THz field-induced electron acceleration before and after photoemission

Spectroscopic techniques provide valuable insights into the material properties. However, interpreting the resulting spectra involves approximations. Particularly, the redistribution of energy and momentum among charge carriers, such as electrons and holes, during the measurement process can alter the material's initial state. Distinguishing whether induced changes observed in spectroscopy originate from the material's initial state, or the excited modified state remains an important fundamental question. In this work, we investigate the streaking energy of photoelectrons in the presence of a THz electric field with a well-defined amplitude and phase to understand the field-induced changes before and after photoemission time. The experimental data presented here help to distinguish the interaction of the THz field before and after the photoemission event. To address this, gold, with its high reflectivity and weak material response to THz radiation, was used, revealing that changes in electron energy occur predominantly after photoemission (final state). Continuous decrease/increase in streaking energy (based on the THz electric field polarity) as a function of the kinetic energy of photoelectrons was observed (streaking energy of \(\approx\)60 meV corresponding to a perpendicular THz field of \(\approx\)5 kV/cm). In contrast, using the quantum material, 1\( \textit{T}-\text{TaS}_{2} \), which exhibits a metallic phase at 300 K and an insulating phase at 155 K, showed distinct behavior. Although the streaking in the metallic phase resembled the streaking of gold, i.e. a continuous decrease in streaking as a function of the kinetic energy of photoelectrons, the insulating phase displayed entirely different dynamics, highlighting THz-induced changes within the material itself before photoemission (initial state). The streaking occurred in photoelectrons from the lower Hubbard band (LHB) of 1\( \textit{T}-\text{TaS}_{2} \) at 155 K with a constant streaking value (streaking energies of \(\approx\)40 meV corresponding to a perpendicular field of \(\approx\)4 kV/cm). The experimental result is explained by considering field-induced changes both before and after the photoemission time (initial and final state). Before the photoemission time, the electrons in the LHB (single energy band with higher electron effective mass (4.6\(m_e\)) in comparison with electron effective mass in gold (1.1\(m_e\))), experience THz field, and after they are photoemitted with the UV beam, the absolute value of streaking energy increases as a function of the kinetic energy of the electrons. Taking into account both effects, the experimental energy streaking of 1\( \textit{T}-\text{TaS}_{2} \) at 155 K is explained. Here we report such an experimental measurement, in which the THz field was used as pump and the electrons are probed using UV pulses with sub-picosecond time resolution. The THz field is generated at 250 kHz repetition rate using a photoconductive (PC) emitter \cite{dreyhaupt2005high}, with which we obtained a field with 1 THz center frequency, 0.5 ps duration, and spectral width of 3.5 THz. We have synchronized the THz pulse with an ultrashort UV pulse for time-resolved photoemission spectroscopy. The probe 6 eV pulses were generated by two-stage second harmonic generation of 1.5 eV pulses. The experimental data presented in this thesis provide insights into the interaction of THz fields with a defined amplitude and phase with solid-state systems. Determination of the pump-induced changes in initial and final states lay the groundwork for future studies on ultrafast electron dynamics, and the exploration of phase transitions in quantum materials.