高性能太赫兹发射:从拓扑绝缘体到拓扑自旋电子

High-performance THz emission:From topological insulator to topological spintronics

利用飞秒激光脉冲激发铁磁/非磁异质结构有望实现高效太赫兹辐射,从而打破制约太赫兹技术快速发展的瓶颈.拓扑绝缘体是一种新型二维材料,其自旋霍尔角远大于重金属材料,可以与铁磁层结合构成自旋太赫兹发射器.为了研究拓扑绝缘体/非磁异质结中的太赫兹产生和调控机理,本综述从飞秒激光激发的超快光电流响应入手,结合拓扑绝缘体的晶体结构与电子结构,分析了拓扑绝缘体薄膜中的太赫兹发射机理,揭示了不同非线性效应产生的超快光电流随外界条件的依赖关系,证实了使用多种手段调控拓扑绝缘体出射非线性太赫兹辐射的可能性;以铁磁/重金属异质结为例,探究了自旋太赫兹发射器的优势与调控方法.结合这两种发射机理,通过非线性太赫兹与自旋太赫兹的合成作用,可以实现在拓扑绝缘体/铁磁异质结中偏振可调谐的太赫兹发射.

Ferromagnet/nonmagnet(FM/NM)heterostructure under the excitation of femtosecond laser has proved to be a potential candidate for high-efficiency terahertz(THz)emission.Topological insulator(TI)is a novel two-dimensional(2D)material with a strong spin-orbital coupling,which endows this material with an extremely large spin-Hall angle.Thus,TI appears to be an attractive alternative to achieving higherperformance spintronic THz emitter when integrated with ferromagnetic material.In this paper,we discuss the ultrafast photocurrent response mechanism in TI film on the basis of the analysis of its crystal and band structures.The discussion of the mechanism reveals a relationship between THz radiation and external conditions,such as crystal orientation,polarized direction and chirality of the laser.Furthermore,we review the spintronic THz emission and manipulation in FM/NM heterostructure.The disclosed relationship between THz radiation and magnetization directions enables an effective control of the THz polarization by optimizing the system,such as by applying twisted magnetic field or fabricating cascade emitters.After integration,the FM/TI heterostructure presents a high efficiency and easy operation in THz radiation.This high-performance topological spintronic THz emitter presents a potential for the achievement of arbitrary polarization-shaping terahertz radiation.