at very low excitation densities [3]. Notably, the hidden, metallic phase has no equivalent in the equilibrium phase diagram and shows decay dynamics on ultrafast timescales, but can also be retained and metastable [4].<br/><br/><b>Time-resolved SNOM in the visible at kHz repetition rates</b>: The compatibility of SNOM with pulsed sources is hampered by the requirement of a high-repetition rate imposed by lock-in detection. We developed a sampling method, called quadrature-assisted discrete (quad) demodulation [5], which releases this constraint. It will be shown how this enables the usage of tuneable kHz sources not only for near-field imaging in pseudoheterodyne mode, but also for ultrafast pump-probe studies.<br/><br/>[1] S. Calati et al., <i>Phys. Chem. Chem. Phys.</i> <b>23</b> 22640 (2021)<br/>[2] S. Calati et al., <i>Phys. Rev. B</i>, submitted (2023)<br/>[3] L. Gierster et al., <i>Nat. </i><i>Commun.</i> <b>12</b> 978 (2021)<br/>[4] L. Gierster et al., <i>Faraday Disc.</i> <b>237</b> 58 (2022)<br/>[5] S. Palato et al., <i>Appl. Phys. Lett.</i> <b>120</b> 131601 (2022)