[论文解读] Room temperature broadband coherent terahertz emission induced by dynamical photon drag in graphene

Jean Maysonnave, Simon Huppert+10 more|HAL (Le Centre pour la Communication Scientifique Directe)|May 25, 2014

Terahertz technology and applications参考文献 26被引用 38

一句话总结

该论文通过飞秒光激发诱导的动态光致拖曳效应，在外延石墨烯中实现了室温、超宽带、相干的太赫兹（THz）辐射。通过倾斜入射光子将动量传递给电子，该方法生成了覆盖0.1–4 THz的相干太赫兹辐射，其贡献来自次近邻电子跃迁和非对称的电子-空穴动力学，使辐射可扩展至60 THz，为无需低温冷却的超宽带太赫兹源提供了新途径。

ABSTRACT

Nonlinear couplings between photons and electrons in new materials give rise to a wealth of interesting nonlinear phenomena. This includes frequency mixing, optical rectification or nonlinear current generation, which are of particular interest for generating radiation in spectral regions that are difficult to access, such as the terahertz gap. Owing to its specific linear dispersion and high electron mobility at room temperature, graphene is particularly attractive for realizing strong nonlinear effects. However, since graphene is a centrosymmetric material, second-order nonlinearities a priori cancel, which imposes to rely on less attractive third-order nonlinearities. It was nevertheless recently demonstrated that dc-second-order nonlinear currents as well as ultrafast ac-currents can be generated in graphene under optical excitation. The asymmetry is introduced by the excitation at oblique incidence, resulting in the transfer of photon momentum to the electron system, known as the photon drag effect. Here, we show broadband coherent terahertz emission, ranging from about 0.1-4 THz, in epitaxial graphene under femtosecond optical excitation, induced by a dynamical photon drag current. We demonstrate that, in contrast to most optical processes in graphene, the next-nearest-neighbor couplings as well as the distinct electron-hole dynamics are of paramount importance in this effect. Our results indicate that dynamical photon drag effect can provide emission up to 60 THz opening new routes for the generation of ultra-broadband terahertz pulses at room temperature.

研究动机与目标

• 探索石墨烯中绕过二阶非线性性中心对称限制的非线性光-物质相互作用。
• 开发一种利用石墨烯高电子迁移率和独特能带结构实现高效、超宽带、相干室温太赫兹辐射的机制。
• 研究电子动力学与晶格耦合在通过光子动量传递实现相干太赫兹辐射中的作用。
• 展示一种替代三阶非线性过程的可行方案，用于在二维材料中生成太赫兹辐射。

提出的方法

• 飞秒光脉冲以倾斜角度入射到外延石墨烯上，以诱导光子向电子的动量转移。
• 动态光致拖曳效应通过光子动量的转移产生瞬态电流，从而导致相干太赫兹辐射。
• 理论建模结合了次近邻跃迁积分和不同的电子-空穴动力学，以解释观测到的超宽带辐射。
• 采用时间分辨太赫兹辐射光谱测量技术，测定所辐射的光谱和时间特性。
• 系统在室温下运行，无需低温冷却。
• 分析聚焦于倾斜入射引起的破缺反演对称性与狄拉克锥能带结构中电子动力学的相互作用。

实验结果

研究问题

• RQ1 能否通过一种绕过二阶非线性性抵消的机制，在室温下实现石墨烯中相干超宽带太赫兹辐射？
• RQ2 次近邻电子跃迁和非对称电子-空穴动力学如何影响太赫兹辐射的效率与带宽？
• RQ3 在飞秒激发下，动态光致拖曳效应在石墨烯中是否显著主导于其他非线性过程？
• RQ4 通过光子动量传递在外延石墨烯中生成的太赫兹辐射的光谱范围和相干性如何？
• RQ5 该机制是否可支持超过4 THz的辐射？其上限频率截止由何种物理极限决定？

主要发现

• 在室温下，通过飞秒光激发，实验观测到外延石墨烯中0.1–4 THz范围内的超宽带相干太赫兹辐射。
• 该辐射机制归因于动态光致拖曳效应，即倾斜入射光子的动量转移驱动了瞬态电流。
• 研究表明，次近邻电子跃迁积分对于解释观测到的辐射谱和效率至关重要。
• 电子与空穴的显著不同动力学显著影响所辐射太赫兹波的时域和频域特性。
• 理论分析表明，辐射可扩展至60 THz，提示其在超宽带太赫兹源方面具有潜在应用前景。
• 结果表明，可在不依赖低温运行或复杂异质结构的情况下实现石墨烯中的相干太赫兹辐射。