Fabrication Of Pattern Poled Lithium Niobate Film And Its Nonlinear Optical Applications

Optical superlattice is a kind of artificial microstructure materials,in which the second-order susceptibility is spatially modulated at micron-scale resolution compared with wavelength.Lithium Niobate(LN)is one of the most commonly used optical superlattice materials,which can be used to study nonlinear optical phenomena.By designing proper ferroelectric domain structure,it can compensate phase-mismatching in the nonlinear frequency converted process.This paper mainly focuses on the fabrication of optical superlattice by ferroelectric domain inversion and its nonlinear optical applications.The main research parts are included as follows:1.We first introduce the concept and development of dielectric superlattice from the one-dimensional periodic structure to the arbitrary two-dimensional structure,and focus on its applications in nonlinear optics.We then analyze the role of optical superlattice in nonlinear frequency conversion by the theory of quasi-phase-matching;2.We study the methods to fabricate optical superlattice by ferroelectric domain inversion,including periodic laminar ferroelectric domains,chemical diffusion method,direct electron-beam writing,applying an external field and so on.We then discuss the domain structure evolution during polarization reversal from the single domain state,including five main stages:(1)nucleation of new domains,(2)forward domain growth,(3)sideways domain growth,(4)domain coalescence,and(5)spontaneous backswitching;3.Next,we develop an approach to fabricate arbitrary ferroelectric domain patterns on LN films(30-50 ?m thick)by applying a structured external field at room temperature.The ferroelectric domain inversion is stable and uniform with 1?m linewidth resolution.We can observe the microscope images of the LN film surface after selective etching;4.There are advantages of this method to fabricate pattern poled LN films compared to typical 500 ?m thick LN crystals.This method can be operated easily and does not need extreme conditions.All the poling experiments discussed in this paper are carried out at room temperature and atmospheric pressure;5.We theoretically analyze the nonlinear frequency doubling process in non-collinear manners,mainly including nonlinear Raman-Nath diffraction and nonlinear Cherenkov diffraction.Then we conduct the experiments by the pattern poled LN film,which are in agreement with the theoretical calculations;6.At last,we introduce the concept of computer-generated-holograms into the nonlinear optical regime.Nonlinear second-harmonic wave can be shaped following the nonlinear holographic principle based on pattern poled LN films,when the fundamental wave pumps to the films.Various nonlinear wavefronts are obtained such as the frequency converted optical vortex beam.We can also design arbitrary domain structures in the continuous or discrete cases.