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The development of efficient terahertz (THz) radiation sources is driven by the scientific and technological applications. To date, as far as the radiation of THz pulses is concerned, the widely used methods are biased semiconductor, electro-optical crystal and air plasma, which are excited separately by femtosecond laser pulses. The mechanisms involved in these THz sources are photo-carrier acceleration, second order nonlinear effect, and plasma oscillations, respectively. Here, we report the generation of coherent THz radiation in the designed ferromagnetic/non-magnetic metallic W/CoFeB/Pt and Ta/CoFeB/Pt trilayers on SiO2 substrates, excited separately by ultrafast laser pulses. The transient THz electric field is fully inverted when the magnetization is reversed, which indicates a strong connection between THz radiation and spin order of the sample. We present the THz radiation results of the bilayers, CoFeB/W, CoFeB/Pt and CoFeB/Ta, which are comprised of the trilayer heterostructures used in our experiments. We find that all experimental results are in good agreement with the results from the inversed spin-Hall effect (ISHE) mechanism. Owing to the ISHE, the transient spin current converts into a transient transverse charge current, which launches the THz electromagnetic wave. In our experiments, W or Ta has an opposite spin Hall angle to Pt. Therefore, the amplitude of the THz emission can be increased by a constructive superposition of two charge currents in metallic layers. Our results indicate that the peak-values of the THz radiation covering the 0-2.5 THz range from W/CoFeB/Pt and Ta/CoFeB/Pt are stronger than that from 0.5 mm thick ZnTe (110) crystal, under very similar excitation conditions. Finally, we investigate the dependence of peak-to-peak values for two different heterostructures on the pump fluence. The saturations of THz pulse at pump fluences of~0.47 mJ/cm2 and~0.61 mJ/cm2 are found for W/CoFeB/Pt and Ta/CoFeB/Pt heterostructures, respectively. The saturation can be generally attributed to the spin accumulation effect and laser-induced thermal effect. Our results indicate that the spin accumulation effect, by which the density of spin-polarized electrons is restricted in a non-magnetic metallic layer, is slightly less pronounced for Ta/CoFeB/Pt system at high fluences. Our findings provide a new pathway for fabricating the spintronic THz emitter, which is comparable to the conventional nonlinear optical crystals.
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Keywords:
- THz radiation /
- ultrafast spectroscopy /
- spin current /
- inversed spin-Hall effect
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[42] Li Y, Wang X, Xiong Z, Cao L, Chen J, Wang X, Shen C, Peng L, Zhao Y, Li W, Deng Q, Wang J, Yu J, Yin H, Wu W 2016 J. Alloy. Compd. 686 841
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[44] Zhang S, Jin Z, Liu X, Zhao W, Lin X, Jing C, Ma G 2017 Opt. Lett. 42 3080
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[1] Tonouchi M 2007 Nat. Photon. 1 97
[2] Ferguson B, Zhang X C 2002 Nat. Mater. 1 26
[3] Jin Z, Tkach A, Casper F, Spetter V, Grimm H, Thomas A, Kampfrath T, Bonn M, Klui M, Turchinovich D 2015 Nat. Phys. 11 761
[4] Ulbricht R, Hendry E, Shan J, Heinz T F, Bonn M 2011 Rev. Mod. Phys. 83 543
[5] Fischer B M, Walther M, Jepsen P U 2002 Phys. Med. Biol. 47 3807
[6] Siegel P H 2004 IEEE Trans. Microw. Theory Tech. 52 2438
[7] Zhang R, Li H, Cao J C, Feng S L 2009 Acta Phys. Sin. 58 4618 (in Chinese) [张戎, 黎华, 曹俊诚, 封松林 2009 物理学报 58 4618]
[8] Jin Z, Mics Z, Ma G, Cheng Z, Bonn M, Turchinovich D 2013 Phys. Rev. B 87 094422
[9] Lewis R A 2014 J. Phys. D:Appl. Phys. 47 374001
[10] Wang W M, Zhang L L, Li Y T, Sheng Z M, Zhang J 2018 Acta Phys. Sin. 67 124202 (in Chinese) [王伟民, 张亮亮, 李玉同, 盛政明, 张杰 2018 物理学报 67 124202]
[11] Nahata A, Weling A S, Heinz T F 1996 Appl. Phys. Lett. 69 2321
[12] Zhong K, Yao J Q, Xu D G, Zhang H Y, Wang P 2011 Acta Phys. Sin. 60 034210 (in Chinese) [钟凯, 姚建铨, 徐德刚, 张会云, 王鹏 2011 物理学报 60 034210]
[13] Matsuura S, Tani M, Sakai K 1997 Appl. Phys. Lett. 70 559
[14] Shi W, Yan Z J 2015 Acta Phys. Sin. 64 228702 (in Chinese) [施卫, 闫志巾 2015 物理学报 64 228702]
[15] Beaurepaire E, Turner G M, Harrel S M, Beard M C, Bigot J Y, Schmuttenmaer C A 2004 Appl. Phys. Lett. 84 3465
[16] Hilton D J, Averitt R D, Meserole C A, Fisher G L, Funk D J, Thompson J D, Taylor A J 2004 Opt. Lett. 29 1805
[17] Shen J, Fan X, Chen Z, de Camp M F, Zhang H, Xiao J Q 2012 Appl. Phys. Lett. 101 072401
[18] Nishant K, Hendrikx R W, Adam A J, Planken P C 2015 Opt. Express 23 14252
[19] Gorelov S D, Mashkovich E A, Tsarev M V, Bakunov M I 2013 Phys. Rev. B 88 220411
[20] Mikhaylovskiy R V, Hendry E, Kruglyak V V, Pisarev R V, Rasing T, Kimel A V 2014 Phys. Rev. B 90 184405
[21] Mikhaylovskiy R V, Hendry E, Secchi A, Mentink J H, Eckstein M, Wu A, Pisarev R V, Kruglyak V V, Katsnelson M I, Rasing T, Kimel A V 2015 Nature Commun. 6 8190
[22] 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 Nature Nanotech. 8 256
[23] Wu Y, Elyasi M, Qiu X, Chen M, Liu Y, Ke L, Yang H 2017 Adv. Mater. 29 1603031
[24] Yang D, Liang J, Zhou C, Sun L, Zheng R, Luo S, Wu Y, Qi J 2016 Adv. Opt. Mater. 4 1944
[25] Qiu H S, Kato K, Hirota K, Sarukura N, Yoshimura M, Nakajima M 2018 Opt. Express 26 15247
[26] 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, Mnzenberg M, Klui M, Kampfrath T 2016 Nature Photon. 10 483
[27] Seifert T, Jaiswal S, Sajadi M, Jakob G, Winnerl S, Wolf M, Klui M, Kampfrath T 2017 Appl. Phys. Lett. 110 252402
[28] Seifert T, Martens U, Gnther S, Schoen M A W, Radu F, Chen X Z, Lucas I, Ramos R, Aguirre M H, Algarabel P A, Anadn A, Krner H S, Walowski J, Back C, Ibarra M R, Morelln L, Saitoh E, Wolf M, Song C, Uchida K, Mnzenberg M, Radu I, Kampfrath T 2017 Spin 7 1740010
[29] Torosyan G, Keller S, Scheuer L, Beigang R, Papaioannou E T 2018 Sci. Rep. 8 1311
[30] Sasaki Y, Suzuki K Z, Mizukami S 2017 Appl. Phys. Lett. 111 102401
[31] Battiato M, Carva K, Oppeneer P M 2010 Phys. Rev. Lett. 105 027203
[32] Eschenlohr A, Battiato M, Maldonad P, Pontius N, Kachel T, Holldack K, Mitzner R, Fhlisch A, Oppeneer P M, Stamm C 2013 Nat. Mater. 12 332
[33] 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
[34] Rudolf D, Chan L O, 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 Nature Commun. 3 1037
[35] Cramer J, Seifert T, Kronenberg A, Fuhrmann F, Jakob G, Jourdan M, Kampfrath T, Klaui M 2018 Nano Lett. 18 1064
[36] Zhang S, Jin Z, Zhu Z, Zhu W, Zhang Z, Ma G, Yao J 2018 J. Phys. D:Appl. Phys. 51 034001
[37] Hao Q, Xiao G 2015 Phys. Rev. Appl. 3 034009
[38] Tanaka T, Kontani H, Naito M, Naito T, Hirashima D S, Yamada K, Inoue J 2008 Phys. Rev. B 77 165117
[39] Huisman T J, Mikhaylovskiy R V, Costa J D, Freimuth F, Paz E, Ventura J, Freitas P P, Blugel S, Mokrousov Y, Rasing T, Kimel A V 2016 Nat. Nanotechnol. 11 455
[40] Huisman T J, Rasing T 2017 J. Phys. Soc. Jpn. 86 011009
[41] Wang X M, Zhao Y, Wang X M, Jiang T, Shen C L, Li W H, Peng L P, Yan D W, Zhan Z Q, Deng Q H, Wu W D, Tang Y J 2015 Mater. Lett. 153 81
[42] Li Y, Wang X, Xiong Z, Cao L, Chen J, Wang X, Shen C, Peng L, Zhao Y, Li W, Deng Q, Wang J, Yu J, Yin H, Wu W 2016 J. Alloy. Compd. 686 841
[43] Kinoshita Y, Kida N, Sotome M, Miyamoto T, Iguchi Y, Onose Y, Okamoto H 2016 ACS Photon. 3 1170
[44] Zhang S, Jin Z, Liu X, Zhao W, Lin X, Jing C, Ma G 2017 Opt. Lett. 42 3080
[45] Barnes M E, Berry S A, Gow P, McBryde D, Daniell G J, Beere H E, Ritchie D A, Apostolopoulos V 2013 Opt. Express 21 16263
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