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自旋太赫兹源作为一种新型太赫兹辐射源, 以其高效率、超宽带、低成本、易集成等优点已成为太赫兹科学与应用领域的研究热点. 本实验报道了晶圆级磁控溅射生长的多晶拓扑绝缘体Bi2Te3和铁磁体CoFeB双层异质结纳米薄膜发射太赫兹电磁波, 并对太赫兹辐射特性进行了深入而系统的实验研究. 在飞秒激光放大级脉冲作用下, 该异质结呈现出高效率的太赫兹发射, 且辐射偏振可通过外加磁场方向控制. 通过与Pt/CoFeB对比, 研究发现Bi2Te3/CoFeB的发射性能与Pt/CoFeB双层异质结相当. 实验还对生长在不同衬底上的Bi2Te3/CoFeB的发射性能进行了对比研究, 发现MgO衬底上制备的样品具有相对较好的太赫兹辐射性能. 本实验研究不仅对自旋太赫兹发射机理有更加深入的认识, 而且通过样品和结构的优化, 有望获得更高的发射效率, 且该发射器具有大尺寸批量生长、成本较低的优势, 具备商业化应用的潜力.
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关键词:
- 太赫兹辐射 /
- 拓扑绝缘体/铁磁异质结 /
- 飞秒激光
High-performance terahertz emitters, which convert the femtosecond laser pulses into terahertz pulses, are essential for terahertz spectroscopy technology and terahertz wireless communication. Spintronic terahertz emitters based on ferromagnet/nonmagnet bilayers have attracted tremendous attention due to their high efficiency, ultra-broadband, low cost and high flexibility. Here, we systematically investigate the terahertz emission from polycrystalline topological insulator Bi2Te3/ferromagnetic CoFeB heterostructure grown by magnetron sputtering. The Bi2Te3/CoFeB heterostructure exhibits high efficiency of terahertz emission, and the polarization of terahertz waves can be controlled by the external magnetic field direction. The performance of Bi2Te3/CoFeB heterostructure is almost comparable to that of the Pt/CoFeB bilayer. In contrast, no terahertz emission is observed in the pure Bi2Te3 or CoFeB film driven by femtosecond laser pulses, probably because the Bi2Te3 prepared by sputtering is polycrystalline and the thickness of CoFeB is too thin. We also compare the performances of Bi2Te3/CoFeB grown on MgO, glass and high-resistivity silicon substrates, and find that the samples grown on MgO substrates exhibit the best emission performances. The glass substrate absorbs more terahertz waves than MgO substrate, resulting in a slightly weaker terahertz signal emitted from the Bi2Te3/CoFeB grown on the glass substrate. Although the absorption coefficient of high-resistivity silicon to terahertz waves is very small, the residual pump light excites the high-resistivity silicon to generate the photo-generated carriers, which change the conductivity of the high-resistivity silicon and reduce the transmittance of terahertz wave. We attribute the mechanism of the terahertz emission to the spin-charge conversion at the interface of Bi2Te3/CoFeB. The terahertz emission efficiency of our sample is expected to be able to be further improved by optimizing the samples. Moreover, with the sputtering method, it is possible to fabricate large area samples at low cost, which is critical for commercial applications.-
Keywords:
- terahertz emission /
- topological insulator/ferromagnetic heterostructures /
- femtosecond laser
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图 1 (a)实验装置示意图; (b) Bi2Te3(4)/CoFeB(2)异质结的结构示意图
Fig. 1. (a) Schematic diagram of experimental setup; (b) schematic illustration of BiTe/CoFeB heterostructure structure information
图 2 生长在MgO、高阻硅和玻璃衬底上的Bi2Te3(4)/CoFeB(2)异质结的太赫兹发射性能比较
Fig. 2. Comparison of the terahertz waveforms generated from the Bi2Te3(4)/CoFeB(2) heterostructure grown on MgO, high resistivity silicon, and glass substrates.
图 3 (a) CoFeB(2), Bi2Te3(4)和Bi2Te3(4)/CoFeB(2)辐射的太赫兹波形; (b) Bi2Te3(4)/CoFeB(2)辐射的太赫兹频域谱
Fig. 3. (a) Terahertz waveforms generated from CoFeB(2), Bi2Te3(4), and Bi2Te3(4)/CoFeB(2), respectively; (b) terahertz spectra obtained from Bi2Te3(4)/CoFeB(2).
图 4 Bi2Te3(4)/CoFeB(2)异质结的太赫兹辐射 (a) 抽运光从Bi2Te3(4)/CoFeB(2)样品正面和背面入射以及磁场反向时Bi2Te3(4)/CoFeB(2)辐射的太赫兹波形; (b) Bi2Te3(4)/CoFeB(2)异质结发射的太赫兹脉冲的峰值振幅与施加的外磁场方向的关系
Fig. 4. Terahertz emission from Bi2Te3(4)/CoFeB(2) heterostructure: (a) Terahertz waveforms emitted from the Bi2Te3(4)/CoFeB(2) heterostructure measured with front and back sample excitation and reversed magnetic field; (b) the peak amplitude of the terahertz signal emitted from the Bi2Te3(4)/CoFeB(2) heterostructure as a function of magnetic field angle θ, with respect to the x-axis.
图 5 Bi2Te3(4)/CoFeB(2), W(4)/CoFeB(2.2), Pt(4)/CoFeB(2.2)的太赫兹发射性能比较
Fig. 5. Comparison of the terahertz waveforms generated from the Bi2Te3(4)/CoFeB(2), W(4)/CoFeB(2.2), and Pt(4)/CoFeB(2.2).
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[1] Kampfrath T, Battiato M, Maldonado P, Eilers G, Notzold J, Mahrlein S, Zbarsky V, Freimuth F, Mokrousov Y, Blugel S, Wolf M, Radu I, Oppeneer P M, Munzenberg M 2013 Nat. Nanotechnol. 8 256
Google Scholar
[2] 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. Photon. 10 483
Google Scholar
[3] Yang D, Liang J, Zhou C, Sun L, Zheng R, Luo S, Wu Y, Qi J 2016 Adv. Opt. Mater. 4 1944
Google Scholar
[4] Wu Y, Elyasi M, Qiu X, Chen M, Liu Y, Ke L, Yang H 2017 Adv. Mater. 29 1603031
Google Scholar
[5] Seifert T, Jaiswal S, Sajadi M, Jakob G, Winnerl S, Wolf M, Kläui M, Kampfrath T 2017 Appl. Phys. Lett. 110 252402
Google Scholar
[6] Zhou C, Liu Y P, Wang Z, Ma S J, Jia M W, Wu R Q, Zhou L, Zhang W, Liu M K, Wu Y Z, Qi J 2018 Phys. Rev. Lett. 121 086801
Google Scholar
[7] Qiu H S, Kato K, Hirota K, Sarukura N, Yoshimura M, Nakajima M 2018 Opt. Express 26 15247
Google Scholar
[8] Feng Z, Yu R, Zhou Y, Lu H, Tan W, Deng H, Liu Q, Zhai Z, Zhu L, Cai J, Miao B, Ding H 2018 Adv. Opt. Mater. 6 1800965
Google Scholar
[9] Li G, Medapalli R, Mikhaylovskiy R V, Spada F E, Rasing T, Fullerton E E, Kimel A V 2019 Phys. Rev. Mater. 3 084415
Google Scholar
[10] Kong D, Wu X, Wang B, Nie T, Xiao M, Pandey C, Gao Y, Wen L, Zhao W, Ruan C, Miao J, Li Y, Wang L 2019 Adv. Opt. Mater. 7 1900487
Google Scholar
[11] Kondou K, Yoshimi R, Tsukazaki A, Fukuma Y, Matsuno J, Takahashi K S, Kawasaki M, Tokura Y, Otani Y 2016 Nat. Phys. 12 1027
Google Scholar
[12] Wang Y, Deorani P, Banerjee K, Koirala N, Brahlek M, Oh S, Yang H 2015 Phys. Rev. Lett. 114 257202
Google Scholar
[13] Jamali M, Lee J S, Jeong J S, Mahfouzi F, Lv Y, Zhao Z, Nikolic B K, Mkhoyan K A, Samarth N, Wang J P 2015 Nano Lett. 15 7126
Google Scholar
[14] Pesin D, MacDonald A H 2012 Nat. Mater. 11 409
Google Scholar
[15] Soumyanarayanan A, Reyren N, Fert A, Panagopoulos C 2016 Nature 539 509
Google Scholar
[16] Wang X, Cheng L, Zhu D, Wu Y, Chen M, Wang Y, Zhao D, Boothroyd C B, Lam Y M, Zhu J X, Battiato M, Song J C W, Yang H, Chia E E M 2018 Adv. Mater. 30 1802356
Google Scholar
[17] Braun L, Mussler G, Hruban A, Konczykowski M, Schumann T, Wolf M, Munzenberg M, Perfetti L, Kampfrath T 2016 Nat. Commun. 7 13259
Google Scholar
[18] Fang Z, Wang H, Wu X, Shan S, Wang C, Zhao H, Xia C, Nie T, Miao J, Zhang C, Zhao W, Wang L 2019 Appl. Phys. Lett. 115 191102
Google Scholar
[19] Torosyan G, Keller S, Scheuer L, Beigang R, Papaioannou E T 2018 Sci. Rep. 8 1311
Google Scholar
[20] van Exter M, Grischkowsky D 1990 Appl. Phys. Lett. 56 1694
Google Scholar
[21] Battiato M, Carva K, Oppeneer P M 2010 Phys. Rev. Lett. 105 027203
Google Scholar
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