石墨烯高能态处光激发载流子数目和能量的弛豫过程研究

The unique electronic band structure gives rise to remarkable electrical and optical properties for graphene, resulting in its important application in novel optoelectronic and photonic devices. For the development of graphene-based optoelectronic and photonic devices, a thorough understanding of the processes that govern number and energy relaxation of photoexcited carriers is essential. At present, the studies on ultrafast carrier dynamics of graphene mainly concentrate on the energetically low state with linear dispersion (<0.8 eV, nIR-THz). However, the study at the energetically high state with trigonal warping and nonlinear dispersion (>0.8 eV, UV-visible) is scarce. This anisotropic electronic band curvature at the energetically high state will change the scatterings of carriers, it is necessary to study the carrier relaxation at the energetically high state. In this project, we will focus on the fundamental issues of carrier relaxation processes, which should be met when the graphene photoelectronic devices are used in UV-Visible Spectroscopy. We will study on the number and energy relaxation processes of photoexcited carriers at energetically high state of graphene. Including: 1.Tracking the evolution of carrier distribution in momentum space during photoexcited carriers relaxing from excitation state to energetically lower and higher states and the physical mechanism; 2. Relaxation of carrier number distribution in energy space and carrier multiplication; 3. Studying the explanation on negative differential transmittance of graphene based on the carrier number distribution in energy space. From this research, we will pay the path for application of graphene photoelectronic devices at UV-Visible Spectroscopy, and reveal the physical mechanism of transient absorption enhancement of photoexcited graphene.

优异的光电性质使石墨烯在光电子器件上有着重要的应用前景。研究石墨烯光激发载流子数目和能量的弛豫过程对发展石墨烯光电子器件是非常必要的。目前，石墨烯载流子弛豫过程的研究主要集中在线性色散关系的低能态区(<0.8 eV,近红外-THz)。然而，在包含三角翘曲和非线性色散关系的高能态区(>0.8 eV,紫外-可见)的研究还很缺乏，这种能带结构将会影响载流子的散射，因而有必要研究高能态区光激发载流子的弛豫过程。本项目将围绕石墨烯光电子器件在紫外-可见光谱区应用相关的基础问题，对高能态处光激发载流子数目和能量的弛豫过程展开研究。内容包括：1.光激发载流子在动量空间数目分布的演化和机制；2.能量空间热载流子数目分布的弛豫和载流子倍增；3.基于能量空间热载流子的分布，研究可见光负差分透射信号的机制。期望通过该研究，为石墨烯光电子器件在紫外-可见光谱区的应用提供指导，并揭示激发态石墨烯瞬态吸收增强的机制。

光电子器件（如光调制器，光探测器和激光锁模）是现代科技产品的重要组成部分，在国防、工业，甚至人们的日常生活中有着越来越广泛的应用。光电子器件性能的提高往往要求材料具有优异的光电性质及快的响应过程。由于具有独特的光学和电学性质，二维材料在光电子器件上有着潜在的应用。揭示这些二维材料的光学性质和响应过程是极其重要的，本课题立足于新颖二维材料在光调制器和光开关等器件应用上的基础问题，对这些材料的非线性光学性质和载流子动力学过程开展了研究。这包括：1. 对基底上光激发石墨烯瞬态光反射和瞬态光透射的偏振依赖性进行了研究。发现倾斜入射和载流子各向异性分布都能赋予石墨烯瞬态光响应偏振依赖性。这两种机制可以单独或共同作用来赋予石墨烯瞬态光学响应偏振依赖性，共同作用时可产生多样化的偏振依赖性。2. 使用外差探测-光学克尔门技术测量了单层MoS2的三阶极化率张量元的值，并揭示其非线性来源于光致电子云扰动。3. 我们对层状半导体TlGaS2的光学非线性进行了研究，结果表明TlGaS2在800nm处同时具有大的双光子吸收和非共振电子非线性，并获得TlGaS2的非线性光学参数值。4. 研究了直接CVD生长石墨烯玻璃中的载流子冷却过程和饱和吸收，发现石墨烯玻璃的饱和光强和载流子冷却速率随着厚度的增加而增强，缺陷引起的超级碰撞能加快载流子的冷却。5. 揭示了少层ReS2和ReSe2在400nm下具有饱和吸收，而在800nm下具有饱和吸收与激发态吸收，瞬态光学测量证实了这些吸收过程，观察到激子形成与激子复合。这些研究结果对认识二维材料光学非线性的超快过程有着重要的科学意义，有助于掌握这些材料在光电子器件上的应用。