太瓦激光驱动下铌酸锂光生载流子的超快太赫兹光谱研究

Optical rectification of high power laser with tilted-pulse-fronts in LiNbO3 crystal has emerged as one of the most efficient THz generation technique. However, the enhancement of the THz pulse energy is limited by the photon induced carriers absorption and other nonlinear effect in LiNbO3 crystal with the pump laser energy increasing. Because various kinds of nonlinear effect exist simultaneously with the THz pulses generation based on tilted-pulse-fronts technique, it can only study the effect of the photon induced carriers in LiNbO3 crystal on the THz pulses qualitatively. While the typical optics pump-THz probe spectroscopy is difficult to acquire the dynamics coefficient because of the limit of the stability and measurement precision of the pump laser..We propose a new technique using large spot and high energy pump-THz probe spectroscopy with Z-scan technique to get the dynamics coefficient of the photon induced carriers in LiNbO3 crystal, and research the carriers transport properties and its effect on intense THz pulses. We will further research the photon induced carriers dynamics properties of two kinds LiNbO3 crystal, and find a method to limit the absorption of photon induced carriers. After these studies, we will be able to provide a guideline for enhancing the THz conversion efficiency and THz pulse energy.

基于倾斜波前技术，采用高能量飞秒激光泵浦铌酸锂晶体是目前产生强脉冲太赫兹辐射的一种有效手段。但随着泵浦激光能量的增加，铌酸锂晶体的光生载流子及其它非线性效应限制了太赫兹脉冲能量的进一步提高。采用倾斜波前技术产生太赫兹脉冲过程中，多种非线性效应同时存在、互相影响，只能定性分析光生载流子对太赫兹辐射的影响；而传统光泵浦-太赫兹探测技术由于激光能量稳定性和测量精度的限制，难于获取铌酸锂晶体光生载流子的动力学参数。.本项目提出采用大光斑高能量激光泵浦-太赫兹时域光谱结合Z扫描技术，避免晶体其它非线性效应的影响，直接研究并获取铌酸锂晶体光生载流子的动力学参数，探索载流子输运性质及其对强太赫兹脉冲的影响；研究两类铌酸锂晶体在不同掺杂类型和掺杂浓度下载流子的动力学性质，在保证晶体抗激光损伤阈值的前提下，寻找有效抑制光生载流子对太赫兹波吸收的方法，为研究强太赫兹脉冲产生提供理论和实验支撑。

本项目开展了铌酸锂晶体光生载流子动力学特性及其对太赫兹辐射产生和传输影响机理的理论建模和研究工作；完成了Z扫描实验系统的搭建，并开展了铌酸锂晶体三光子吸收的研究工作，获得了铌酸锂晶体的三光子吸收系数；利用高能激光泵浦-太赫兹时域光谱实验系统开展了铌酸锂晶体光生载流子动力学特性研究及其对太赫兹脉冲辐射产生的影响研究；进一步采用近化学计量比铌酸锂晶体高效率地产生了双模态（窄带和宽带）可调谐的太赫兹辐射。以上研究为mJ量级强脉冲太赫兹辐射的产生提供了必要的理论和实验支撑。