Spin angular momentum of femtosecond striped space-time surface plasmon polaritons

Surface-bound electromagnetic waves, such as surface plasmon polaritons (SPPs), carry transverse spin angular momentum (SAM) oriented perpendicular to their propagation direction. In ideal monochromatic plane waves, this transverse SAM arises from spatial gradients in the energy flow density and remains confined to the in-plane component. However, when the surface field has a spatial structure, the additional gradient along the in-plane direction gives rise to an out-of-plane SAM component, allowing the spin vector to attain a fully three-dimensional orientation. In this study, we investigate the three-dimensional spin texture of striped space-time SPPs (ST-SPPs)—structured SPP wave packets generated by optical pulses with transverse spatial periodicity and temporal confinement along the propagation axis. Through numerical simulations, we analyze how the SAM texture depends on the transverse periodicity of striped ST-SPPs. Our results reveal that varying the transverse period alters the balance between in-plane and out-of-plane field gradients inherent to SPPs, enabling the emergence of qualitatively distinct spin structures via periodicity control. A comparison between the simulated electric fields and experimentally observed near-field patterns—obtained using two-photon fluorescence microscopy—validates our theoretical model. These findings indicate that striped ST-SPPs offer a versatile platform for robust SAM control on metallic surfaces, with promising applications in chiral-sensitive molecular spectroscopy and surface-bound molecular manipulation.