Growth Of Potassium Tantalate Niobate Single Crystals And Their Dynamic Electro-optic Properties

Electro-optic（EO）effect is an important method to control and modulate light signal by an applied electric field,widely used in optical communication,imaging and storage.It is of great significance and value to grow crystals with good EO properties and study the origin of large EO effect for improving the optical modulation efficiency and promoting the development of EO applications.Potassium tantalate niobate(KT_a1–xNb_xO₃,KTN)crystal has excellent EO properties: large quadratic electro-optic coefficient as high as 10^-14–10^-13 m²/V² in magnitude;a fast response time of nanoseconds;a broad transmittance of 400–4000 nm;the high diffraction efficiency and large electrostrictive coefficient.Also,its phase transition can be controlled by Ta/Nb component ratio.Hence,KTN is considered as one of the best EO materials,and is a research hotspot in the field of EO crystals,having a good potential for optical and electromechanical applications.However,material and physical limitations prevent the good EO properties of KTN.In terms of materials,high optical quality KTN crystals are still difficult to grow,quality control lacks normative basis,and composition control technology is not mature yet;From a physical perspective,the excellent EO performance of KTN is closely related to the dynamic characteristics of polar nanoregions（PNRs）,but the mechanisms of field-induced evolution of PNRs,dynamics EO features of PNRs,and the relationship between PNRs and macroscopic EO properties are still unclear,needing breaktroughs.How to solve these two problems are the key to further improving the EO performance of KTN crystal and boosting the development of high-quality EO devices.This work aims to solve the existing problems of the EO effect of KTN,and study the growth,design and dynamics EO characteristics of KTN crystal.Crystal morphology and size uniformity of cross-section are important factors to consider for obtaining high-quality crystals with excellent properties.Here,based on top-seeded solution growth method,this work combines natural crystallization models and first principle calculations to study KTN morphology and size-uniformity control,revealing the effect of growth temperature and other factors on crystal growth,proposing an effective temperature-control methiod to avoid diameter fluctuation and optimize crystal growth method.The size-controlled growth is achieved,and a series of high-quality KTN crystals with good crystalizaiton and EO properties are successfully prepared.Gradient refractive index（GRIN）optical materials have received substantial attention because of their applications in integrated imaging and fiber optics.This work proposes variable GRIN engineering method.Based on the segregation effect in crystal growth,the effective composition regulation method is proposed,and the composition control method is optimized,and the compositional gradient KTN crystals are designed and grown.Using the EO measuring technique,the quadratic EO coefficient and its gradient distribution are characterized.Further,the variable GRIN is realized under different electric fields.A novel EO def lection scanner with the large defletion angle and high response rate at MHz is developed utilizing the compotion gradient KTN sample.PNRs are considered to be responsible for the giant EO effect in KTN.In this work,the dynamic EO characteristics are comprehensively studied.Combined the EO measurements and temperature dependence of dielectrc permittivity,the orderdisoirder states of PNRs near the ferro-paraelectric phase boundary are investigated.Through the electric-field-dependent EO properties,the intrinsic EO characteristics of PNRs are revealed,and the vibration and reorientation response modes of PNRs are proposed.The enhanced mechanism of activated reorientation mode on quadratic EO properties of KTN are explained.Based on polarization and thermal-shock treatments,the structure evolution of PNRs is induced,and the effect of PNR structural features on EO properties is studied.The stable polarization state with the same orientation in ferroelectric phase is demonstrated to be the key to improving linear EO effect.In addition,this study indicates the contributions of PNRs and other dipole clusters to EO properties.Correlated disorder,a type of crystallographic state,widely exists in complex materials systems and is beneficial for the improvement of material properties.The stress and strain gradient control the features of correlated disorder.Here,the effect of stress and strain gradient on PNRs in KTN is studied firstly by the tertiary pyroelectric effect and temperature-field-induced birefringence effect.Moreover,the macroscopic strain gradient in KTN crystal is designed and implemented,and initial PNR state is controlled by the flexoelectric field.Then,based on the strong electricfield-depedences of quadratic EO effect,the effect of strain gradient on PNR-induced dynamic EO properties is mainly investigated.The studies of dielectric,Raman and absorption spectrum demonstrate the decisive role of strain gradient in controlling local correlated dipoles.Using the high-resolution imaing method,the local strain gradients of 10^-3–10^-2 nm^-1 in magnitude are shown,and the effects of strain gradient caused by composition-disorder-induced local structural heterogeneity on the formation and driving of PNRs and are analyzed.This work indicates the key control effect of strain gradient on the performances of ferroelectric solid solution.