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自铁磁金属在飞秒激光泵浦下的超快退磁效应发现以来, 电子的自旋属性逐渐被应用于太赫兹电磁波的产生. 利用逆Rashba-Edelstein效应产生太赫兹辐射首先在Ag/Bi界面得到证实, 而LaAlO3/SrTiO3界面通过该效应产生直流的自旋-电荷转换效率要高于Ag/Bi界面约一个数量级, 但利用该结构转化自旋流来产生太赫兹的有效性尚待系统的研究. 本文制备了NiFe/LaAlO3//SrTiO3(001)系列样品, 在飞秒激光泵浦下观察到了太赫兹辐射的产生及其对磁场方向的依赖效应, 并通过改变LaAlO3层的厚度验证了超扩散模型与光学传输模型的有效性, 观察到了在LaAlO3/SrTiO3界面由于多次反射导致太赫兹波的减弱, 为进一步优化太赫兹波的产生提供了实验和理论支持.
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关键词:
- 太赫兹辐射 /
- 自旋流 /
- 逆Rashba-Edelstein效应 /
- 氧化物异质结
Since the discovery of the ultrafast demagnetization of the ferromagnetic metal, the spin degree of electrons is gradually used to generate terahertz radiation. The terahertz radiation generated by the inverse Rashba-Edelstein effect was confirmed first at the interface of Ag/Bi. However, the spin-to-charge conversion efficiency of the LaAlO3/SrTiO3 interface is one order of magnitude lager than that of the Ag/Bi interface under equilibrium or quasi-equilibrium condition. Whether the LaAlO3/SrTiO3 heterostructures can be used to convert spin current to generate terahertz radiation remains to be systemically studied. In this work, we fabricate the NiFe/LaAlO3//SrTiO3 heterostructures and investigate the generation of terahertz radiation by femtosecond laser pumping and its dependence of the magnetic field direction. We change the thickness of the LaAlO3 to show the applicability of the superdiffusive spin transport model and optical transmission model. We find the multireflections at the LaAlO3/SrTiO3 interface weaken the terahertz radiation intensity. This work provides experimental and theoretical support for further optimizing the generation of terahertz electromagnetic waves.-
Keywords:
- terahertz radiation /
- spin current /
- inverse Rashba-Edelstein effect /
- oxide heterostructures
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图 1 (a) STO(001)衬底生长LAO薄膜的RHEED振荡谱图和衍射图; (b) LAO//STO(001)薄膜形貌图; (c) 太赫兹发射示意图
Fig. 1. (a) The RHEED spectrum for the growth process of LAO on STO substrate (001), and the RHEED patterns before and after the growth of the LAO films; (b) the surface morphology of LAO//STO films; (c) the schematic diagram of the terahertz emission.
图 2 太赫兹辐射实验装置. 放大的虚线框表示样品与永磁体的关系
Fig. 2. Schematic diagram of terahertz radiation experimental configuration. The zoomed area shows the relations between the sample and magnetic field.
图 3 (a)不同厚度的LAO样品辐射的太赫兹时域波形; (b)对应的频谱图
Fig. 3. (a) Typical terahertz temporal waveforms for LAO samples with different thicknesses, and (b) the corresponding spectra.
图 4 (a)太赫兹辐射极性随外加磁场方向的改变而反转; (b) NiFe//STO与NiFe/LAO (10 uc)//STO辐射太赫兹波的大小比较
Fig. 4. (a) Radiated terahertz polarity reversal when varying the applied magnetic field direction; (b) comparison of the terahertz radiation between NiFe//STO and NiFe/LAO (10 uc)//STO.
图 5 真空环境下, 10 uc的LAO太赫兹辐射时域波形(a)和对应的频谱(b)
Fig. 5. (a) Emitted terahertz temporal waveform from the LAO (10 uc)//STO, and (b) its corresponding spectrum under vacuum environment.
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[1] Smith P R, Auston D H, Nuss M C 1988 IEEE J. Quantum Electron. 24 255
Google Scholar
[2] Beaurepaire E, Merle J C, Daunois A, Bigot J Y 1996 Phys. Rev. Lett. 76 4250
Google Scholar
[3] Dornes C, Acremann Y, Savoini M, et al. 2019 Nature 565 209
Google Scholar
[4] Pierce D T, Meier F 1976 Phys. Rev. B 13 5484
Google Scholar
[5] Battiato M, Carva K, Oppeneer P M 2010 Phys. Rev. Lett. 105 027203
Google Scholar
[6] Battiato M, Carva K, Oppeneer P M 2012 Phys. Rev. B 86 024404
Google Scholar
[7] Battiato M, Maldonado P, Oppeneer P M 2014 J. Appl. Phys. 115 172611
Google Scholar
[8] Battiato M, Held K 2016 Phys. Rev. Lett. 116 196601
Google Scholar
[9] Melnikov A, Razdolski I, Wehling T O, Papaioannou E T, Roddatis V, Fumagalli P, Aktsipetrov O, Lichtenstein A I, Bovensiepen U 2011 Phys. Rev. Lett. 107 076601
Google Scholar
[10] Rudolf D, La-O-Vorakiat C, Battiato M, Adam R, Shaw J M, Turgut E, Maldonado P, Mathias S, Grychtol P, Nembach H T, Silva T J, Aeschlimann M, Kapteyn H C, Murnane M M, Schneider C M, Oppeneer P M 2012 Nat. Commun. 3 1037
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[12] Ando K, Morikawa M, Trypiniotis T, Fujikawa Y, Barnes C H W, Saitoh E 2010 Appl. Phys. Lett. 96 082502
Google Scholar
[13] Isella G, Bottegoni F, Ferrari A, Finazzi M, Ciccacci F 2015 Appl. Phys. Lett. 106 232402
Google Scholar
[14] Kampfrath T, Battiato M, Maldonado P, Eilers G, Nötzold J, Mährlein 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
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[19] 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
[20] Cheng L, Wang X, Yang W, Chai J, Yang M, Chen M, Wu Y, Chen X, Chi D, Johnson K E, Zhu J X, Sun H, Wang S, Song C W J, Battiato M, Yang H, Chia E E M 2019 Nat. Phys. 15 347
Google Scholar
[21] Lesne E, Fu Y, Oyarzun S, Rojas-Sánchez J C, Vaz D C, Naganuma H, Sicoli G, Attané J P, Jamet M, Jacquet E, George J M, Barthélémy A, Jaffrès H, Fert A, Bibes M, Vila L 2016 Nat. Mater. 15 1261
Google Scholar
[22] Huisman T J, Mikhaylovskiy R V, Tsukamoto A, Rasing T, Kimel A V 2015 Phys. Rev. B 92 104419
Google Scholar
[23] Huang L, Kim J W, Lee S H, Kim S D, Tien V M, Shinde K P, Shim J H, Shin Y, Shin H J, Kim S, Park J, Park S Y, Choi Y S, Kim H J, Hong J I, Kim D E, Kim D H 2019 Appl. Phys. Lett. 115 142404
Google Scholar
[24] Beaurepaire E, Turner G M, Harrel S M, Beard M C, Bigot J Y, Schmuttenmaer C A 2004 Appl. Phys. Lett. 84 3465
Google Scholar
[25] Yang H, Zhang B, Zhang X, Yan X, Cai W, Zhao Y, Sun J, Wang K L, Zhu D, Zhao W 2019 Phys. Rev. Appl. 12 034004
Google Scholar
[26] Puebla J, Auvray F, Yamaguchi N, Xu M R, Bisri S Z, Iwasa Y, Ishii F, Otani Y 2019 Phys. Rev. Lett. 122 256401
Google Scholar
[27] Song Q, Zhang H R, Su T, Yuan W, Chen Y Y, Xing W Y, Shi J, Sun J R, Han W 2017 Sci. Adv. 3 e1602312
Google Scholar
[28] Sing M, Berner G, Goß K, Müller A, Ruff A, Wetscherek A, Thiel S, Mannhart J, Pauli S A, Schneider C W, Willmott P R, Gorgoi M, Schäfers F, Claessen R 2009 Phys. Rev. Lett. 102 176805
Google Scholar
[29] Han J, Wan F, Zhu Z, Zhang W 2007 Appl. Phys. Lett. 90 031104
Google Scholar
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