Recently, a paper published in Advanced Functional Materials reports our study on the ultrafast and ultrasensitive photoactive terahertz modulator based on the 3D Dirac semimetal (Cd3As2) thin film. In this work, utilizing the optical pump terahertz probe technique, we experimentally demonstrate that the conductivity of Cd3As2 thin film increases with the increasing pump fluences and shows a metallic behavior. A long momentum scattering time (~157 fs) and high terahertz electron mobility (~7200 cm2/Vs) at terahertz frequencies are discovered. These superior characters allow Cd3As2 thin film to be an ideal photo-tunable terahertz modulator, which possesses a low optical threshold (~2.54 μJ/cm2) and ultrafast (<15.5 ps) broadband (0.2-1.5 THz) modulation of terahertz waves with modulation depth up to 63% at a pump fluence of 63.5 μJ/cm2. Additionally, integrated with metasurface structures, Cd3As2 thin film could potentially be used as sensitive and ultrafast reconfigurable terahertz resonant plasmonic/meta-devices. Our findings provide a robust platform (Cd3As2) for a new class of 3D Dirac topological semimetal-based energy-efficient, all-optical, ultrafast tunable, compact photonic and terahertz electronic devices. 

      This paper is a collaborative result with Prof. Weiwei Liu from Nankai Univ., Prof. Faxian Xiu from Fudan Univ., and Prof. Ranjan Singh from Nanyang Technological Univ. Please read more on: https://doi.org/10.1002/adfm.202011011.

    (a)基于三维狄拉克半金属Cd3As2薄膜的光控太赫兹调制器示意图; (b)不同泵浦光功率下测得的调制深度和载流子动力学过程（插图）。 

    (a)Schematic diagram of the photoactive terahertz modulator based on Cd3As2 thin film. (b)The measured modulation depth (MD) and relaxation dynamics (Inset) at selected pump fluences. 

    近年来，随着6G等下一代高速无线通信进入太赫兹（Terahertz）波段，开发开启光功率低、调制速度快的光控太赫兹调制器件成为相关研究的重点。作为一种新的量子材料，狄拉克半金属在三维动量空间表现出线性色散关系、以及受拓扑保护的费米弧状表面态，呈现出高载流子迁移率、手性反常效应等诸多新奇的物理特性，这也为高性能光控太赫兹调制器件的研究提供了新的契机。 

    近日，南开大学现代光学研究所刘伟伟教授团队和复旦大学修发贤教授、新加坡南洋理工大学Ranjan Singh教授合作，实现了一种基于三维狄拉克半金属Cd3As2薄膜的光控太赫兹调制器。光泵浦太赫兹探测结果表明，Cd3As2薄膜的电导率随泵浦光的增强而增大，并且具有长散射时间（τs ≈157 fs）以及高载流子迁移率（μ ≈7200 cm2/Vs），由此构造的光控太赫兹调制器的开启光功率为2.54 μJ/cm2，调制速度 <15.5 ps，工作带宽为0.2-1.5 THz、调制深度为63%（@63.5 μJ/cm2）；此外，结合电磁超表面，仿真证明了Cd3As2薄膜支持太赫兹表面等离基元模式传输。 

    本研究成果发表在《先进功能材料》期刊上（Adv. Funct. Mater. 2021, 31(17): 2011011），文章链接https://doi.org/10.1002/adfm.202011011。