Optical studies of ultrafast carrier dynamics in high temperature superconductors

In this thesis, I present time-resolved femtosecond optical pump-probe spectroscopy experiments and ultrafast photoresponse experiments for carrier dynamics studies in high temperature superconductors. After demonstrating experimental setup and general review of carrier dynamics in superconductors, the main experimental results are presented.; In pump-probe measurements, the optical excitation from the pump pulse perturbs the electron distribution near the Fermi surface and induces changes of optical properties. Consequently, by measuring the optical properties using probe pulses, tune-resolved carrier dynamics can be directly measured. I present pump-probe studies in superconducting HgBa2Ca2Cu 3O8 + delta thin films and MgB2 thin films. The measured optical signals are strongly related to the presence of a superconducting energy gap in the electronic energy spectrum. Carrier dynamics is studied at different temperatures and different excitation levels. Signatures of superconducting phase transition at the critical temperature (Tc) are observed in both materials. In HgBa2Ca2Cu3 O8 + delta, measured signals in superconducting state are only related to the Cooper pair breaking and recombination dynamics under weak excitation. In MgB2 however, the measured signals are always composed of several different processes and it is necessary to separate and study each process individually, in order to elucidate the dynamics in the superconducting state.; In ultrafast photoresponse measurements of YBCO microbridges, voltage transients of a current-biased YBCO structure are measured using an electro-optic sampling system. At temperatures much lower than Tc, the nonequilibrium kinetic inductive response due to the change of the superfluid is observed. A new model based on second quantization is proposed to account for damped oscillations observed in experimentally measured voltage transients. The model shows excellent agreement with the experimental data. The high excitation situation is also discussed, and a combination of a nonequilibrium kinetic inductive response and a resistive response is used to account for the measured voltage transients.