Twisted Surface Plasmons with Spin-Controlled Gold Surfaces

© 2019 WILEY-VCH Verlag GmbH & Co. KGaA, WeinheimTwisted photon, associated with orbital angular momentum (OAM), is a physical notion that has long captivated the intriguing imagination and wide applications. Owing to the native orthogonality between different topological charges of the vortices, it will be of significant value to generate, access, and discriminate the vortex on integrated chips. Archimedean spirals or multiple split gratings are commonly employed to generate OAMs on plasmonic films. However, the single-crystalline plasmonic surface sets a very stringent condition of probing the on-chip OAM dynamics at sub-femtosecond scale. In previous reports, spins of the incident light and actual topological charge of the on-chip OAM generator are also hybridized due to the intrinsic spin-to-orbital angular momentum conversion, making the direct discrimination of plasmonic vortex impossible. Here, a paradigm of generating twisted surface plasmons is presented in a fully spin-controlled fashion. With the two-photon photoemission electron microscopy, the dynamics of OAM formation is demonstrated at subwavelength spatial resolution and sub-femtosecond temporal resolution simultaneously, revealing its OAM-dependent angular velocity. In addition, this scheme of twisting on-chip plasmons shows that the challenging crystalline requirement of the thin film can be significantly alleviated. The results open up a distinct way to multiplex, record, and read the information with plasmons.