UC San Diego

Comprehending the electron behaviors within condensed matter systems holds significant importance, as these systems play a fundamental role in various applications that hinge on electron interactions. This is particularly evident in fields like opto-electronics, semiconductors, and topological insulators. Nonetheless, electrons exhibit rapid changes on the femtosecond to picosecond timescale, making it imperative to employ ultrafast Extreme Ultraviolet (EUV) spectroscopy for capturing electron dynamics. Through EUV excitation, this method can furnish highly specific insights into the elemental, spin, and oxidation state characteristics of valence-level electrons, thus enabling precise measurements of the carrier evolution after photo excitation.In the first part of this thesis, we delve into the formation of polarons within different iron compounds, namely hematite and akageneite, using transient EUV spectroscopy. Our findings reveal distinctive characteristics in the polaron formation process for these two materials. Specifically, the polaron state in akageneite exhibits greater stability compared to that in hematite. In the second part, we investigate the factors that influencing changes in ferroelectricity within the multiferroic material, BiFeO3. We monitored these changes using transient SFG spectroscopy. Our investigations reveal that upon photoexcitation, there is a reduction in the internal polarization, followed by a rapid relaxation process occurring on the picosecond timescale. We further discern that this relaxation originates from the formation of polarons within the FeO6 octahedral structure and the shear strain within the domain. In the third part, we studied the ultrafast carrier dynamics in Ta2NiSe5. Our results indicate that the carrier dynamics at top surface is different from that at sub-surface or bulk region. In addition, we claimed that after photo excitation, the change in structure is more obvious than the change in electronic properties. Our findings provide a new sight in understanding the mechanism of the excitonic condensation process in Ta2NiSe5. Lastly, we showed our experimental setup could reach to monolayer sensitivity, which will allow more interesting investigations in physics in two dimensional materials.