| This thesis presents a detailed study of structural and optoelectronic properties of Hg-based high temperature superconductors (HBCCO). The optical and terahertz characterization techniques used include time-resolved pump-probe spectroscopy, photoimpedance measurements, transmission terahertz time-domain spectroscopy (THz-TDS), and optical-pump and THz-probe spectroscopy (OPTP).;We used atomic force microscopy, X-ray diffraction, and scanning electron microscopy to characterize the surfaces and structures of the studied HBCCO thin films and microbridges.;In optical pump-probe spectroscopy, we measured the temperature- and power-dependent transient reflectivity signals. In the superconducting state, the amplitude and rise time of the signals followed anisotropic BCS-like energy gap dependence. The rise time in the normal state was faster than that in the superconducting state. The relaxation time of the superconducting components just below Tc followed 1/Delta(t) dependence since it was decided by the phonon anharmonic decay process. Below 0.8 Tc, instead of being limited by the "phonon bottleneck" effect, typical for conventional superconductors, the relaxation time was decided by the quasiparticle recombination process, and was in a single-picosecond range. The latter shows that HBCCO is a good candidate for ultrafast photodetectors.;In photoimpedance measurements, we characterized the voltage-current property of our microbridge structures. The temperature dependence and laser fluence dependence of photoresponse signals were measured. At low temperatures, and under low bias and low excitation conditions, we observed ∼90-ps (system-limited), bi-polar shape, kinetic-inductive response, associated with Cooper-pair breaking and quasiparticle recombination processes. At high temperatures, close to Tc, the photoresponse was a superposition of the fast kinetic-inductive and much slower resistive signals, with the fast part essentially the same as the one observed at low temperatures and under low bias. The slow resistive component (bolometric effect) corresponded to the heat transfer from the film to the substrate.;In the transmission THz-TDS experiments, we measured and derived temperature-dependent complex conductivity of the HBCCO material. We observed that sigma re, showed a peak below Tc which shifted to lower temperatures with lower frequencies. At the same time, sigma im had a sharp increase below Tc, due to the increase of Cooper-pair density and formation of a superconducting condensate. The time-resolved quasiparticle relaxation of HBCCO measured by the OPTP technique also exhibited an intrinsic single-picosecond dynamics and confirmed the lack of the phonon-mediated rebreaking of Cooper pairs in HBCCO (the phonon-bottleneck effect). |
