非z切铌酸锂薄膜光学超晶格的全光极化研究

Single crystal lithium niobate thin film (LNOI) is a very promising platform for integrated optics that enables efficient nonlinear optical frequency conversion through quasi-phase matching (QPM) technique based on ferroelectric domain inversion. Currently, the electric field poling is mainly used for poling LNOI, and most of the fabricated ferroelectric domain structures are one-dimensional with big poling periods. As for the on-chip microstructures such as non-z-cut LNOI microresonators, direct and effective poling technique is still needed. This project proposes to use near-infrared femtosecond laser poling technology to achieve ferroelectric domain inversion in non-z-cut LNOI. Firstly, through theoretical analysis, the nonlinear absorption process of near-infrared femtosecond laser and ferroelectric crystal under intense focusing condition will be studied, and the all-optical poling model of ferroelectric crystals represented by lithium niobate will be constructed. Then, through precise manipulation of the parameters of laser direct writing, real-time monitoring of ferroelectric domain inversion and three-dimensional non-destructive imaging after poling, the influence of laser writing parameters on the ferroelectric domain inversion in x-cut LNOI will be clarified, and finally all-optical poling of two-dimensional aperiodic optical superlattices in x-cut LNOI and on-chip microstructures will be fabricated. The research of this project can provide theoretical and technical support for the fabrication of important on-chip LNOI photonic devices and has important implications.

单晶铌酸锂薄膜(LNOI)是一种极具应用前景的集成光学平台，可通过基于铁电畴反转的准相位匹配(QPM)技术实现高效的非线性光学频率变换。目前LNOI的极化主要采用电场极化法，制备的铁电畴结构多为周期较大的一维周期性结构。而非z切LNOI微谐振腔等片上微结构仍缺少直接有效的极化手段。本项目提出采用近红外飞秒激光极化技术，在非z切LNOI中实现铁电畴反转。首先通过理论分析，深入研究强聚焦条件下，近红外飞秒激光与铁电晶体的非线性吸收过程，构建以铌酸锂为代表的铁电晶体的全光极化模型。随后通过精准操控激光直写的各项参量，结合对铁电畴反转的实时监控和后期三维无损成像，阐明直写参量变化对x切LNOI中铁电畴反转的影响规律，并最终在x切LNOI和微谐振腔等片上微结构中制备全光极化的二维非周期性光学超晶格结构。本项目的研究可为LNOI重要片上光子器件的制备提供理论和技术支持，具有重要意义。

铁电畴的反转（极化）会导致材料所有的奇数阶张量（例如二阶非线性张量和压电张量）反号。通过人工调制铁电畴结构可以形成非线性光子晶体、压电声子晶体等结构，具有广泛的应用前景。目前铁电畴的主要极化手段是电场极化法，该方法存在着无法实现三维结构，对晶向敏感等不足之处，极大地限制了人工铁电畴结构的应用。. 本项目以铌酸锂、单畴铌酸钙钡（CBN）和单畴铌镁酸铅-钛酸铅（PMN-PT）等铁电晶体为研究对象，以近红外飞秒激光直写为技术手段，研究铁电晶体的高精度全光极化及应用。项目获得以下重要结果：①首次在单畴铁电晶体中制备了三维非线性光子晶体，并实现了基于准相位匹配的三维非线性频率变换，获得的倍频转换效率为多畴情况的四倍。②首次利用激光直写技术在单畴PMN-PT晶体中实现了三维铁电畴反转及擦除，解决了该晶体在电场极化过程中容易产生90°畴转或是发生晶体开裂的难题。③首次在PMN-38PT晶体中制备了周期铁电畴结构，实现了基于准相位匹配的倍频输出，并测量了该晶体的所有非零二阶非线性极化率张量元。④利用激光直写技术在y切铌酸锂中实现了二维周期极化。 . 项目获得了激光直写参量对铁电畴反转影响的规律，建立了激光极化三维铁电畴的理论模型，设计并实验表征了激光极化非线性光子晶体的非线性频率转换性能，推进了激光极化铁电畴技术的发展。通过本项目发展的激光极化技术可以在包括铌酸锂在内的多种铁电材料中制备高精度的铁电畴结构，为发展集成非线性光子器件奠定基础。