关键词: 太赫兹波/
超快光谱/
自旋流/
逆自旋霍尔效应
English Abstract
Coherent terahertz radiation via ultrafast manipulation of spin currents in ferromagnetic heterostructures
Zhang Shun-Nong1,Zhu Wei-Hua2,
Li Ju-Geng1,
Jin Zuan-Ming1,3,
Dai Ye1,
Zhang Zong-Zhi2,
Ma Guo-Hong1,3,
Yao Jian-Quan4
1.Department of Physics, Shanghai University, Shanghai 200444, China;
2.Department of Optical Science and Engineering, Fudan University, Shanghai 200082, China;
3.STU & SIOM Joint Laboratory for Superintense Lasers and the Applications, Shanghai 201210, China;
4.College of Precision Instrument and Opto-electronics Engineering, Tianjin University, Tianjin 300110, China
Fund Project:Project supported by the National Natural Science Foundation of China (Grant Nos. 11604202, 11674213, 61735010, 51671057, 11774220), the Young Eastern Scholar, China (Grant No. QD2015020), Chen Guang Project of the Shanghai Municipal Education Commission of China and the Shanghai Education Development Foundation of China (Grant No. 16CG45), and the Shanghai Rising-Star Program, China (Grant No. 18QA1401700).Received Date:15 June 2018
Accepted Date:26 July 2018
Published Online:05 October 2018
Abstract: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.
Keywords: THz radiation/
ultrafast spectroscopy/
spin current/
inversed spin-Hall effect
