硅基自旋光电子学太赫兹辐射源特性

Terahertz emission characterization of silicon based ferromagnetic heterostructures

将光电子器件集成到硅片上是光电子集成器件研发的首要步骤.自旋光电子学太赫兹辐射源,通常是由纳米厚度的铁磁/非磁性金属多层膜结构组成,在飞秒激光辐照下能产生高质量、宽带太赫兹脉冲辐射.本文利用飞秒激光脉冲在生长于硅衬底上的Ta/CoFeB/Ir铁磁/非磁性金属异质结中实现了高效、宽带的太赫兹相干脉冲辐射.首先,Ta/CoFeB/Ir异质结的太赫兹脉冲的极性随外加磁场的反转而反转,太赫兹辐射的物理机制可以归结为超快自旋流-电荷流转换.其次,通过改变抽运激光的激发能量密度,研究了Ta/CoFeB/Ir异质结的太赫兹辐射饱和现象.此外,通过研究Ta/CoFeB/Ir异质结的太赫兹发射特性随Ir层厚度的依赖关系,不仅优化了器件的辐射强度,而且获得了Ir层在太赫兹频率下的自旋扩散长度(~(0.59±0.12)nm).该值小于通过自旋抽运技术获得的GHz频率下的自旋扩散长度(1.34 nm),表明不同频率范围对应于不同的电子输运机理.

Terahertz spectroscopy and imaging have many applications,so the generation of broadband terahertz radiation is very important,but now it faces some challenges.Opto-spintronic terahertz emitters,composed of nanometer-thin magnetic multilayer,can produce high-quality broad-band terahertz pulses.Integration of opto-spintronic terahertz emitters onto the silicon wafers is the first step towards their usage in modern photonic devices.In this work,Ta/CoFeB/Ir heterostructures are deposited on thermally oxidized silicon wafers by dc magnetron sputtering.Under the illumination of a femtosecond laser pulse on the Ta/CoFeB/Ir trilayer heterostructure grown on silicon substrate,a spin current can be generated in the ferromagnetic layer due to the ultrafast demagnetization.The spin current is transported and injected into the neighboring non-magnetic metal layers of Ta and Ir.Consequently,the spin current can be converted into the charge current due to the strong spin-orbit coupling.The sub-picosecond transient charge current gives rise to the terahertz radiation that enters into the free space.The terahertz electric field is fully inverted when the magnetization is reversed,which indicates a strong connection between THz radiation and spin order of the heterostructure.The THz radiation from Ta/CoFeB/Ir heterostructure covers the 0.1–2.5 THz frequency range with a maximum value of about 0.64 THz.We also investigate the dependence of THz peak-to-peak value on the pump fluence.The THz emission is found to be saturated at a pump fluence of~0.73 mJ/cm2.Our results demonstrate the existence of the strong spin-orbit coupling in the heavy metal Ir.Furthermore,we optimize the THz emission from the Ta/CoFeB/Ir heterostructure by changing the thickness of Ir layer.According to the thickness dependence of THz emission from the heterostructure,the propagation length of the spin current at THz frequencies is extracted to be about(0.59±0.12)nm,which is shorter than the GHz experimental measurement(~1.34 nm).Our experimental observation is consistent with that in the antiferromagnet IrMn layer,which may be attributed to different transport regimes.Theoretically,the optimized thickness values for CoFeB and Ir layers are 2.4 nm and 1.1 nm,respectively.