Ferroelectric Domain Reversal And Micro Fabrication In Lithium Niobate

Lithium niobate (LiNbO3, LN) is a ferroelectric crystal of great interest for material scientists due to its excellent electro-optic, acousto-optic, elasto-optic, piezoelectric, pyroelectric, and nonlinear properties. Ferroelectric domain engineering of LN has received much attention for its wide applications, such as nonlinear frequency conversion devices, optical parametric oscillators, photonic band-gap devices, and electro-optic Bragg modulators. Until now the most common domain reversal technique for creating domain patterns is electric poling, where a photolithographically patterned electrode provides a spatially modulated electric field across the crystal along z-axis. Because high poling electric field provides high energy, domains become very easy to grow outside the predesigned region, result to the difficulty of obtaining small domain structures. Thus, some improved poling methods are proposed to enhance this technique. Light-induced domain reversal, which combines application of an external field and laser illumination, has developed to a promising method for domain engineering in recent years. Because the reverse field is reduced within the illuminated areas, a light pattern can be applied to induce and control domain pattern at a much precise scale. However, light-induced domain reversal has its own disadvantages, e.g. limitation of domain depth and pinning effect, which block the further application of these domain structures. The objective of this work is to investigate the light and mater interaction in lithium niobate, in order to explore new method for domain reversal, and then enhance the current micro domain fabrication technique.This thesis begins with an introduction to lithium niobate crystal and its ferroelectric properties. Ferroelectric is one of the most important characters of lithium niobate, based on this property, some applications are introduced, e.g. holographic data storage and quasi-phase-matched technique.A review of domain fabrication technique is presented in Chapter 2. Firstly, we introduce the kinetics of domain wall, and then we discuss some visualization techniques, e.g. Birefringence, Hydrofluoric acid etching, Harmonic generation and Polarized light detection method. Each technique has its own advantages and disadvantages, thus usually we should combine some of them at one time to obtain more detail information of domain structure. Second, we introduce a new fabrication technique——light-induced domain reversal. Mechanism of light-induced domain reversal, light wavelength and intensity influence on domain reversal have been discussed in details. Besides, we also investigate the relation between light-induced effect and defect concentration inside the crystal. Finally, we introduce some UV induced domain micro structure fabrication technique.In Chapter 3 we demonstrate the theoretical and experimental result of 532nm laser induced domain reversal. In 1 mol% Mg doped near-stoichiometric lithium niobate, we succeed a superlow electric field for light-induced domain reversal process. At the same time, the origin of superlow field and the domain grow behavior under superlow field have been investigated in details. What's more, we first time report the observation of pinning effect in lithium niobate. Applying light-induced domain reversal technique, we succeed two dimensional domain pattern fabrications, and then investigate the domain depths of these structures.Green laser inhibited domain reversal is presented in Chapter 4. Firstly, mechanism of green light-induced domain inhibition is investigated. Second, partially transcribed and totally transcribed domain processes are analyzed in details. Finally, two dimensional transcribed domain structure is fabricated. This method brings a powerful technique for solving the long term disadvantages of light-induced domain engineering.Chapter 5 presents research on domain wall. Domain wall has a huge influence to micro fabrication, and also provides much information about the material, thus we discuss domain structure in details. Perpendicular and horizontal screening fields have been calculated, while horizontal screening field is considered to be closely related to domain marks generation process under some specific illuminations. At the same time, a domain mark erase process is investigated and found to be so sensitive to defect concentration inside lithium niobate. Besides, domain wall bending experiment is presented. Surface screening field and domain wall bending mechanism are discussed in details. This phenomenon provides a useful method for controlling domain wall bending by adding bubbles inside liquid electrode, at the same time, gives an inspiration in the field of surface particle control.Finally, Chapter 6 summarizes the results of this thesis, and presents future research directions. The appendix includes a list of papers and presentations based on this research.