Efficient metallic spintronic emitters of ultrabroadband terahertz radiation

Abstract

Terahertz electromagnetic radiation is extremely useful for numerous applications, including imaging and spectroscopy. It is thus highly desirable to have an efficient table-top emitter covering the 1–30 THz window that is driven by a low-cost, low-power femtosecond laser oscillator. So far, all solid-state emitters solely exploit physics related to the electron charge and deliver emission spectra with substantial gaps. Here, we take advantage of the electron spin to realize a conceptually new terahertz source that relies on three tailored fundamental spintronic and photonic phenomena in magnetic metal multilayers: ultrafast photoinduced spin currents, the inverse spin-Hall effect and a broadband Fabry–Pérot resonance. Guided by an analytical model, this spintronic route offers unique possibilities for systematic optimization. We find that a 5.8-nm-thick W/CoFeB/Pt trilayer generates ultrashort pulses fully covering the 1–30 THz range. Our novel source outperforms laser-oscillator-driven emitters such as ZnTe(110) crystals in terms of bandwidth, terahertz field amplitude, flexibility, scalability and cost.

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Figure 1: Metallic spintronic terahertz emitter.
Figure 2: Impact of the NM material.
Figure 3: Impact of stack geometry on emitter performance.
Figure 4: Spintronic emitter performance and spectroscopic application.

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Acknowledgements

The authors thank S. Winnerl for providing a TeraSED3 emitter and for stimulating discussions. The authors acknowledge the German Science Foundation for funding through SPP 1538/SpinCaT (Berlin, Greifswald, Jülich and Mainz groups) and through SFB TRR 173/Spin+X (Mainz group). The European Union is acknowledged for funding through ERC H2020 CoG project TERAMAG/grant no. 681917 (T.K.), FP7 project CRONOS/grant no. 280879 (T.K. and M.W.), FP7 Marie Curie ITN WALL project/grant no. 608031 (J.S., G.J. and M.K.), Career Integration Grant LIGHTER/grant no. 334324 (D.T.) and FP7 project FemtoSpin/grant no. 281043 (P.M. and P.M.O.). The authors are grateful for support from the Max Planck Society (T.K. and D.T.), the Swedish Research Council, the Röntgen-Ångström Cluster, and the K. and A. Wallenberg Foundation (P.M. and P.M.O.), the Samsung SGMI programme, the EFRE Program of the state of Rhineland Palatinate, TT-DINEMA, the Excellence Graduate School MAINZ (GSC 266) and the state research centre CINEMA (Mainz group). F.F. and Y.M. acknowledge Jülich Supercomputing Centre for computing time.

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T.K. and T.S. conceived the experiments. T.K. and I.R. carried out preliminary measurements. S.J., U.M., A.K., J.He., E.B., M.J., G.J., M.M. and M.K. fabricated the spintronic emitters and optimized the fabrication process. L.B., T.S. and T.K. built the terahertz emission set-up. J.Ha. and L.M.H. fabricated the LAPC electrooptic detectors. T.S. performed the terahertz experiments and optical sample characterization. T.S. and T.K. analysed experimental data with contributions from D.T. and M.K. T.K. and T.S. developed the analytical terahertz-emitter model with contributions from F.F. and Y.M. F.F. and Y.M. calculated spin-Hall conductivities. P.M. and P.M.O. conducted calculations of the ultrafast spin transport. T.K., T.S., D.T., M.W. and M.K. co-wrote the paper. All authors contributed to discussing the results and writing the paper.

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Correspondence to T. Kampfrath.

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Seifert, T., Jaiswal, S., Martens, U. et al. Efficient metallic spintronic emitters of ultrabroadband terahertz radiation. Nature Photon 10, 483–488 (2016). https://doi.org/10.1038/nphoton.2016.91

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