1.Department of Physics, Shanghai University, Shanghai 200444, China 2.Terahertz Technology Innovation Research Institute, Shanghai Key Lab of Modern Optical System, Engineering Research Center of Optical Instrument and System (Ministry of Education), Terahertz Spectrum and Imaging Cooperative Innovation Center, University of Shanghai for Science and Technology, Shanghai 200093, China 3.Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China 4.STU & SIOM Joint Laboratory for Superintense Lasers and the Applications, Shanghai 201210, China 5.Shanghai Institute of Intelligent Science and Technology, Tongji University, Shanghai 200092, China 6.College of Precision Instrument and Opto-electronics Engineering, Tianjin University, Tianjin 300110, China
Fund Project:Project supported by the National Natural Science Foundation of China (Grant Nos. 61975110, 11674213, 61735010, 11604202), the Shanghai Rising-Star Program of Science and Technology Commission of Shanghai Municipality, China (Grant No. 18QA1401700), the Chen Guang Project of Shanghai Educational Development Foundation, China (Grant No. 16CG45), and the Young Eastern Scholar Project ofShanghai Municipal Education Commission, China (Grant No. QD2015020)
Received Date:15 May 2020
Accepted Date:11 June 2020
Available Online:12 June 2020
Published Online:20 October 2020
Abstract:Recently, ferromagnetic/non-magnetic heterostructures have been widely studied for the generation of terahertz (THz) emitter based on spin-to-charge conversion. Actually, thermal spintronics effectively combines thermal transport with magnetism for creating and detecting non-equilibrium spin transport. A spin current or voltage can be induced by a temperature bias applied to a ferromagnetic material, which is called spin Seebeck effect (SSE). In this paper, we present a SSE based THz emission by using the heterostructures made of insulating ferrimagnet yttrium iron garnet (Y3Fe5O12, YIG) and platinum (Pt) with large spin orbit coupling. Upon exciting the Pt layer with a femtosecond laser pulse, a spin Seebeck current arises, applying a temperature gradient to the interface. Based on the inverse spin Hall effect, the spin Seebeck current is converted into a transient charge current and then yields the THz transients, which are detected by electrooptic sampling through using a ZnTe crystal at room temperature. The polarity of the THz pulses is flipped by 180° when the direction of the external magnetic field is reversed. By changing the direction of the pump beam excitation geometry to vary the sign of the temperature gradient at the YIG/Pt interface, the polarity of the THz signal is reversed. Fast Fourier transformation of the THz signals yields the amplitude spectra centered near 0.6 THz with a bandwidth in a range of 0.1–2.5 THz. We systematically investigate the influence of annealing effect on the THz emission from different YIG/Pt heterostructures. It can be found that the THz radiation is achieved to increase ten times in the YIG/Pt grown on a Gd3Ga5O12 (GGG) substrate through high-temperature annealing. The mechanism of annealing effect can be the increase of the spin mixing conductance of the interface between YIG and Pt. Finally, we investigate the pump fluence dependent THz peak-to-peak values for the annealed YIG/Pt grown on the Si substrate. Due to the spin accumulation effect at the interface of the YIG/Pt heterostructure, the THz radiation intensity gradually becomes saturated with the increase of pump fluence. Our results conclude that annealing optimization is of importance for increasing the THz amplitude, and open a new avenue to the future applications of spintronic THz emitters based on ultrafast SSE. Keywords:THz radiation/ ultrafast spectroscopy/ spin Seebeck effect/ inversed spin-Hall effect
表15种不同结构样品的制备过程及其归一化THz振幅对比 Table1.Preparation processes of five different sample structures and their normalized THz amplitudes.
THz发射实验光路如图1(a)所示. 使用钛宝石激光放大器系统(Spitfire Pro), 飞秒激光单脉冲能量为2 mJ, 中心波长为800 nm, 重复频率为1 kHz, 脉冲宽度为120 fs. 实验光路中飞秒脉冲被 9∶1 的分束器分为两路, 一路为激发光(pump pulse, 90%), 一路为探测光(probe pulse, 10%). 准直光束垂直入射到样品表面(脉冲能量为0.1 mJ)用以产生超快自旋流. 使用泡沫板过滤激光脉冲, 只让THz脉冲通过. 离轴抛物镜将THz脉冲和经过延迟线的探测光脉冲(脉冲能量为0.05 μJ)同时汇聚到1 mm 厚(110)取向的THz探测电光晶体ZnTe上. 通过自由空间电光取样(EOS)记录下THz相干辐射信号. 实验中, 使用平衡差分探测器, 通过记录THz电场所诱导探测光时间分辨的椭圆率信号来反映THz辐射信号场强的大小. 如图1(b)所示, 沿z轴施加约200 mT的外加磁场, YIG样品为面内磁化. 该外加磁场强度足够强, 能够使YIG的磁化强度达到饱和. 所有实验都在室温及干燥氮气氛围中进行. 图 1 (a) THz发射光谱实验装置图; (b) 在YIG/Pt双层膜结构中, 沿z轴方向外加面内磁场H = ± 200 mT, 飞秒激光诱导铁磁绝缘体和非磁性金属界面产生瞬态温度梯度$ \nabla T $(沿着–y轴; 红色表示高温, 蓝色表示低温), 超快SSE产生一个从YIG进入Pt层的自旋流(沿着–y轴), 基于ISHE, 在–x轴方向上产生瞬态电荷流; (c) 样品GGG//Pt(10)和GGG//YIG(60)/Pt(10)双层膜的THz辐射信号, +N和–N分别表示激光脉冲从Pt膜一侧和GGG衬底一侧辐照样品; (d), (e), (f) 分别表示(c)中GGG//Pt(10)和GGG//YIG(60)/Pt(10)的3种激发构置下的THz辐射原理图 Figure1. (a) Schematic of experimental setup for THz generation; (b) schematic of the YIG/Pt bilayer sample placed in the static in-plane magnetic field of ± 200 mT. A femtosecond laser pulse excites the YIG/Pt bilayer, a temperature gradient $ \nabla T $is created at the interface of ferromagnetic insulator YIG and nonmagnetic metal Pt, launching a spin current (along the –y direction; the red part means the high temperature side and the blue part describes the low temperature side) from YIG layer into the Pt layer based on the SSE. Within the Pt layer, the spin current is converted into a charge current (along the –x direction) via ISHE; (c) measured electrooptic signal of THz emission from GGG//Pt(10) and GGG//YIG(60)/Pt(10) bilayer. THz emission signals are radiated with front (+N, red) and back (–N, blue) pumps; (d), (e), (f) the THz emission schematics of the three sample cases in (c).
图 1 (a) THz发射光谱实验装置图; (b) 在YIG/Pt双层膜结构中, 沿z轴方向外加面内磁场H = ± 200 mT, 飞秒激光诱导铁磁绝缘体和非磁性金属界面产生瞬态温度梯度
图 2 (a) GGG//YIG(40)/Pt(3), Si//YIG(40)/Pt(3), GGG//YIG(40)/Pt(3), Si//YIG(40)/Pt(3), GGG//YIG(40)/Pt1st(3)/Pt2nd(3)和Si//YIG(40)/Pt1st(3)/Pt2nd(3)不同结构样品所产生的THz辐射脉冲; (b) 飞秒激光脉冲激发YIG/Pt1st/Pt2nd结构辐射THz信号示意图; (c) 将图 (a) 中GGG//YIG(40)/Pt1st(3)/Pt2nd(3)和Si//YIG(40)/Pt1st(3)/Pt2nd(3)的时域谱线进行傅里叶变换后的归一化频谱图, 插图为THz发射光谱的半高全宽(ΔF)和中心频率(fc)
图 3 (a) 外加磁场+H (蓝线)和–H (红线)时, GGG//YIG(40)/Pt1st(3)/Pt2nd(3)结构辐射的THz脉冲; (b) GGG//YIG(40)/Pt1st(3)/Pt2nd(3)结构在不同激光激发构置下产生的THz脉冲, 此时外加磁场固定为+H, 插图为飞秒脉冲激发样品的方向
图 4 Si//YIG(40)/Pt1st(3)/Pt2nd(3)异质结构所产生的THz脉冲峰峰值与入射光能量密度的依赖关系. 图中紫色圆圈为实验数据点, 黑色曲线为拟合结果