Research On Dynamic Electro-Optic Modulation Characteristics Of Potassium Tantalate Niobate Crystal

With the development of electro-optic modulation technology,electro-optic crystals that can realize ultra-high-speed electro-optic modulation have been paid more attention and studied.Potassium Tantalate Niobate(KTa_1-xNb_xO₃)crystal has the largest known Quadratic electro-optic coefficient in the paraelectric phase（about 70 times that of lithium niobate）,and its excellent electro-optical properties make electro-optic modulation devices based on KTN crystals.Whether it is used as a modulation device or a deflection device,it has obvious performance advantages,and has wide application prospects in the fields of optical communication,biomedical imaging and radar scanning.Theoretical and experimental studies on many properties of KTN crystals contribute to its performance optimization and application in electro-optical modulation.Based on the existing theoretical and experimental research of KTN crystals at home and abroad,the electro-optical properties of KTN crystals are studied as follows:KTN crystal is a solid solution mixed crystal of KTaO₃（KT）and KNbO₃（KN）,which has the inherent characteristics of continuous solid solution,so it has been difficult for a long time to grow KTN crystals with very uniform optical quality even under laboratory conditions.In this thesis,the proportion of raw material components is controlled before growing crystals.By improving the traditional top seeding method growth process,large-sized KTN crystals with no obvious macroscopic defects are grown.The test shows that the Curie point of the crystal is near room temperature.In a transparent paraelectric phase state.Structural design of KTN crystals,design and processing of optical wafers suitable for electro-optic modulation,to ensure that the surface roughness and parallelism of the wafers meet the requirements of optical experiments.The electrode materials are Pt,Au,Ag,Cu,Al and other metals,and the metal electrodes are made by magnetron sputtering.The dielectric properties of the crystals are measured by a dielectric thermometer.The electrical constant is significantly improved.The electro-optic modulation characteristics of KTa_0.65Nb_0.35O₃crystal were systematically characterized,the change of the Quadratic electro-optic coefficient of the crystal during the cooling process was measured,and the growth composition fringes caused by the inconsistent melting behavior of KTN crystal growth,that is,the crystal memory,were explored.The components fluctuate.By measuring the Quadratic electro-optic coefficients of KTN crystals under different modulation voltages and electric field frequencies,it is found that high modulation voltages will reduce the Quadratic electro-optic properties of KTN crystals,especially when the crystal temperature drops to close to the Curie temperature,high modulation voltages make the two The Quadratic electro-optic coefficient decreases rapidly.In addition,as the frequency of the applied electric field increases,the Quadratic electro-optical effect of KTN crystals also decays,and the decay is more pronounced as it gets closer to the Curie temperature.We believe that the applied electric field changes the size structure of PNRs（polar nano-domains）and affects their response.In view of the existence of nanometer-sized polar microdomains in KTN crystals near the Curie temperature of the paraelectric phase,which can have a significant impact on the electro-optical modulation properties of KTN crystals,we characterized the dynamic change characteristics of the domain structure near the Curie point of KTN.From the point of view of mechanism,some characterization methods of ferroelectric domain structure in KTN crystal are briefly introduced.According to the characteristics of KTN crystal domain structure and experimental requirements,the polarization imaging observation method is selected and the experimental optical path system is designed.The evolution of the ferroelectric domain structure of KTN crystals with temperature was observed,and the uniform group distribution in KTN crystals was also observed.The evolution process of the ferroelectric domain structure of KTN crystals under DC electric field and AC electric field is studied.Under the action of a strong DC electric field,the domain structure in the KTN crystal is effectively and collectively rearranged to form a single-domain ferroelectric state with a more uniform size,and the ferroelectric domain in the crystal will be polarized by the strong DC electric field,and the inner crystal will be polarized along the polarization electric field.The directional energy potential well is further deepened,which severely restricts the reorientation of the ferroelectric single-domain structure and makes its orientation tend to be consistent.The dielectric loss induced by the strong AC electric field promotes the crystal phase transition,which reduces the internal domain structure of the crystal until it disappears into unobservable PNRs.