Research Article

Ultrafast vector imaging of plasmonic skyrmion dynamics with deep subwavelength resolution

See allHide authors and affiliations

Science  24 Apr 2020:
Vol. 368, Issue 6489, eaba6415
DOI: 10.1126/science.aba6415

You are currently viewing the abstract.

View Full Text

Log in to view the full text

Log in through your institution

Log in through your institution

Watching plasmonic skyrmions

Skyrmions are stable topological textures that arise from solutions of the electromagnetic field. Because these “hedgehog”-like textures are robust, can be manipulated, and can interact, there is an interest in pursuing them for memory and logic applications. Skyrmions can also be generated in thin metal layers under optical excitation, but detailed information about the vectorial dynamics of these surface plasmon polariton skyrmions is so far lacking. Davis et al. used a time-resolved photoelectron vector microscope to image their spatiotemporal dynamics, piecing together movies as the skyrmions propagated across the surface of a perfect gold crystal. Access to dynamics with such high spatial and temporal resolution could help in controlling other nanophotonic systems.

Science, this issue p. eaba6415

Structured Abstract

INTRODUCTION

Topology is the study of geometric properties that are unaffected by continuous changes in shape and size. Skyrmions are examples of topological defects in vector fields. Skyrmions exhibit a characteristic vector structure. When excited by electromagnetic near fields on thin metal films, they are called plasmonic skyrmions. These fields exist at sub–100-nm scales and oscillate with periods of a few femtoseconds and thus are difficult to measure.

RATIONALE

Two-photon photoemission electron microscopy studies were previously able to image the local plasmon fields with femtosecond time resolution, but the vector information of the local electric fields was missing. Here we introduce a new technique, time-resolved vector microscopy, that enables us to compose entire movies on a subfemtosecond time scale and a 10-nm spatial scale of the electric field vectors of surface plasmon polaritons (SPPs). We use this technique to image complete time sequences of propagating surface plasmons, demonstrating their spin-momentum locking, as well as plasmonic skyrmions on atomically flat single-crystalline gold films that have been patterned using gold ion beam lithography.

RESULTS

The key technique to obtain vector information is to take two sequences of the entire process with two different probe beam polarizations. Hence, the electric field vectors will be projected onto the probing electric field by the two-photon photoemission process. The spatial dependence of the two in-plane vector components coupled with Maxwell’s equations then permits the retrieval of the out-of-plane component. This allows us to unambiguously resolve all vector components of the electric field as well as their time dynamics, enabling the retrieval of the experimental time-dependent skyrmion number and indicating the periodic transformation from skyrmion number +1 to −1 and back on a time scale of a few femtoseconds. Additionally, all three magnetic field vectors of the surface can be obtained from the electric field vectors by using Maxwell’s curl equation.

CONCLUSION

With our vector microscopy technique, we are able to image plasmonic spin-momentum locking and plasmonic skyrmion dynamics. In the future, other topological nanophotonic systems should be in reach as well; these include plasmonic merons or short-range skyrmions, where the dispersion of plasmons in extremely thin films is used. This research will open the door to creating linear optical features on the few-nanometer length scale.

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.

Abstract

Plasmonic skyrmions are an optical manifestation of topological defects in a continuous vector field. Identifying them requires characterization of the vector structure of the electromagnetic near field on thin metal films. Here we introduce time-resolved vector microscopy that creates movies of the electric field vectors of surface plasmons with subfemtosecond time steps and a 10-nanometer spatial scale. We image complete time sequences of propagating surface plasmons as well as plasmonic skyrmions, resolving all vector components of the electric field and their time dynamics, thus demonstrating dynamic spin-momentum coupling as well as the time-varying skyrmion number. The ability to image linear optical effects in the spin and phase structures of light in the single-nanometer range will allow for entirely novel microscopy and metrology applications.

View Full Text

Stay Connected to Science