Time-resolved spectroscopic studies of mode-specific electron-phonon coupling in solids

Interactions among constituent particles in many-body systems govern the physics of condensed matter systems. In this thesis, we have demonstrated a novel two-dimensional electron phonon coupling spectroscopy (2D-EPC spectroscopy) to determine the mode-resolved coupling between optical accessible phonon modes with electronic degrees of freedom. By observing a benchmark sample, methylammonium lead halide perovskite (MAPI), we determined the phonon modes coupled to the near-gap electronic states and their relative coupling strengths. Then, we obtained the relative coupling strengths of each phonon modes with the electronic states near the band gap. In order to investigate the role of the particular phonon mode that sticks out from the 2D-EPC spectroscopy in changing the electronic states near the band gap, we employed intense THz pulses whose frequency can be tuned to excite the specific phonon mode in MAPI then monitored its electronic band edge absorption with sub-ps time resolution. Additionally, when a material is non-centrosymmetric, after optical excitation its terahertz radiation with the electronic excited state as the intermediate state is dominated by EPCs. From this terahertz radiation, one can learn which phonon modes participate in the electronic relaxation process after over gap excitation. In the final chapter, an optical pump Terahertz emission spectroscopy is performed on a charge ordering system, α-(BEDT-TTF)₂I₃, to investigate its phonon-assisted charge dynamics.