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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.
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Keywords:
- broadband terahertz radiation /
- ferromagnetic heterostructure /
- inverse spin hall effect /
- spin diffusion length
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图 1 (a)透射式太赫兹发射光谱实验系统示意图; (b)硅基Ta/CoFeB/Ir异质结的太赫兹辐射物理机理示意图, 飞秒激光脉冲激发铁磁层中的自旋流js注入相邻的非磁性金属Ta和Ir层中, 转换成垂直于磁化强度M的电荷流jc
Figure 1. (a) Experimental setup for THz emission spectroscopy in the configuration of transmission; (b) in silicon-based Ta/CoFeB/Ir THz spintronic heterostructures, a femtosecond laser pulse excites CoFeB FM layer, the spin current js injects into the adjacent Ta and Ir layers, and then is transformed into transverse charge current jc, which is perpendicular to M.
图 2 (a) Ta(4 nm)/CoFeB(8 nm)/Ir(0.4 nm)和(b) Ta(4 nm)/CoFeB(8 nm)/Ir(0.8 nm)三层异质结典型的太赫兹辐射信号, 蓝色和红色曲线分别代表磁场方向为+H和–H时的太赫兹脉冲信号; 经傅里叶变换得到的归一化振幅谱(c) Ta(4 nm)/CoFeB(8 nm)/Ir(0.4 nm)和(d) Ta(4 nm)/CoFeB(8 nm)/Ir(0.8 nm)
Figure 2. (a) THz emission pulses of Ta(4 nm)/CoFeB(8 nm)/Ir(0.4 nm) and (b) Ta(4 nm)/CoFeB(8 nm)/Ir(0.8 nm) three-layer heterostructures under ±H; the corresponding normalized amplitude spectra by Fourier transform for (c) Ta(4 nm)/CoFeB(8 nm)/Ir(0.4 nm), (d) Ta(4 nm)/CoFeB(8 nm)/Ir(0.8 nm).
图 3 (a) Ta(4 nm)/CoFeB(8 nm)/Ir(0.8 nm)异质结在不同抽运光能量密度下的太赫兹脉冲时域信号, 抽运激光的能量密度改变范围为0.42—3.36 mJ/cm2; (b)太赫兹辐射脉冲的峰峰值随入射激光脉冲能量密度的关系, 图中的实心方块为实验数据, 红色虚线为拟合结果
Figure 3. (a) Time domain signals emitted from a Ta(4 nm)/CoFeB(8 nm)/Ir(0.8 nm) heterostructure measured under different pump fluences in a range of 0.42–3.36 mJ/cm2; (b) the peak to peak values of THz emission as a function of incident pump fluence, the symbols are raw data and the red dashed line is a fitting curve.
图 4 (a) Ta(4 nm)/CoFeB(8 nm)/Ir(x = 0.4—10 nm)异质结的太赫兹辐射信号时域图; (b)蓝色圆圈为太赫兹脉冲峰峰值随Ir层厚度的依赖关系; 红色曲线为(1)式的拟合曲线
Figure 4. (a) THz emission from Ta(4 nm)/CoFeB(8 nm)/Ir (x = 0.4—10 nm); (b) the peak to peak amplitudes of THz emission as a function of thickness of Ir layer, the red line is a fit using a spin transport model Eq.(1).
图 5 (a)基于不同厚度CoFeB/Ir异质结的模拟太赫兹辐射强度的等高线图; (b) Ir层为优化厚度1.1 nm时, 太赫兹辐射强度随CoFeB层厚度的依赖关系; (c) CoFeB层为优化厚度2.4 nm时, 太赫兹辐射强度随Ir层厚度的依赖关系
Figure 5. (a) Contour plot of the simulated terahertz amplitude based on the CoFeB and Ir layer thickness; (b) terahertz amplitude of CoFeB/Ir heterostructure in terms of CoFeB thickess variation, measured at thickness of Ir is 1.1 nm; (c) terahertz amplitude of CoFeB/Ir heterostructure as a function of Ir thickess, measured at thickness of CoFeB is 2.4 nm.
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[1] Neu J, Schmuttenmaer C A 2018 J. Appl. Phys 124 231101
Google Scholar
[2] Lyu J M, Shen S Y, Chen L, Zhu Y M, Zhuang S L 2023 Photoni X 4 31
Google Scholar
[3] Zhu Y, Zang X F, Chi H X, Zhou Y W, Zhuang S L 2023 Light Adv. Manuf. 4 104
Google Scholar
[4] Zang X F, Yao B S, Chen L, Xie J Y, Guo X G, Balakin A V, Shkurinov A P, Zhuang S L 2021 Light Adv. Manuf. 2 148172
Google Scholar
[5] 韩张华, 孙开礼, 蔡阳健 2021 光学学报 41 258270
Google Scholar
Han Z H, Sun K L, Cai Y J 2021 Acta Opt. Sin. 41 258270
Google Scholar
[6] 吴晓君, 任泽君, 孔德胤, 郝思博, 代明聪, 熊虹婷, 李培炎 2022 中国激光 49 1914001
Google Scholar
Wu X J, Ren Z J, Kong D Y, Hao S B, Dai M C, Xiong H T, Li P Y 2022 Chin. J Lasers 49 1914001
Google Scholar
[7] Chen Q, Zhang X C 1999 Appl. Phys. Lett. 74 3435
Google Scholar
[8] Tu C M, Ku S A, Chu W C, Luo C W, Chen J C, Chi C C 2012 J. Appl. Phys. 112 093110
Google Scholar
[9] Zhang Z L, Chen Y P, Cui S, He F, Chen M, Zhang Z, Yu J, Chen L M, Sheng Z M, Zhang J 2018 Nat. Photonics 12 554
Google Scholar
[10] Fedorov V Y, Tzortzakis S 2020 Light Sci. Appl. 9 186
Google Scholar
[11] Zhang Z L, Chen Y P, Chen M, Zhang Z, Yu J, Sheng Z M, Zhang J 2016 Phys. Rev. Lett. 117 243901
Google Scholar
[12] Jin Z M, Tkach A, Casper F, Spetter V, Grimm H, Thomas A, Kampfrath T, Bonn M, Kläui M, Turchinovich D 2015 Nat. Phys. 11 761
Google Scholar
[13] Jin Z M, Guo Y Y, Ji B Y, Li Z S, Ma G H, Cao S X, Peng Y, Zhu Y M, Zhuang S L 2022 Acta Opt. Sin. 51 0751410 [金钻明, 郭颖钰, 季秉煜, 李章顺, 马国宏, 曹世勋, 彭滟, 朱亦鸣, 庄松林 2022 光子学报 51 0751410]
Google Scholar
Jin Z M, Guo Y Y, Ji B Y, Li Z S, Ma G H, Cao S X, Peng Y, Zhu Y M, Zhuang S L 2022 Acta Opt. Sin. 51 0751410
Google Scholar
[14] Kampfrath T, Tanaka K, Nelson K A 2013 Nat. Photonics 7 680
Google Scholar
[15] Matsuda T, Kanda N, Higo T, Armitage N P, Nakatsuji S, Matsunaga R 2020 Nat. Commun. 11 909
Google Scholar
[16] Jin Z M, Li J G, Zhang W J, Guo C Y, Wan C H, Han X F, Cheng Z X, Zhang C, Balakin A V, Shkurinov A P, Peng Y, Ma G H, Zhu Y M, Yao J Q, Zhuang S L 2020 Phys. Rev. Appl. 14 014032
Google Scholar
[17] Kampfrath T, Battiato M, Maldonado P, Eilers G, No¨tzold J, Ma¨hrlein S, Zbarsky V, Freimuth F, Mokrousov Y, Blügel S, Wolf M, Radu I, Oppeneer P M, Münzenberg M 2013 Nat. Nanotechnol. 8 256
Google Scholar
[18] Seifert T, Jaiswal S, Martens U, Hannegan J, Braun L, Maldonado P, Freimuth F, Kronenberg A, Henrizi J, Radu I, Beaurepaire E, Mokrousov Y, Oppeneer P M, Jourdan M, Jakob G, Turchinovich D, Hayden L M, Wolf M, Münzenberg M, Kläui M, Kampfrath T 2016 Nat. Photonics 10 483
Google Scholar
[19] Liu J Y, Lee K, Yang Y S, Li Z Q, Sharma R, Xi L F, Salim T, Boothroyd C, Lam Y M, Yang H, Battiato M, Chia E E M 2022 Phys. Rev. Appl. 18 034056
Google Scholar
[20] Wang Y K, Li W W, Cheng H, Liu Z, Cui Z Z, Huang J, Xiong B, Yang J W, Huang H L, Wang J L, Fu Z P, Huang Q P, Lu Y L 2023 Commun. Phys. 6 280
Google Scholar
[21] Wu W P, Lendinze S, Kaffash M T, Schaller R D, Wen H D, Jungfleisch M B 2022 Appl. Phys. Lett. 121 052401
Google Scholar
[22] 褚欣博, 金钻明, 吴旭, 李婧楠, 沈阳, 王若愚, 季秉煜, 李章顺, 彭滟 2023 物理学报 72 157801
Google Scholar
Chu X B, Jin Z M, Wu X, Li J N, Shen Y, Wang R Y, Ji B Y, Li Z S, Peng Y 2023 Acta Phys. Sin. 72 157801
Google Scholar
[23] Agarwal P, Huang L S, Lim S T, Singh R 2022 Nat. Commun. 13 4072
Google Scholar
[24] Chaurasiya A, Li Z Q, Medwal R, Gupta S, Mohan J R, Fukuma Y, Asada H, E. M. Chia E, Rawat R S 2022 Adv. Opt. Mater. 10 2201929
Google Scholar
[25] Jin Z M, Guo Y Y, Peng Y, Zhang Z Y, Pang J Y, Zhang Z Z, Liu F, Ye B, Jiang Y X, Ma G H, Zhang C, Balakin A V, Shkurinov A P, Zhu Y M, Zhuang S L 2023 Adv. Phys. Res. 2 2200049
Google Scholar
[26] Levchuk A, Juvé V, Otomalo T O, Chirac T, Rousseau O, Solignac A, Vaudel G, Ruello P, Chauleau J P, Viret M 2023 Appl. Phys. Lett. 123 012407
Google Scholar
[27] Chen S H, Feng Z, Li J, Tan W, Du L H, Cai J W, Ma Y C, He K, Ding H F, Zhai Z H, Li Z R, Qiu C W, Zhang X C, Zhu L G 2020 Light Sci. Appl. 9 99
Google Scholar
[28] Cocker T L, Jelic V, Gupta M, Molesky S J, Burgess J A J, Reyes G D L, Titova L V, Tsui Y Y, Freeman M R, Hegmann F A 2013 Nat. Photonics 7 620
Google Scholar
[29] Wu Y, Elyasi M, Qiu X P, Chen M J, Liu Y, Ke L, Yang H 2017 Adv. Mater. 29 1603031
Google Scholar
[30] Sasaki Y, Kota Y, Iihama S, Suzuki K Z, Sakuma A, Mizukami S 2019 Phys. Rcv. B 100 140406
Google Scholar
[31] Zhang S N, Jin Z M, Zhu Z D, Zhu W H, Zhang Z Z, Ma G H, Yao J Q 2018 J. Phys. D: Appl. Phys. 51 034001
Google Scholar
[32] Vogel T, Omar A, Mansourzadeh S, Wulf F, Sabanés N M, Müller M, Seifert T S, Weigel A, Jakob G, Kläui M, Pupeza L, Kampfrath T, Saraceno C J 2022 Opt. Express 30 20451
Google Scholar
[33] Jin Z M, Peng Y, Ni Y Y, et al. 2022 Laser Photonics Rev. 16 210068
Google Scholar
[34] 张顺浓, 朱伟骅, 李炬赓, 金钻明, 戴晔, 张宗芝, 马国宏, 姚建铨 2018 物理学报 67 197202
Google Scholar
Zhang S N, Zhu W H, Li J G, Jin Z M, Dai Y, Zhang Z Z, Ma G H, Yao J Q 2018 Acta Phys. Sin. 67 197202
Google Scholar
[35] Torosyan G, Keller S, Scheuer L, Beigang R, Papaioannou E T 2018 Sci. Rep. 8 1311
Google Scholar
[36] Belmeguenai M, Apalkov D, Gabor M, Zighem F, Feng G, Tang G 2018 IEEE T. Magn. 54 11
Google Scholar
[37] Seifert T S, Tran N M, Gueckstock O, et al. 2018 J. Phys. D: Appl. Phys. 51 364003
Google Scholar
[38] Saito Y, Tezuka N, Ikeda S, Endoh T 2021 Phys. Rev. B. 104 064493
Google Scholar
[39] Sun R, Li R, Xie Z K, Li Y, Zhao X T, Liu W, Zhang Z D, Zhu T, Cheng Z H, He W 2020 J. Magn. Magn. Mater. 497 165971
Google Scholar
[40] Gueckstock O, Seeger R L, Seifert T S, Auffret S, Gambarelli S, Kirchhof J N, Bolotin K I, Baltz V, Kampfrath T, Nádvorník L 2022 Appl. Phys. Lett. 120 062408
Google Scholar
[41] Panahi O, Yahyaei B, Mousavi S Y, Ghiasabad A M 2020 Laser Phys. 30 055001
Google Scholar
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