Research On Terahertz Modulator Based On ABO₃-type Ferroelectric Thin Films

With the rapid development of terahertz communication technology,relevant researches on terahertz modulator as a middle part of the communication system are urgent to be carried out.Ferroelectric thin films have been widely used in microwave communication because of their excellent properties,such as dielectric,piezoelectricity and ferroelectric properties.In this thesis,the properties of ABO₃ type ferroelectric thin films based on silicon substrate in terahertz range were studied by terahertz time-domain spectrometer.The micro-mechanisms behind observed effects of the ferroelectric thin films under external field were further discussed.This thesis provides a reference for the preparation of terahertz modulation devices.Below are key research findings:?1?The dielectric tunability of ABO₃ type lead titanate?PbTiO₃,PT?and lead zirconate titanate?PbZrTiO₃,PZT?ferroelectric single layer film were compared with PZT/PT superlattice under the effect of external optical field by terahertz time-domain spectrometer.Experimental results showed that the optical field realized a modulation of 39.42%and69.86%for the real part of the dielectric constant for the PT and PZT ferroelectric thin film,respectively.However,the dielectric tunability of the superlattice reached up to 174.77%.In order to investigate the internal mechanism of the interaction between the optical field and the thin films,lattice dynamics of above three films were analyzed with the assistance of the Lorenz-Debye coupling model.According to fitting results,when ferroelectric thin films were illuminated by the 532 nm laser,the electrons were excited and redistributed,which led to the formation of a built-in electric field.Such an electric field promoted the hardening of the soft mode.The oscillator strength and damping coefficient were increased along with optical powers.?2?The dielectric tunability of the silicon-based barium titanate?BaTiO₃,BTO?thin film in the terahertz range was studied under the effect of electric field.When the applied bias voltage was tuned from 0 V to 45 V,the real part of the complex dielectric constant was tuned by 57.6%,corresponding to the imaginary part tuned by 35.4%.In order to analyze the lattice dynamics of the BTO film under the effect of electric field,the dielectric spectra was fitted by Lorenz-Debye coupling model.It is found that the offset of the Ti ion relative to the center position decreased as the bias electric field increased,which hardened the soft mode.In addition,by comparing the modulation of the non-illuminated and illuminated silicon-based BTO film under the electric field,it is found that the optical field shielded the effect of the electric field on the BTO film.?3?The modulation characteristics of silicon-based lead titanate/strontium titanate?PbTiO₃/SrTiO₃,PT/STO?ferroelectric superlattice with different periodic structures were studied under the effect of power-adjustable optical field.When the optical power was tuned from 0 mW to 450 mW,the transmission modulation of the samples PT/STO/Si and?PT/STO?₃/Si can reach up to 46.3%and 56.2%,respectively.For the sample PT/STO/Si,when the optical power was tuned to 300 mW,the variation in refractive index tended to be saturated,indicating the generation of the photovoltaic field.The built-in electric field is equal in magnitude to the photovoltaic field,yet in the opposite direction.In addition,the built-in electric field can effectively modulate refractive index closely related to the excellent electro-optic properties of the materials.?4?The modulation depth of non-irradiated and irradiated PT/STO ferroelectric superlattice are studied with the positive and negative bias.It was found that the two samples were not exposed to light,electric field had a little modulation on the sample in terahertz range.However,when samples were illuminated by 532 nm laser,the transmission of the sample PT/STO/Si showed a'diode'effect under the effect of positive and negative bias voltages.Furthermore,the sample?PT/STO?₃/Si realized a continuous modulation of terahertz waves.When the illuminated optical powers tuned to 400 mW,the electric field can achieve a modulation depth of 48.7%and 78.5%for the two samples,respectively.