The ability of light to carry and deliver orbital angular momentum (OAM) in the form of optical vortices has attracted much interest. The physical properties of light with a helical wavefront can be confined onto two-dimensional surfaces with subwavelength dimensions in the form of plasmonic vortices, opening avenues for thus far unknown light-matter interactions. Because of their extreme rotational velocity, the ultrafast dynamics of such vortices remained unexplored. Here we show the detailed spatiotemporal evolution of nanovortices using time-resolved two-photon photoemission electron microscopy. We observe both long- and short-range plasmonic vortices confined to deep subwavelength dimensions on the scale of 100 nanometers with nanometer spatial resolution and subfemtosecond time-step resolution. Finally, by measuring the angular velocity of the vortex, we directly extract the OAM magnitude of light.

Transformation of light carrying spin angular momentum (SAM) to optical field vortices carrying orbital angular momentum (OAM) has been of wide interest in recent years. The interactions between two optical fields, each carrying one of those degrees of freedom, and furthermore, the transfer of the resulting angular momentum product to matter are seldom discussed. Here, we measure the interaction between 3D light carrying axial SAM and 2D plasmon-polariton vortices carrying high-order transverse OAM. The interaction is mediated by two-photon absorption within a gold surface, imprinting the resulting angular-momentum mixing into matter by excitation of electrons that are photo-emitted into vacuum. Interestingly, the spatial distribution of the emitted electrons carries the signature of a subtraction of the spin from the orbit angular momenta. We show experimentally and theoretically that the absorptive nature of this interaction leads to both single and double photon-plasmon angular momentum mixing processes by one-and two-photon interactions. Our results demonstrate high order angular momenta light-matter interactions, provide a glimpse into specific electronic excitation routes, and may be applied in future electronic sources and coherent control.

Orbital angular momentum of light is a core feature in photonics. Its confinement to surfaces using plasmonics has unlocked many phenomena and potential applications. Here, we introduce the reflection from structural boundaries as a new degree of freedom to generate and control plasmonic orbital angular momentum. We experimentally demonstrate plasmonic vortex cavities, generating a succession of vortex pulses with increasing topological charge as a function of time. We track the spatiotemporal dynamics of these angularly decelerating plasmon pulse train within the cavities for over 300 femtoseconds using time-resolved photoemission electron microscopy, showing that the angular momentum grows by multiples of the chiral order of the cavity. The introduction of this degree of freedom to tame orbital angular momentum delivered by plasmonic vortices could miniaturize pump probe–like quantum initialization schemes, increase the torque exerted by plasmonic tweezers, and potentially achieve vortex lattice cavities with dynamically evolving topology.

Surface plasmon polaritons carrying orbital angular momentum are of great fundamental and applied interest. However, common approaches for their generation are restricted to having a weak dependence on the properties of the plasmon-generating illumination, providing a limited degree of control over the amount of delivered orbital angular momentum. Here we experimentally show that by tailoring local and global geometries of vortex generators, a change in circular polarization handedness of light imposes arbitrary large switching in the delivered plasmonic angular momentum. Using time-resolved photoemission electron microscopy we demonstrate pristine control over the generation and rotation direction of high-order plasmonic vortices. We generalize our approach to create complex topological fields and exemplify it by studying and controlling a "bright vortex", exhibiting the breakdown of a high-order vortex into a mosaic of unity-order vortices while maintaining the overall angular momentum density. Our results provide tools for plasmonic manipulation and could be utilized in lab-on-a-chip devices. Main:Surface Plasmon Polaritons (SPPs) are evanescent electromagnetic waves propagating along metaldielectric interfaces. In recent years, their ability to carry surface-confined orbital angular momentum (OAM) and form plasmonic vortices has been of wide interest [1][2][3][4][5][6][7]. Understanding and controlling such vortices opens the door towards a variety of applications. Examples are the unlocking of forbidden multipolar transitions in novel light-matter interactions [8-10] and plasmonic tweezers for biological and chemical purposes [11][12][13][14]. For the latter, the trapping and rotating of both dielectric [13] and metallic [11,12,14] microparticles have been demonstrated. For the manipulation of metallic particles,

Magnetization dynamics on a femtosecond timescale has been observed for a huge variety of magnetic structures. However, the influence of different excitation photon energies has not been studied in detail yet. In our time-resolved magneto-optical Kerr effect setup we excite a Nickel bulk system with 1.55 and 3.1 eV, respectively, leading to different remagnetization dynamics depending on the chosen photon energy. Furthermore we complement our experimental data with a theoretical approach applying appropriate Boltzmann collision integrals including the density of states of Nickel. The comparison between the experimental data and the theoretical approach indicates that photon-energy dependent transport processes play a major role in this setup.