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极性材料中, 电子受激发跃迁会改变材料的电极化矢量, 受到飞秒脉冲激光的激发时, 瞬态变化的电极化矢量会向外辐射电磁波, 产生太赫兹波段的发射谱. 在磁有序体系中, 受到相干激发的自旋波在进动弛豫的过程中, 会辐射相同频率的电磁波. 研究这些材料的太赫兹发射谱, 不仅有助于理解材料的铁电有序和磁有序的动力学过程, 也为寻找新的太赫兹源提供参考. 我们对极性反铁磁体Fe2Mo3O8的太赫兹发射谱进行了研究, 在800 nm 飞秒激光的泵浦下, 材料中的电子发生跨越能隙的激发, 对电极化矢量产生超快调制, 观察到0.1—3.5 THz的宽带太赫兹激发谱, 太赫兹电场方向沿材料的固有电极化方向. 在进入磁有序后, 观测到两个新的单频太赫兹震荡, 分别位于1.25和2.70 THz, 分别对应Fe2Mo3O8的两个反铁磁自旋波.In polar materials, the transition of electrons in momentum space will change the spontaneous polarization. When excited by femtosecond pulse laser, the transient modulation of the electric polarization will radiate electromagnetic wave at terahertz frequency. In a magnetic ordered system, the coherent excited spin wave radiates electromagnetic waves of the same frequency in the process of precession and relaxation. The investigation of the terahertz emission spectra of these materials not only helps us to understand the ferroelectric and magnetic ordered dynamic processes of materials, but also provides a reference for searching for new terahertz sources. We study the terahertz emission spectrum of the polar antiferromagnet Fe2Mo3O8. Under the pumping of 800 nm laser, electrons in the material are excited across the band gap leading the electric polarization to be ultra-fast modulated. The broadband terahertz excitation spectrum from 0.1 to 3.5 THz is observed, and the direction of the terahertz electric field is along the inherent electric polarization direction of the material. After entering into the magnetic order state, two new single-frequency terahertz oscillations are observed, located at 1.25 THz and 2.7 THz respectively, which correspond to the excitation of the two antiferromagnetic spin waves of Fe2Mo3O8.
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Keywords:
- Fe2Mo3O8 /
- polar antiferromagnet /
- THz emission spectrum
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图 2 太赫兹发射谱测量实验示意图 (a) 实验光路图; (b) 泵浦光激发太赫兹的示意图
Fig. 2. schematic diagram of the measurement of the THz emission spectrum: (a) optical path of the experiment system; (b) schematic diagram of the THz emission from optical pump.
图 3 太赫兹发射谱测量结果 (a) ac面样品60 K的太赫兹发射谱的时域谱线; (b) (a)图中时域谱线的傅里叶变换; (c) ac面样品10 K 的太赫兹发射谱的时域谱线; (d) (c)图中时域谱线的傅里叶变换; (e) ab 面样品60 K 的太赫兹发射谱的时域谱线; (f) (e)图中时域谱线的傅里叶变换; (g) ab面样品10 K的太赫兹发射谱的时域谱线; (h) (g)图中时域谱线的傅里叶变换;
Fig. 3. THz emission spectra : (a) Time domain spectrum of ac plane at 60 K; (b) Fourier transform spectrum of (a); (c) time domain spectrum of ac plane at 10 K; (d) Fourier transform spectrum of (c); (e) time domain spectrum of ab plane at 60 K; (f) Fourier transform spectrum of (e); (g) time domain spectrum of ab plane at 10 K; (h) Fourier transform spectrum of (g).
图 4 (a) 太赫兹时域信号峰值随泵浦光功率的变化; (b) 傅里叶变换光谱仪测得的Fe2Mo3O8的电导率谱
Fig. 4. (a) Fluence dependence of the THz intensity; (b) Optical conductivity of Fe2Mo3O8 measured by Fourier transform spectrometer.
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[1] Kawayama I, Zhang C H, Wang H B, Tonouchi M 2013 Supercond Sci. Tech. 26 093002
Google Scholar
[2] Turner G M, Harrel S M, Beard M C 2004 Appl. Phys. Lett. 84 3465
Google Scholar
[3] Klatt G, Hilser F, Qiao W, Beck M, Gebs R, Bartels A, Huska K, Lemmer U, Bastian G, Johnston M B, Fischer M, Faist J, Dekorsy T 2010 Opt. Express 18 4939
Google Scholar
[4] Fiebig M, Lottermoser T, Meier D, Trassin M 2016 Nat. Rev. Mater. 1 16046
Google Scholar
[5] Wang Y Z, Pascut G L, Gao B, Tyson T A, Haule K, Kiryukhin V, Cheong S W 2015 Sci. Rep.-UK 5 12268
Google Scholar
[6] Kurumaji T, Ishiwata S, Tokura Y 2015 Phys. Rev. X 5 031034
[7] Li Y, Gao G, Yao K L 2017 EPL (Europhysics Letters) 118 37001
Google Scholar
[8] Kurumaji T, Takahashi Y, Fujioka J, Masuda R, Shishikura H, Ishiwata S, Tokura Y 2017 Phys. Rev. B 95 020405
Google Scholar
[9] Kurumaji T, Takahashi Y, Fujioka J, Masuda R, Shishikura H, Ishiwata S, Tokura Y 2017 Phys. Rev. Lett. 119 077206
Google Scholar
[10] Yu S K, Gao B, Kim J W, Cheong S W, Man M K L, Madéo J, Dani K M, Talbayev D 2018 Phys. Rev. Lett. 120 037601
Google Scholar
[11] Shi L Y, Wu D, Wang Z X, Lin T, Hu C M, Wang N L 2020 arXiv 2004.05823
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Google Scholar
[13] Strobel P, Page Y L, McAlister S P 1982 J. Solid State Chem. 42 242
Google Scholar
[14] Zhou R Z, Jin Z M, Li G F, Ma G H, Cheng Z X, Wang X L 2012 Appl. Phys. Lett. 100 061102
Google Scholar
[15] Xu L, Zhang X C, Auston D H 1992 Appl. Phys. Lett. 61 1784
Google Scholar
[16] Liu T A, Tani M, Nakajima M, Hangyo M, Sakai K, Nakashima S I, Pan C L 2004 Opt. Express 12 2954
Google Scholar
[17] Sotome M, Nakamura M, Fujioka J, Ogino M, Kaneko Y, Morimoto T, Zhang Y, Kawasaki M, Nagaosa N, Tokura Y, Ogawa N 2019 P. Natl. Acad. Sci. USA 116 1929
Google Scholar
[18] Takahashi K, Kida N, Tonouchi M 2006 Phys. Rev. Lett. 96 117402
Google Scholar
[19] Takahashi K, Tonouchi M 2007 Appl. Phys. Lett. 90 052908
Google Scholar
[20] Rana D S, Kawayama I, Mavani K, Takahashi K, Murakami H, Tonouchi M 2009 Adv. Mater. 21 2881
Google Scholar
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