Study On Nonlinear Frequency Conversion And Field Manipulation In Thin Film Lithium Niobate Waveguides

Lithium Niobate（LN）is renowned as the"optical silicon"due to its outstanding optoelectronic properties,such as strong Electro-optic（EO）effect,high refractive index,strong piezoelectric effect,and large non-linear coefficient.It has become a focus of scientific research and exploration,especially with the successful development of on-chip Lithium Niobate（LNOI）thin films,which provides a new opportunity for the study of Lithium Niobate optoelectronic devices and photonic integration.This has once again become a hot research topic in the field.This paper’s research work is mainly based on the LNOI platform.Using numerical simulation methods and according to the principles of interaction between optical waveguide modes and spatial optical field control,the nonlinear frequency conversion and optical field control in lithium niobate waveguides are studied.The specific content includes:1.Using numerical simulation research methods,we explored the process of generating Second harmonic generation（SHG）through I-type mode phase matching（MPM）in z-cut LNOI rectangular waveguides.By designing the geometric parameters of the waveguide to achieve mode phase matching between the pump light TE₀₀mode at 1.55μm and the second harmonic light TM₀₀mode at 0.775μm.Thanks to the large mode overlap factor between the two fundamental modes,the theoretically calculated normalized conversion efficiency is as high as 72.1%W^-1cm^-2.In this work,we also theoretically calculated a large bandwidth of approximately 50 nm in a 5 mm long LNOI rectangular waveguide in the communication band,with flexible temperature-tunable behavior at the phase-matching wavelength,with a temperature tuning rate of 2.5 nm/K.Then,based on the actual manufacturing process,we explored the process of using the above mode phase matching method to generate second harmonic in a ridge waveguide.A larger bandwidth（100 nm in a 5 mm long LNOI ridge waveguide）was theoretically calculated near the phase-matching wavelength,and the waveguide had better tolerance to manufacturing deviations.This stratagem will inspire new integrated nonlinear frequency conversion methods for versatile applications.2.This study investigates the mechanism of integrated nonlinear optical field control in x-cut thin film lithium niobate waveguides.The evolution of the second harmonic optical field in the waveguide is studied,and a meta-surface antenna is designed based on this evolution law to efficiently scatter and integrate the second harmonic optical field.Through numerical simulation,the integrated second harmonic optical field focusing function is designed and realized.This design does not require nonlinear phase matching in the waveguide,and the pump light is focused in the waveguide and sequentially acts on multiple meta-surface unit structures,which can significantly improve the efficiency of nonlinear conversion.The results of this work demonstrate that meta-surfaces can achieve precise control of nonlinear optical fields in lithium niobate waveguides,providing new ideas and references for nonlinear frequency conversion and integrated control based on lithium niobate waveguides.