| Ferroelectric physics is an important branch of condensed matter physics with a history of more than 100 years.In 1920,Valasek observed the first permanent polarization and P-E loops in Roche salts,which marked the formal discovery of ferroelectric phenomena.Ferroelectric crystals are a class of crystals with spontaneous polarization and the direction of spontaneous polarization can change with the applied electric field.Nowadays,ferroelectric crystals have a wide range of applications in magnetoelectric intermodulation,pressure sensing and data storage.In the 1950s,Devonshire proposed a free-energy based thermodynamic phenomenological theory for ferroelectrics in BaTiO3 crystals.Subsequently,Landau refined the ferroelectric phenomenological theory by relating order parameter changes to symmetry breaking and proposing a structural model for ferroelectric-paraelectric phase transitions.Ginzburg and Devonshire developed the Landau theory by relating the free energy to spontaneous polarization,leading to the widely known Landau-Devonshire theory.Afterwards,Corhan and Anderson proposed a microscopic theory of ferroelectricity based on lattice dynamics,which revealed the common properties of ferroelectric phase transitions and pointed out that the "soft mode" phonon mode is the core feature of ferroelectric phase transitions.The foundation and improvement of these theories are of great significance to the development of ferroelectric physics.The characteristics and scales of functional element in a material determine the different ways it can response to energy.The spatial order of functional element can be enhanced,amplified,or even broken through the performance of the material through coupling to produce more efficient or completely new properties.The domain structure in ferroelectric crystals,as a key functional element at the mesoscopic scale,is one of the core elements that determine the physical properties of the crystal.By artificially modulating(external fields,components,temperature,etc.)the spatial order of the ferroelectric domains inside the crystal,the original properties of the base materials can be enhanced or even broken through.The interaction between ferroelectric domains and force,heat,light and electromagnetic fields can be used to develop multifunctional coupled devices and create a new generation of highly integrated and intelligent functional material system.Therefore,clarifying the correlation between ferroelectric domains and macroscopic properties of crystals is the key to constructing novel properties and opening up new applications of ferroelectric crystals.Curie temperature is an important point to study the domain structure of ferroelectric crystals.The spontaneous polarization of ferroelectric crystals exists below the Curie temperature and disappears above the Curie temperature.Near the Curie temperature,the ferroelectric domains tend to exhibit a variety of exotic spatial orders and thus offering the possibility of designing new optical devices.For most ferroelectric crystals,the dielectric constant changes paradoxically at the Curie temperature,up to 104 or even higher.Therefore,the physical properties of ferroelectric crystals at Curie temperature are abundantly tunable and have become a hot topic of research in the field of functional materials.Perovskite ferroelectric crystals are the most widely studied class of ferroelectric crystals due to their inherent characteristics:asymmetric structure,non-coincident positive and negative charge centers,and the direction of polarization regulated by the applied electric field.KTa1-xNbxO3(KTN)crystal is a typical ABO3 perovskite crystal.Since it is a solid solution mixed crystal of KTaO3 and KNbO3 crystals,its Curie temperature can be adjusted by the Ta/Nb content ratio in the range of 77 K to 500 K,which is the most unique feature of KTN crystal compared with other ferroelectric crystals.Thus,KTN crystals exhibit a variety of novel physical phenomena,such as three-dimensional polarization vortices,self-induced inverse spatial topological transformations,giant broad-band reflectivity in the visible region and three-dimensional nonlinear photonic crystals.In addition,tetragonal KTN have concentration gradients and complex spontaneous polarization distribution of both 900 and 180° domains exist inside.It also has a small coercivity field(0.5 kV/cm)which means good compatibility with material processing such as applied electric field periodic polarization and femtosecond laser direct writing.Consequently,KTN crystals are an ideal vehicle to study the spatial order distribution and macroscopic performance enhancement of ferroelectric domains.In this thesis,we focus on the coupling properties of KTN crystals with thermal,electric and optical fields and investigate the response of KTN crystals with different components to temperature and electromagnetic fields.We reveal the connection between ferroelectric domain order structure and crystal lattice vibration,photoelectric detection,and nonlinear frequency conversion.This work provides a reference for enhancing crystal performance and broadening applications.The main research contents include:1.The inhomogeneity of the polarization in ferroelectric crystals leads to lattice vibrations with both discrete and continuous states,which can be coupled leading to the Fano resonance phenomenon.We measured the temperature depended line shape changes of Raman spectra for KTN crystals with different components and systematically investigated the Fano resonance in its TO2 mode.We fit parameters such as the asymmetry factor q for KTN crystals with different Nb concentrations and found that the q factor is closely related to the Curie temperature Tc and the distribution of Fe polarization in different phases.The effects of Cu-doping and Fe doping on the Fano resonance of KTN crystals were also analyzed.The asymmetry factor q,which characterizes the line shape of the Fano spectrum,can reflect the microscopic polarization distribution in ferroelectric crystals.This indicates that Raman spectroscopy in this particular band is expected to be a novel means of detecting the sequential structure and device properties of ferroelectric crystals.2.The macroscopic polarization of ferroelectric crystals can provide an intrinsic built-in electric field that drives photogenerated carrier separation and enables the crystal device to detect optical signals in a zero-bias state,which is known as a self-powered detector.We use the crystal components to modulate the size and spatial order of the KTN ferroelectric domains and prefer a polarization arrangement type that is favorable for carrier separation and transport.It is found that when the Curie temperature Tc of KTN crystal is slightly higher than room temperature,a large number of "head-to-head"polarization distributions are generated in the crystal,resulting in strong conducting domain walls(CDWs)at the interfaces of different polarization directions.At this time,the electrostatic potential inside the crystal shows a periodic rise and fall,driving the carriers to move in a specific direction and achieving efficient separation and collection of carriers.In this mode,the ferroelectric domains provide an effective built-in electric field to drive carrier separation while reducing the carrier complexation probability,which effectively improves the photocurrent response of the crystal.Benefits from the electron-hole channels generated by the charged domain walls,the KTN crystal achieves an ultra-high 280 nm UV light responsiveness at zero bias,which is about four orders of magnitude higher than that of previously reported ferroelectric materials.The existence of highly conductive domain walls in the crystal was confirmed by a combination of theoretical simulations and PFM and CAFM studies.The conclusion breaks the previous inherent understanding of low photocurrent in ferroelectric oxide single crystals,and provides new ideas for the study of functional element order regulation in the field of optoelectronics.3.By periodically reversing the ferroelectric domain direction of ferroelectric crystals with an applied electric field,dielectric superlattices can be prepared and used for quasi-phase-matching nonlinear optical frequency conversion.Based on the refractive index dispersion equation,we calculated the coherence length and phase matching characteristics of KTN crystals at different wavelengths.We prepared PPKTN devices with different coherence length using the applied electric field polarization method for quasi-phase-matching and measured the second harmonic generation efficiency under 1.0μm,1.5 μm,and 5 μm femtosecond laser irradiation.In addition,we verified the stability of PPKTN devices by achieve angular tuning and measuring the polarization-dependent characteristics of fundamental and second harmonic wave.Compared with other common quasi-phase-matching crystals,KTN crystals have a higher nonlinearity coefficient of d33(84 pm/V)and a wider transmission band(0.5-8 μm),making them a promising candidate for wide-band laser frequency converters. |
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