Research Of Terahertz Modulation Characteristics Based On Ca₃Cu₄TiO₁₂/LiNbO₃Ferroelectric Materials

The terahertz functional device is an important part in the area of terahertz research.Especially, the terahertz modulation characteristics based on ferroelectric materials haveattracted much attention of researchers because of its unique physical property andapplication value. This manuscript focused on the dielectric behavior of CaCu₃Ti₄O₁₂（CCTO） ceramic and different doped near-stoichiometric LiNbO3（near-SLN） crystals inthe terahertz range. We analyses the physical mechanisms of the interaction betweenterahertz wave and ferroelectric materials. By employing a terahertz time-domainspectroscopy system, we measured the complex dielectric constants of CCTO and dopednear-SLN. When applying an external optical field, we have modulated the dielectricconstants of both CCTO and doped near-SLN, the modulation depths reached up to7%and5.5%, respectively. We discussed the micro mechanisms behind the observed effectsand some photo-ferroelectric phenomena. The details of my work are listed as following:（1） We introduced the concepts of THz technology and ferroelectrics. Then we talkedabout the THz application system structure and the THz time-domain spectroscopy system.We investigated the soft-mode theory, which is the most important micromechanism inferroelectric phase transition. The theory indicated that the phase transition was caused bythe frequency drop of an optical transverse mode in the center of Brillouin zone. Weinvestigated the Debye relaxation model and oscillator model for the dielectric behavior offerroelectrics, which corresponded to the displacement ferroelectrics and theorder-disorder ferroelectrics, respectively. We introduced some common photo-ferroelectric phenomena, emphatically talked about anomalous photovoltaic （APV） effectand photorefraction.（2） The dielectric properties of1050°C/12h sintered CaCu₃Ti₄O₁₂（CCTO） ceramicshave been investigated by using terahertz time domain spectroscopy in the frequencyrange of0.2^-1.6THz at room temperature. CCTO do not show giant permittivityproperties in the THz range. When applying an external optical field, an obvious variationof dielectric constant was observed and reached up to7%. From the results, we found thatthe change of refractive index has a linear relationship on scale with the applied light intensity. These findings were attributed to the change of spontaneous polarization in theceramic caused by the excited free carriers.（3） The dielectric properties of near-stoichiometric LiNbO_:Fe and near-stoichiometric LiNbO_:Ce single crystals have been investigated by using a terahertz timedomain spectroscopy （THz-TDS） in a frequency range of0.7^-1.6THz at room temperature.Through a Fourier transform, we got the transmission spectra and the intrinsic phase shiftsof the samples. The complex refractive index and permittivity were calculated from theTHz transmission spectra. In the terahertz range, the dielectric constant of near-SLN:Fe isabout42-46（corresponding refractive index is6.5-6.8）, and the dielectric constant ofnear-SLN:Ce is about40-43（corresponding refractive index is6.3-6.6）. The absorptioncoefficient of near-SLN:Fe in the terahertz range is about60^-100cm^-1, and the absorptioncoefficient of near-SLN:Ce in the terahertz range is about70^-120cm^-1. They are bothhigher than that of undoped CLN.（4） When coupled with an applied external optical field, obvious photorefractiveeffects were observed, resulting in the modulation of the complex dielectric constants forboth near-SLN:Fe and near-SLN:Ce. The variation of refractive index|Δn|has a linearrelationship on scale with the applied light intensity accompanied with a steplike decrease.These findings were attributed to the internal space charge field of photorefraction and thelight-induced domain reversal in the crystals. The light intensity at which the steplikedecrease of photorefraction takes place in near-SLN:Ce is a little higher than that ofnear-SLN:Fe crystal. It means that the coercive field of near-SLN:Ce is a little larger thanthat of near-SLN:Fe, which can also be attributed to the photorefraction resistancebehavior of near-SLN:Ce crystal.