STUDIES OF ULTRAFAST PROCESSES IN THIN FILM HIGH TEMPERATURE SUPERCONDUCTOR AND FERROELECTRIC MATERIAL
Lee, Chi H
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This Ph.D. thesis covers two projects, namely the study of optical responses of high temperature superconductor thin films and the study of polarization switching dynamics in ferroelectric materials, both using a femtosecond laser. We employed the improved Transient Photoimpedance Response method with high signal-to-noise ratio to conduct systematic investigations of the dependence of femtosecond optical responses of YBCO films on current, average laser power, temperature, and film thickness. All of the physical processes embedded in the optical responses were observed. Feature waveforms associated with different physical processes, namely the kinetic inductance, 2-T, and thermal model, were obtained. Taking advantage of the direct relationship between the optical response and the Cooper pair breaking rate, we studied the photon energy dependence of the fast optical response of YBCO films. A resonance around 1.5 eV was observed with a total width of ~100 meV. This narrowness of a spectrum-width has not been reported in the literature. The results are well interpreted by stripe phase theory. We also developed a superconducting sampling gate to perform the electrical correlation measurement of the fast optical response of YBCO films. An estimated ~8 ps pulse was obtained. For the application of ferroelectric materials as high-speed nonvolatile random access memories, the issue of how fast the polarization can be switched is of great importance. The goal of our research is to obtain the intrinsic polarization switching time. By simulating the pulse method utilized in our experiments, we concluded that the rise-time of the input electric pulse and the capacitor size are two limiting factors of this method. We therefore proposed and realized a novel approach for generating jitter-free, sub-100 ps rise-time step-function-like electric pulses, using a femtosecond laser-activated semiconductor photoconductive switch. Quantitative measurements yielded a polarization switching time ts of ~220 ps for a 4.5x5.4 micron square PNZT capacitor, which is to our knowledge the fastest one ever reported. Modeling of the switching transient using the Ishibashi-Merz model gave a characteristic switching time t0 of ~70 ps. Moreover, polarization switching behaviors for various sizes of capacitors and various rise-times of input electric pulses were also investigated.