Study On Optical Properties Of Lithium Niobate Guided-mode Resonant Structure

Lithium niobate,as an excellent material,has become a hot research object in the field of optoelectronics in recent years.It has very excellent physical properties,such as electro-optic,acoustooptic,nonlinear,ferroelectric,etc.,and has high melting point,stable mechanical properties,ultra-wide transparent spectrum,and has important applications in the field of integrated photonics and nonlinear optics.Due to the progress of processing technology,people have prepared lithium niobate film with high refractive index contrast to limit and control the light field.Based on lithium niobate film material,various excellent photoelectric devices can be developed,such as optical resonator,electro-optic modulator,wavelength converter and periodically poled lithium niobate.Grating structure,as a common means of changing the phase,can be used to ultrashallow etch the sample and maintain small damage to the sample.Even this fine-tuning can make the spectra and other characteristics of the sample form a strong contrast with the plate film of the unetched structure,so as to change the near-field and far-field distribution of the structure.Through the combination of lithium niobate thin film and resonant waveguide grating,the transverse leakage guiding mode of visible light can be realized by utilizing the guiding mode resonance characteristics of all-medium grating,and the resonance mode of high-quality factors can be realized to strengthen the interaction between light and matter.In this way,wavelength selection and local field enhancement can be realized.Meanwhile,on the basis of the grating structure,a grating coupler is designed to realize the efficient coupling of optical field to waveguide in visible band.The main contents are as follows:1.A subwavelength grating structure is designed based on lithium niobate film.The near field enhances the electric field and magnetic field,and the far field strengthens the reflection and weakens the transmission at the resonance.The characteristics of guided mode resonance spectra of the ultra-shallow etched subwavelength grating are systematically studied,and the effects of grating period,filling factor,etch depth and other structural parameters on the wavelength position and line width of the formant are studied in detail.The response of different polarization states and incident angles to the spectrum is also studied.The quasi-BIC mode of ultrahigh-quality factor is realized by using an asymmetric grating structure,and this ultrahigh Q value is enhanced locally,while combining the large second-order nonlinear coefficient of lithium niobate to achieve efficient second harmonic conversion.2.Based on the subwavelength grating structure,a space optical coupler is designed to achieve efficient coupling of visible light.For grating couplers,coupling efficiency is an important index to evaluate their performance.According to the target wavelength of visible light,the influence of angle polarization of incident light and structural parameters of the grating on coupling efficiency is studied respectively,and then combined with the particle swarm algorithm to find a higher coupling efficiency.