Structured light, which denotes optical beams with spatially modulated intensity, phase, and polarization, has elucidated the detailed characteristics of optical angular momentum, encompassing both orbital and spin components [[1], [2], [3]]. By tailoring these properties for specific purposes, structured light has enabled a wide range of applications, including novel spectroscopic techniques based on otherwise forbidden atomic and molecular transitions [[4], [5], [6], [7]], the fabrication of micro- and nanostructures through manipulation of solid and liquid states [8,9], optical trapping [10,11], and robust optical communication systems resilient to external perturbations [12,13].

Surface plasmon polaritons (SPPs)— surface electromagnetic waves propagating along metal–dielectric interfaces—are also considered a form of structured light [3,14,15]. Due to their electric field localization near the metal–dielectric interface, SPPs exhibit high sensitivity to environmental changes at the metal surface [16]. This property has led to their widespread use in chemical and biosensing applications, including surface-enhanced Raman scattering (SERS) [[17], [18], [19]].

Beyond their well-known field confinement, recent theoretical studies have revealed that SPPs inherently carry spin angular momentum (SAM), oriented in a right-handed sense with respect to the propagation direction [20,21]. This SAM is predicted to induce circular motion of free electrons at the metal surface, resulting in magnetization via the inverse Faraday effect [[21], [22], [23], [24], [25], [26], [27]]. Notably, such magnetization can occur even in nonmagnetic metals, potentially generating spin-polarized electron currents [28,29]. This intriguing phenomenon holds promise for the development of low-power information processing devices and chiral-sensitive spectroscopic detection of surface-adsorbed molecules [[30], [31], [32]].

The SAM associated with SPPs is classified as transverse SAM (t-SAM) [3,33,34], arising from the evanescent nature of the surface mode—distinct from the longitudinal SAM (l-SAM) associated with circularly polarized light in free space [35,36]. In general, the SAM of structured light emerges from the spin–orbital decomposition of the kinetic momentum density of the electromagnetic field, expressed as p=p0+ps , where p0 is the canonical momentum (linked to orbital angular momentum) and ps is the spin momentum (linked to SAM), which satisfies ps=12∇×S , with S denoting the spin angular momentum density [37].

Further, Shi et al. demonstrated that the spin density S for both TM- and TE-guided modes—including SPPs—satisfies the relation S=(12k2)∇×p [38,39]. This relationship is a hallmark of spin texture in structured light. In electromagnetic fields with steep gradients normal to the surface—as is the case for SPPs—it provides an intuitive explanation for the emergence of transverse SAM. SAM textures have been explored in various structured SPP fields, such as non-diffracting SPP cosine waves, SPP Airy beams, and SPP Bessel beams [38]. More recently, SAM textures have also been studied in topological plasmonic configurations, including plasmonic spin skyrmions and merons [[40], [41], [42], [43], [44], [45], [46]], which feature both in-plane and out-of-plane SAM vector components.

In this study, we examine the SAM texture of a specific type of space-time SPP wave packet (ST-SPP), referred to as the striped ST-SPP [47,48], through theoretical analysis and experimental near-field visualization of its electric field distribution. The ST-SPP is the surface-wave analog of the space-time wave packet (STWP)—a recently developed optical pulse sheet exhibiting non-diffracting propagation in free space [[49], [50], [51]]. Our group has recently shown that femtosecond-duration ST-SPPs propagate nondiffractively on metal surfaces, analogous to STWPs in free space [52].

Unlike previously reported non-diffracting SPP beams generated using monochromatic continuous-wave (CW) laser sources [53,54], ST-SPPs are excited by pulsed laser sources. As a result, ST-SPPs exhibit localization not only in the transverse direction but also along the propagation axis [47,55]. Additionally, since SPPs are inherently confined in the direction normal to the surface, ST-SPPs realize a three-dimensionally localized electromagnetic field—often referred to as a light bullet. For such SPP fields, the approximate relation S≈∇×p suggests that the SAM texture contains nonzero components in both in-plane and out-of-plane directions [56].

In particular, we investigate how the relative strengths of these SAM components depend on the periodicity along the transverse (x) direction. We further simulate microscopic images generated from the electric fields of striped ST-SPPs and the excitation light and compare them with experimentally obtained results using two-photon fluorescence microscopy. The measured and simulated images reveal that multiple stripe-shaped SPP beams, aligned periodically, propagate non-diffractively while maintaining high visibility. The excellent agreement between experiment and theory—based on a model consistent with the SAM texture—strongly supports the validity of our theoretical framework.