Second Harmonic Generation And Its Application On Studying Perovskite-type Oxide Crystals And Films

Nowadays,with the rapid development of surface and interface science and the critical importance of ferroelectric materials,the non-destructive detection techniques for both are urgently needed to be updated and developed.The second harmonic generation?SHG?technology,which came into being after the laser was invented,has attracted more and more attention from researchers due to its characteristics of central symmetry-breaking,higher sensitivity,simpler devices,and bio-friendliness.On the other hand,perovskite-type oxide materials are full of physical and chemical properties and thus have great potential for application.In particular,the perovskite-type oxide thin-film materials have performed well in solar cells,ultraviolet detection,ferroelectric storage,ferroelectric metals,and multiferroic mechanisms in recent studies.Therefore,the research on the characterization and control of these materials continues unabated.The main work of this dissertation is basing on the far-field SHG system to study the structural characteristics of perovskite-type oxide films and the structural phase transition of perovskite ferroelectrics,as well as continuously improving the SHG system during these works.The main results are as follows:1.A series of BiFeO₃?BFO?thin films with different thicknesses were epitaxially grown on the lanthanum strontium titanate-doped substrates using the laser-MBE technique.The results of the piezoresponse force microscopy?PFM?measurement show that the films have good ferroelectricity.X-ray reciprocal space mapping results show that the structure of 60-nm-thick BFO film belongs to the tetragonal phase,while that of the 180-nm-thick BFO film belongs to the monoclinic phase?R-like phase?.The conclusion is consistent with the results obtained from the SHG polarization anisotropy patterns of the thin films.In addition,based on the SHG theory,we conclude that the disappearance of fourfold rotational symmetry in SHG s-out pattern implies for the appearance of R-phase in the BFO thin films.We also derived and calculated the ratios of second-order nonlinear susceptibility tensor?₃₁/?₁₅ and?₃₁/?₁₁.2.The temperature-changing stage was introduced into the typical far-field reflective SHG system,and the structural phase transition properties of BaTiO₃?BTO?single crystals and nanocrystals were studied.For the first time,the coexistence of displacive and order-disorder modes in the ferroelectric phase transition of barium titanate is proved by SHG technology.The behavior of BTO ferroelectric phase transitions with different initial states is different.Based on the evolution of temperature-dependent SHG polarization anisotropy patterns of the samples,we concluded that this difference is the result of the competition between the two modes.The results of PFM demonstrated that the BTO surface domain structure increases after annealing.In addition,the SHG results of BTO nano-particles indicate that BTO nano-particles with an average diameter as small as 2.8 nm can still generate second-harmonic signals.3.The evolution of polarization-dependent anisotropy patterns of BTO single crystals was studied,and it turned out there is a linear relationship between the rotation angle of SHG anisotropy patterns and the polarization angle.It is a direct evidence illustrating that the polarization of BTO crystal can be qualitatively identified from 0°to 180°by SHG technology.This work gives a glance at improving a nonintrusive and convenient method to identify the polarization of perovskite ferroelectric materials.