Observation of second-harmonic generation induced by pure spin currents

Journal name:
Nature Physics
Year published:
Published online

Spintronics, where the spin of electrons is used to carry information, is a rapidly growing area of research1, 2. There are several techniques for generating pure spin currents3, 4, 5, 6, 7, 8, 9, 10; however, there is no method that can directly detect them, largely because they carry no net charge current and no net magnetization. At present, studies of pure spin currents rely on measuring the induced spin accumulation with either optical techniques5, 11, 12, 13 or spin-valve configurations14, 15, 16, 17. However, spin accumulation does not directly reflect the spatial distribution or temporal dynamics of the pure spin current, and therefore does not give a real-time or real-space measurement. Here we demonstrate a second-order nonlinear optical effect of the pure spin current that has never been explored before, and show that it can be used for the non-invasive, non-destructive and real-time imaging of pure spin currents. The detection scheme can be applied in a wide range of materials with different electronic band structures because it does not rely on optical resonances. Furthermore, the control of nonlinear optical properties of materials with pure spin currents may have potential applications in photonics integrated with spintronics.

At a glance


  1. Schematics of the experimental configuration to observe the second-harmonic generation induced by pure spin currents.
    Figure 1: Schematics of the experimental configuration to observe the second-harmonic generation induced by pure spin currents.

    a,b, The configuration in real space (a) and energy space (b). The GaAs sample is simultaneously illuminated by two laser pulses (red and green waves). Quantum interference between the transition pathways driven by the two pulses (vertical red and green arrows in b) causes electrons with opposite spin orientations to be excited to energy states with opposite momenta (orange and blue spheres). As the two spin systems move along opposite directions, a pure spin current is formed. The nonlinear optical effect of the injected pure spin current is studied by detecting second-harmonic generation (EJ) from a probe pulse (Ep).

  2. Second-harmonic generation induced by the pure spin current.
    Figure 2: Second-harmonic generation induced by the pure spin current.

    a, The ΔP measured as functions of the probe delay, τ, and the relative phase, Δφ, with the probe and the current-injection spots overlapped (x=0). b,c, Cross-sections of a, with τ=−0.06ps (b) and Δφ=0 (c). d, The ΔP measured as functions of x and Δφ, with a fixed τ=−0.06ps. e, A cross-section of d, with Δφ=0.

  3. Time evolutions of [Delta]P at various carrier densities.
    Figure 3: Time evolutions of ΔP at various carrier densities.

    The ΔP is measured with x=0 and Δφ=0. The carrier densities are (from bottom (black) to top (green)) 3.6, 4.8, 6.0, 7.2, 9.6 and 12×1017cm−3. The inset shows the height of the peak as a function of carrier density.


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  1. Department of Physics and Astronomy, The University of Kansas, Lawrence, Kansas 66045, USA

    • Lalani K. Werake &
    • Hui Zhao


L.K.W. constructed the experimental apparatus and carried out the measurements; H.Z. proposed the topic, provided guidance on the experiments and prepared the manuscript. Both authors contributed to data analysis and interpretations.

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The authors declare no competing financial interests.

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