Research Article

Ultrafast vector imaging of plasmonic skyrmion dynamics with deep subwavelength resolution

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Science  24 Apr 2020:
Vol. 368, Issue 6489, eaba6415
DOI: 10.1126/science.aba6415
  • Ultrafast time-resolved vector microscopy of plasmonic skyrmions.

    Femtosecond laser pump-probe techniques using polarized beams combined with two-photon electron emission in an electron microscope enables the retrieval of all vector components of the electric field of propagating SPPs as a function of time. We used this technique to image the vectorial time dynamics of the plasmonic skyrmion field. Hexagons are milled into single-crystalline gold flakes via ion beam lithography. A circularly polarized femtosecond laser pulse excites surface plasmon waves on the gold flakes that interfere to create an SPP skyrmion lattice. The SPPs are detected by interference with a second laser pulse that is first polarized in the x direction to retrieve the Ex component of the SPP wave and then is polarized in the y direction to produce the Ey component. These fields are combined to obtain the characteristic in-plane pattern of the skyrmion lattice: E. Use of the measured field components in Maxwell’s equations enables the vertical field component Ez to be calculated. From these data, we reconstruct the vector field of the SPP skyrmion and, by varying the laser pump-probe delay time (Δτ), gain time-resolved information (top right), allowing us to create vector movies that show plasmonic spin-momentum locking and plasmonic skyrmions (bottom right). SEM, scanning electron microscopy; TR-PEEM, time-resolved photoemission electron microscopy.

  • Fig. 1 2PPE-PEEM process used to obtain vector and time information from surface plasmons.

    (A) The 2PPE-PEEM process involves a pump-probe excitation of surface plasmons from grooves etched in a single-crystal gold flake, the interference of the propagating surface plasmon with a probe pulse, and the subsequent imaging of the ejected photoelectrons in a photoemission electron microscope. The pulse duration is typically 16 fs, and the delay time (Δτ) is varied in steps of 0.16 fs. (B) Scanning electron microscopy image showing examples of single-crystal gold flakes and the hexagonal boundary shapes milled by an ion beam. (C) The hexagonal boundary milled into a single-crystal gold flake, as imaged in the electron microscope. The arrow points to the side that is displaced by a half wavelength of the SPP (λspp) to create the skyrmion lattice.

  • Fig. 2 Method for extracting vector information from the 2PPE-PEEM experiment.

    (A) The excitation of photoelectrons involves a two-photon process to overcome the work function of the metal film. A submonolayer of cesium is deposited on the gold surface to reduce the work function below 3 eV to facilitate the two-photon absorption. The polarization direction of the probe pulse determines the component of the SPP electric field that is measured. (B) Vector fields in the plane of the SPP are obtained from the interference between the orthogonal probe fields during two separate measurements. From the spatial dependence of these two vectors, we derive the out-of-plane vector.

  • Fig. 3 2PPE-PEEM measurement of the time evolution of the electric field vectors of an SPP traveling wave.

    (A) An image of the wave taken at one pump-probe delay time. The arrow shows the wave propagation direction. A time-invariant background signal has been removed using a differencing procedure described in the supplementary text (section II). (B) Full vectorial reconstruction of the SPP electric field at the pump-probe time delay τ = 58.29 fs. The image beneath the vectors shows the vertical component of the SPP electric field that is proportional to the SPP surface charge (white, positive; black, negative), whereas the PEEM image (A) probes the in-plane component of the plasmon field. See Movie 1. (C) Four profiles through the experimental wavefront depicting the SPP vector configurations at different relative time delays. The sloped dashed line and gray arrows highlight the propagation of the wavefront with time associated with the SPP traveling wave. The vertical dashed line and blue, cyan, and yellow arrows highlight the rotation of the SPP electric field vector at one position in space that gives rise to transverse spin.

  • Fig. 4 Vector and time measurement of the SPP skyrmions.

    (A and B) Two images taken at the same pump-probe delay times but with orthogonal polarization states of the probe field. A time-stationary background has been removed, as described in the supplementary text (section II). (C and D) The in-plane Es=Esx2+Esy2 and out-of-plane Esz components of the SPP skyrmion field extracted from the experimental data. (E) Experimentally derived vectors along the dashed line in (D) for three relative time delays. The vertical dashed line highlights the standing wave nature of the SPP field at this location. (F) Time dependence of the SPP skyrmion lattice, as obtained from experiment (Movie 2). The background image is scaled to the normal component, which provides a representation of the SPP surface charge (white, positive; black, negative; gray, zero).

  • Fig. 5 Skyrmion number density.

    (A and B) Regions of the SPP skyrmion lattice at extrema of the SPP wave cycle, at times equivalent to a π phase shift. (C) The skyrmion number density Ns is calculated from the data in (A) and compared with a theoretical calculation. The color codes the density and is slightly negative in the red regions (≈−0.05 μm−2) and peaks just above 5 μm−2 in the blue regions. (D) The skyrmion winding number W per skyrmion for the regions in (C) is obtained from the experiment for two complete SPP wave periods. At the extrema, the winding number is ±1, with the minus sign indicating that the SPP skyrmion vector rotates over a complete sphere but in the opposite sense. The theoretical curve is calculated from an analytical model of interfering SPP waves (see supplementary text section V for details).

  • Movie 1. Vector dynamics of the propagating SPPs.
  • Movie 2. Vector dynamics of the spinning plasmonic skyrmion electric field vectors.

Supplementary Materials

  • Ultrafast vector imaging of plasmonic skyrmion dynamics with deep subwavelength resolution

    Timothy J. Davis, David Janoschka, Pascal Dreher, Bettina Frank, Frank-J. Meyer zu Heringdorf, Harald Giessen

    Materials/Methods, Supplementary Text, Tables, Figures, and/or References

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    • Supplementary Text
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