Regulation Mechanism Of Ferroelectric Domain In Lithium Niobate And High-quality Periodically Poled Crystals

Nonlinear optical frequency conversion using nonlinear optical crystals enables the acquisition of different laser wavelengths,which cannot be achieved with conventional laser gain media,thereby presenting possibilities for applications of lasers.Nonlinear optical frequency conversion needs to meet the condition of phase matching.Birefringence phase matching(BPM)fails to utilize the maximum nonlinear optical coefficients of the crystal due to its birefringent nature,leading to decreased optical conversion efficiency.Quasi-Phase matching(QPM)accounts for the phase mismatch between lightwaves due to material dispersion by periodically modulating the direction of the nonlinear optical coefficients in the nonlinear optical crystal to meet the phase matching condition.QPM theoretically enables frequency conversion over the entire transparency range and can take advantage of the crystal’s maximum nonlinear optical coefficient,making it highly advantageous in practical applications.Periodically poled lithium niobate(PPLN)serves as an ideal QPM-based nonlinear optical crystal material due to its high nonlinear optical coefficient,wide transmission range,and low fabrication cost.However,lattice defects in lithium niobate(LN)crystals,the non-uniform distribution of the poling electric field,and unclear kinetic mechanisms of ferroelectric domain reversal make it challenging to achieve optimum periodic poling domain structures.The dynamic control mechanism of lithium niobate ferroelectric domains,considering the combined effects of forces,heat,light,electricity,and other fields,provides insights into the artificial manipulation of lithium niobate ferroelectric domains.This holds the potential to become a breakthrough opportunity for high-quality PPLN crystal devices.This thesis aims to achieve a technological breakthrough in high-quality PPLN crystal devices by concentrating on the theoretical basis and technology of LN ferroelectric domain reversal dynamics.The work focuses on optimizing PPLN matrix crystal quality,developing new schemes and devices for domain structure regulation,using real-time monitoring and microscopic domain structure analysis to investigate the domain structure growth kinetics process,and combining multiphysics field simulations to analyze the spatial electric field design and optimize the PPLN crystal fabrication method to obtain high-quality PPLN crystals.The following work has been mainly carried out:(1)Growth of high-quality MgO:LN crystal:Through numerical simulation to optimize the temperature field structure during crystal growth,high-quality optical-grade 3-inch MgO:LN crystals were successfully obtained,which appeared transparent with no appeared transparent with no polycrystalline,cracking,and macroscopic defects such as bubbles and inclusions.The wafer-level optical uniformity of the crystal was measured by a laser interferomter and reached 4.4×10-5 cm-2,indicating excellent optical uniformity.Defect analysis of the crystal was conducted using an optical weak absorption measurement instrument,which showed few lattice defects.The distribution of Mg was found to be uniform by electron probe microanalysis.The coercive field of the MgO:LN crystal was measured by hysteresis loop measurement to be 4.5 kV/mm,much lower than that of CLN,providing a good material basis for the fabrication of high-quality PPLN crystals.(2)Real-time monitoring of high-temperature periodic poling and investigation of domain nucleation and growth kinetics:A comparative study of the effect of spontaneous polarization intensity and coercive field on MgO:LN crystal properties under different temperatures(140℃to 190℃)was conducted.It was found that the spontaneous polarization intensity of the crystal decreased at high temperatures,resulting in a reduction of the internal electric dipole moment and ion displacement for poling reversal.At higher temperatures,ions are in a more active state and more easily undergo relative displacement under an electric field,which lowers the coercive field and increases the poling rate.A dynamic model for longitudinal growth and transverse expansion of ferroelectric domains in MgO:LN crystals under different high temperatures was developed,and the relationship between the transverse and longitudinal growth rates of reversed domains was summarized.As the temperature increases,the transverse expansion of reversed domains weakens,while the longitudinal growth rate increases,which helps to obtain high-quality PPLN crystals.A highly uniform PPLN crystal with a thickness of 1 mm,a poling period of 28 μm,and a duty cycle of 45.1%was obtained at the temperature in 190℃ and applying a poling voltage of 1.49 kV.A real-time monitoring system for LN ferroelectric domain reversal under high temperature was designed using indium tin oxide(ITO)transparent electrodes and electro-optic imaging method.By studying the poling process and effect,the reliability of the real-time monitoring system was verified.The system was combined with microscopic domain structure characterization to show the process of point nucleation,growth,merging,and formation of stable periodic domains at the electrode edge.The macroscopic process of poling reversal from the edge of the frame electrodes to the center was also demonstrated,which complements the macroscopic model of ferroelectric domain reversal.(3)Research on ferroelectric domain regulation of LN crystals based on multihole electrode structures:Qualitative and quantitative analysis of the poling spatial electric field distribution was conducted using COMSOL Multiphysics simulation software.In traditional frame electrodes poling,the uneven distribution of spatial electric fields leads to preferential nucleation of reverse domains at the electrode edges.Based on this,a novel design scheme for multihole electrodes was proposed,and numerical analysis of the spatial electric field distribution in multihole electrodes was conducted using COMSOL.It was found that multihole electrodes can regulate the poling spatial electric field intensity,and the local edge electric field strength increased by 1.7 times.The design of multihole electrodes significantly increases the high electric field at the edges,achieving a relatively uniform distribution of high electric fields,which is beneficial for increasing the nucleation density of reversed domains.The combination of the multihole electrodes structure and real-time monitoring was experimentally verified to increase the reversed ferroelectric domain nucleation density by 3.6 times and reduce the lateral expansion during the ferroelectric domain reversal process,effectively improving the overall uniformity and penetration depth of the PPLN crystal.By using multihole electrodes to enhance domain nucleation density and domain growth uniformity,combining with a real-time monitoring scheme of the poling process,a 2 mm-thick PPLN crystal with a period of 30.5 μm was successfully achieved.The duty cycle of the crystal+Z-surface domain was 49.5%,and the Y-surface domain was uniform with a duty cycle maintained within the range of 45±5%,which meet the requirements of the first-order QPM.The nonlinear optical performance of the 2 mm-thick PPLN crystal device was characterized using single resonant optical parametric oscillation technology,and nonlinear optical frequency conversion from 1064.2 nm to 3402.4 nm with a conversion efficiency of 26.2%was achieved.The multihole electrodes design provides an effective way to fabricated large-aperture PPLN crystals.(4)Research on ferroelectric domain regulation of LN crystals based on surface acoustic waves(SAW):A regulation scheme for LN ferroelectric domain reversal using SAW combined with an external electric field is proposed.By constructing a suitable interdigital transducer structure on the surface of the LN crystal and applying a sinusoidal wave excitation voltage at an appropriate resonant frequency,a stable SAW output of 200 mV is obtained.The fabrication of a small-period PPLN with a period of 9μm is achieved by using the SAW-assisted poling method.Through comparative experiments on the poling voltage,Y-domain structure,and duty cycle of PPLN crystals with and without SAW,it is confirmed that SAW can reduce the poling reversal voltage of LN crystals by 10%and obtain more complete and uniform periodic domain structures.Using COMSOL to simulate the effect of SAW on the internal deformation and internal electric field of LN crystal,the mechanism of SAW regulate of LN ferroelectric domain reversal is further analyzed.The study found that SAW can promote local ion activation,thereby optimizing the uniformity of domain structures and providing a new approach for external field regulation of domain structure regulation.The ferroelectric domain regulation based on SAW provides an effective way to fabricated small-period PPLN crystals.