基于三维狄拉克半金属的中红外可饱和吸收器件及其参数调控研究

Saturable absorber is a key device for ultra-short pulse generation (also known as laser mode-locking). However, conventional Semiconductor Saturable Absorber Mirrors (SESAMs) operate only within a relatively narrow band and is not operating beyond 3 μm. On the other hand, although low-dimensional materials, such as carbon nanotubes and graphene, exhibit broadband nonlinear optical absorption, the defect-prone preparation processes inevitably lead to poor repeatability and reliability, and it has been challenging to tune the intrinsic nonlinear optical response in these material systems..In view of the lack of a robust, wideband and tunable saturable absorber in the mid-wave infrared region, we propose to investigate a novel three-dimensional Dirac semimetal-based saturable absorber in this project. The goal is to demonstrate precise tuning of key optical parameters of the device (including modulation depth, saturable intensity and non-saturable losses) via structure and material engineering. We will also study the effect of an externally applied electric field on the linear and nonlinear optical parameters of this emerging device, which is likely to lead to active and precise control of the absorber as well as the mode-locked pulsation characteristics. This project will enable ultrafast lasers to access spectral range beyond 3 μm and will facilitate the development of more practical lasers and spectroscopic systems in the important 3-5 μm range.

可饱和吸收体是超短脉冲生成的关键器件。半导体可饱和吸收镜（SESAM）具有生长工艺成熟、参数可灵活调控的优势，但其工作波长难以拓展到中红外波段（3-20 μm）。另一方面，以碳纳米管和石墨烯为代表的低维材料展现出了超宽带的可饱和吸收特性，然而其光学参数容易受到材料缺陷以及制备工艺的影响，在参数可控性方面尚不能完全满足实用激光工程的要求。.针对中红外超快光源对参数可调谐可饱和吸收体的需求，本项目提出一种基于新型三维狄拉克半金属材料的中波红外可饱和吸收器件。拟通过对狄拉克半金属材料及器件结构的设计，获得关键光学参数在3 μm以上波段可灵活调控的锁模器件。项目还将研究外加电场对器件非线性光学参数的主动、精细调控规律。拟利用该新型器件所提供的显著扩大的可饱和吸收参数空间，实现长波长外腔半导体激光器的锁模输出并研究其锁模动力学规律。本项目对中红外脉冲激光器的实用化以及相关光谱技术的发展具有重要意义。

中红外波段的超短脉冲激光器在材料光谱学，遥感探测等诸多领域具有重要的应用价值。本项目从中红外波段高性能可饱和吸收器件缺失这一现状出发，重点研究一种基于狄拉克半金属Cd3As2材料的参数宽带可调控可饱和吸收器件。探索并揭示其光学性质尤其是载流子寿命，可饱和吸收等非线性光学参数的调控规律，并展示该新型光开关器件在长波长激光器中的脉冲运转可行性。项目按照既定研究计划推进，取得了如下研究结果：..（1）厘清了多种不同掺杂元素（Cr，Mn，Zn）对狄拉克体系Cd3As2载流子动力学的调控规律。尤其是利用Cr、Mn元素掺杂，实现了体系狄拉克费米子弛豫时间从亚皮秒至纳秒，接近四个数量级的大范围调制。（2）成功制备了带有DBR/光学共振结构的类SESAM器件，通过对器件结构和线性光学参数（如可饱和吸收层厚度，反射镜反射率）的控制，实现了可饱和吸收器件核心参数的调控，与传统器件相比获得了更大的参数可选范围。此外，利用具有上下金电极的共振结构，实现了Cd3As2三阶非线性系数的显著（大于400倍）增强。（3）制备了具有横向电场的可饱和吸收器件，发现了体系内受热效应驱动的小信号吸收调制效应。在该过程中体系的载流子寿命基本保持不变，实现了载流子寿命和吸收系数的独立调控。（4）利用所制备的可饱和吸收材料，在多个波段实现锁模/调Q运转，尤其在GaSb基2微米外腔激光器内，实现了锁模运转。首次在长波长量子级联激光外腔激光器内，观测到了Cd3As2引致的独特的光谱调制效应。