Skyrmions are topological defects in vector fields that exhibit a characteristic vector structure. When excited by electro-magnetic near fields on thin metal films, they are called plasmonic skyrmions. These fields exist at the sub–100-nm scale, oscillate with periods of a few femtoseconds, and thus are difficult to measure. So far, two-photon photoemission electron microscopy was able to image local plasmon fields with femtosecond time resolution. We now extend this technique to obtain time-resolved vector information 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 [1].
The estimation of electric field amplitudes of surface plasmon polaritons, or at least ratios of field amplitudes, can be cumbersome if they need to be determined from time-resolved photoemission electron microscopy measurements. We discuss a Fourier–based strategy for the data analysis of two-photon photoemission electron microscopy images that adresses this issue and allows distinguishing surface plasmon polariton pulses that overlap in space but propagate in different directions. The Fourier-based strategy is applied to determine the coefficients of transmission and reflection of plasmonic Bragg reflectors. The reflection coefficient increases with the number of reflector grooves. Our work demonstrates that 2PPE–PEEM can be routinely used to quantitatively analyze the performance of plasmonic Bragg reflectors.
A circular grating coupler on a single-crystalline Au platelet is used to create surface plasmon polariton waves with spherical phase fronts that form a focal point in the center of the circle. In a time-resolved photoemission microscopy experiment, the propagation and interaction of plasmon waves in the focusing structure is investigated with sub-femtosecond time-resolution. Several space-time signatures of propagating and transiently formed standing plasmon waves are identified. The method of contrast enhancement for the data-analysis is discussed, and all observed experimental features are explained. In addition to the known electron emission from the nonlinear superposition of light and plasmon field, we also observe plasmoemission signatures, i.e., emitted electrons that arise from a nonlinear emission process driven purely by the plasmonic field.
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