| Summary: | 博士 === 國立交通大學 === 電子物理系所 === 93 === In this dissertation, three major parts are included. First, the observation of terahertz generation and detection from current-biased superconducting YBCO thin films excited by femtosecond optical pulses by using a free-space electro-optic sampling (FSEOS) technique is reported. The transient terahertz pulse was found to originate from the nonequilibrium superconductivity. Secondary, we also carried out systematic comparisons between the emissions generated with different operating parameters when using the semiconducting photoconductive switches as radiation source. Lastly, in relation to terahertz applications, we studied the terahertz frequency response in superconducting YBCO thin films by carrying out the terahertz time-domain spectroscopy (THz-TDS) measurements.
The characteristics of optically induced bipolar terahertz radiation in biased photoconductive switches and the emitted frequency spectrum distribution were found to remain unchanged even when the optical excitation fluence, strength of the biased field and the emitter gap spacing were varied. This suggests that the terahertz radiation obtained in the current setup must have originated from the same mechanism which is believed to be associated with the ultrafast charge transport process during pulsed laser illumination.
On the other hand, the efficiency of terahertz detection strongly depends on the orientation of the terahertz polarization with respect to the ZnTe crystal (001) axis and on the angle between the polarization of the probe beam and that of the terahertz radiation. These results give the optimal operating parameters for terahertz pulse detection using the ZnTe sensor crystal.
The origin of photogenerated terahertz radiation pulse emitted from current-biased superconducting YBCO thin films excited by femtosecond optical laser pulses is delineated using a FSEOS technique. Picosecond electromagnetic pulses about 450 fs wide were obtained. The frequency spectrum derived by Fourier transforming the picosecond pulses spans over 0.1-4 THz. By investigating the performances of the transient terahertz radiation generated under different operating parameters, pulse reshaping in the measured terahertz electric field caused by the kinetic inductance of the superconducting charge carriers is identified. After recovering the original waveforms of the emitted terahertz pulses, the transient supercurrent density directly correlated to the optically excited quasiparticle dynamics is obtained. A fast decreasing component of about 1.0 ps and a slower recovery process with a value of 2.5 ps are unambiguously delineated in the optically induced supercurrent modulation. The radiation mechanism of the transient terahertz pulse related to nonequilibrium superconductivity is discussed.
Finally, we have used the established coherent terahertz time-domain spectroscopy technique to investigate the properties of the electrodynamics of superconductor at terahertz frequencies. The temperature and frequency-dependent complex index of refraction of the materials, such as NdGaO3 substrate and YBCO thin films, were determined by analyzing the complex transmittance. We used these analysis to explain the variation of the amplitude and phase of pulse shape of terahertz generation in current-biased YBCO thin film a FSEOS technique, and in observing the effect of pulse reshaping of terahertz transmission in YBCO thin films detected in the THz-TDS measurements. The temperature dependence of the real component of complex conductivity revealed a large broad peak over the whole measured terahertz range. It is attributed to a competition between an increase in the normal carrier relaxation time and a decrease in the number of normal carriers with decreasing temperature below Tc. By fitting the measured complex optical conductivity with the aid of a two-fluid model, we were able to extract the temperature dependent of the London penetration depth and the temperature dependence of quasiparticle scattering rate
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