Periodically Poling Of Single-Crystal Lithium Niobate Thin Films And Its Application

Integrated optics has been developed rapidly since the end of the 20th century.It aims to integrate a series of Micro/Nano optical devices on a substrate material,and it has a very wide range of applications in optical communications,environment,sensing and other fields.Lithium niobate has excellent electro-optical,nonlinear-optical,acoustic-optical,ferroelectric,photorefractive,and piezoelectric properties.It has been widely used in integrated optics.Recently,single-crystal lithium niobate thin films fabricated by ion implantation and direct bonding became an important substrate material for integrate optics.As the refractive index difference between the lithium niobate thin film and the silica is large,the optical waveguides and the optoelectronic devices based on the LNOI have a smaller cross-section size and the bending radius,which can greatly improve the integration density.At present,many devices have been successfully developed on LNOI,such as electro-optical modulator,micro-disk resonator,and filters.Periodically poled lithium niobate(PPLN)is an important non-linear optical material.Using quasi-phase matching,high-efficiency optical frequency conversion can be realized.At present,the external electric field method to make PPLN has been well developed.If the lithium niobate thin film can be periodically poled,it will have many important applications in integrated optics.On X-cut LNOI,to make domain inversion,there is no need to make electrode under the lithium niobate thin film,which simplify the LNOI fabrication process,while Z-cut LNOI generally needs to add a metal electrode underneath the LN thin film,and the metal electrode may absorb light,causing optical loss.There are several groups which can make X-cut PP-LNOI,such as University of Paderborn,University of California at Santa Barbara,and University of Central Florida.In this dissertation,periodic domain inversion was realized on X-cut LNOI and the poling process was optimized.Two kinds of non-linear waveguides(proton exchanged,and TiO2 strip-loaded)were designed,then the domain period for second harmonic generation was calculated.The main contents are as follows:1.A poling setup was designed and built.X-cut PP-LNOI was successfully realized using pulsed external electrical field.The reversed domain length was 10 ?m.The domain periodicity was 20 ?m.2.Using the selective etching method(HF:HNO3 = 1:2),the periodically poled domain was verified.Using a piezoelectric microscope(PFM),the domain inversion region was observed.The poling process was optimized.The effects of poling time and annealing temperature were studied.PFM results showed that the increasing of polarization time would result in the expansion of domain area.After poling,the PP-LNOI samples were annealed at different temperatures,and the reversed domain was examined by PFM.It was found that the PFM signal became smooth when the annealing temperature was above 400 ?.3.The PP-LNOI periodicities for SHG were designed on the waveguides formed by proton exchange and TiO2 strip-loaded methods,respectively.The fundamental wavelength was 1.26?1.62 ?m.In the proton exchange LNOI waveguide,theperiodicity was 3.44?4.08 ?m.In the TiO2 strip-loaded waveguide,the periodicity was 3.94?4.21 ?m.