飞秒激光双色场全光诱导铌酸锂晶体畴反转的应用基础研究

Lithium niobate (LiNbO3) crystal is one of the most important platform materials in integrated photonics; many of its applications are based on its ferroelectric domain patterns. The limitations of previous domain pattern fabrication techniques, i.e., ferroelectric domain reversal techniques, can not fulfill the requirement of sub-micron domains and various domain patterns on different crystal surfaces for integrated photonics applications, thus it has great importance to study on convenient and controllable domain reversal technique to produce sub-micron domain structure on arbitrary crystal surfaces. . Femtosecond two-color laser field, which is synthesized by the superstition of the femtosecond laser pulse of fundamental frequency and its second harmonic pulse, has unique property of controllable asymmetric electric field oscillation along the direction of laser polarization. This special two-color field has broad applications in the study of atomic and molecular optics, and it has been proved that the two-color field can be used for all-optical poling of glasses.. We thus apply for the research proposal on the basic study on all-optical domain reversal of lithium niobate crystal using femtosecond two-color laser field. The key scientific issue of this project is the micro-dynamics and controllability of the all-optical domain reversal of lithium niobate induced by the femtosecond two-color laser field. We plan to build up a parameter-controllable femtosecond two-color laser field setup and a time-resolved imaging system for in situ observation of lithium niobate domain reversal induced by the two-color field. With this experimental system, we will systematically study the influences of the parameters of the femtosecond two-color laser field and the components of the lithium niobate on the ferroelectric domain inversion processes in lithium niobate. Corresponding theoretical simulation and analysis will be also performed. . From these investigations, we expect to reveal the underlying mechanism of the all-optical ferroelectric domain reversal of lithium niobate induced by the femtosecond two-color laser field. Meanwhile, we expect to summarize the technical parameters and experimental conditions of controllable domain reversal, thus to realize the fabrication of sub-micro domain structure on arbitrary crystal surfaces. The implementation of this proposal will significantly benefit the exploration of novel technique of all-optical induced lithium niobate ferroelectric domain reversal, and will also benefit the extension of the applications of lithium niobate in integrated photonics.

铌酸锂晶体是最重要的光子学基础材料之一，其许多应用都基于铁电畴微结构。传统畴结构制备工艺存在一些局限，不能满足集成光子学对亚微米尺度及特殊晶面上畴结构的应用需求，因此探索简单可控的制备铌酸锂任意晶面亚微米畴结构的畴反转技术具有重要的研究价值。飞秒激光基频和倍频光合成的双色场具有可控的非对称光电场振动特性，在原子分子光学中有广泛应用，并被证实能实现玻璃的极化。为此我们提出用飞秒激光双色场全光诱导铌酸锂畴反转的研究课题，其关键科学问题是飞秒激光双色场实现铌酸锂铁电畴反转的微观机制和可控性。我们将通过搭建参数可调的飞秒激光双色场和铌酸锂畴反转的超快实时探测平台，系统研究双色场的参数和铌酸锂的组分等对畴反转的影响，从而明确其中的物理机制，并总结出飞秒激光双色场可控畴反转的工艺条件，实现任意晶面上的畴结构制备。该研究对开发铌酸锂铁电畴调控的全光实现新工艺、拓展铌酸锂在集成光子学领域的应用有重要意义。

当两束频率分别是w和2w的激光相干合成时，其电场在振动方向可以是非对称的，从而可以驱动带电粒子的定向运动，研究发现这种非对称光电场在与原子分子相互作用时会产生定向光电离等新颖的物理现象。本项目借鉴这些研究经验，提出了用飞秒激光双色场诱导铌酸锂晶体畴结构反转的创新性研究方案。在项目支持下，我们搭建了功率、相位、偏振等多维可调的飞秒激光双色场系统、铁电畴结构实时成像系统和飞秒时间分辨泵浦探测系统等实验系统，详细研究了飞秒激光双色场和单色飞秒激光与石英玻璃、铌酸锂晶体、紫外光刻胶、二硫化钼等材料的相互作用。我们观测到了双色场在石英玻璃中诱导的白光超连续谱的相干振荡，论证并实现了一类基于克尔透镜效应的新型飞秒时间分辨快门技术，用显微拉曼技术表征了光刻胶在聚合前后的结构变化，发现了飞秒激光诱导二硫化钼表面的纳米褶皱结构的功率阈值效应。项目取得了一定的创新性研究成果，发表了学术期刊论文4篇，学术会议论文/报告6篇/次，培养毕业了2名博士生、2名硕士生和3名本科生，基本完成了项目的成果目标。.但实验研究迄今未能观测到预期的铌酸锂铁电畴反转的直接证据，主要原因可能在于：a、瞬态的局域畴反转在周围反向畴的影响下又弛豫回复到了初始状态；b、由于克尔非线性等效应的存在，飞秒激光双色场的两种波长的光在介质中相互影响，未能达到畴反转所需要的非对称电场的要求，后继研究拟通过飞秒激光直写加工工艺切割出微米尺度的铌酸锂晶体条带、同时优化飞秒激光双色场相位关系的方式进行脊背波导区域内的畴结构调控。.尽管我们目前尚未获得铌酸锂晶体畴结构反转的实验证据，项目的研究将为人们研究非对称飞秒激光双色场与固体材料、薄膜材料等的相互作用，以及飞秒激光直写构建新型的光子学器件提供一定的研究经验。研究中实现的基于克尔透镜效应的新型飞秒时间分辨快门技术，将为飞秒时间分辨光谱测量提供一种更为简单实用的表征手段。