| Summary: | 碩士 === 國立清華大學 === 光電工程研究所 === 103 === In this work, we studied two methods to generate stronger THz pulses: (1) by tilted-pulse-front pumping in highly-doped stoichiometric lithium niobate (sLN) and (2) by optical rectification (OR) in ZnTe by using dual-color pulses. Firstly, we build up the system with 5 mol% Mg: sLN. When this unique source was applied in THz-TDS, it can increase signal-noise ratio in sub-THz region, such as the extraction of the optical constant of Teflon and fused silica to 50 GHz. Further, the saturation effect which limits efficiency of THz generation can be much more unobvious than from the sLN with small Mg doping level with negligible increase of THz absorption in 0.1-1.5 THz region. Our explanation is that more phonons are scattered by the more defects in the highly-doped crystal. However, THz phonon-polaritons decay into pair of phonons. But, the less number of the phonons decreases the decay rate of the THz polaritons.
Besides, to explore the possibility of enhancement in THz generation, we demonstrate the experimental and theoretical THz pulses generation by applying OR in ZnTe crystal semiconductor with coherent-controlled two-color laser. The phase inverse phenomenon of THz signals generated by fundamental wave (FW) and the second harmonic (SH) pulses can be observed. With different power ratios and delay times between two-color pulses, the competition of the THz generations between FW and SH pulses was revealed. The possible explanation of the competition is that the shift current which is one of the OR process is modulated by the different color pulse. Since the shift current is established by the excitation of the photons whose energy is above the energy bandgap of crystal and the state of the electromagnetic field of same frequency wave, the SH pulses whose photon energy is below energy bandgap also affect the generation of the shift current. Consequently, the shift current is reduced by the different color pulses.
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