Surface-Normal Electric Spin in Obliquely Excited Parallel-Slit SPP Fields

Handedness-dependent light-matter interactions require control of local electric-field rotation, but generating controlled surface-normal spin textures is difficult under oblique illumination. Here, we show that oblique s-polarized illumination of a periodic array of parallel slits produces spatially alternating surface-normal electric spin, with its period tunable by the incidence angle. We further show that the spin texture changes qualitatively between substrate- and air-side excitation depending on whether the SPP field interferes with the excitation field. Under substrate-side excitation, the spin distribution follows the conventional spin-flow relation, whereas under air-side excitation, interference between the SPP and excitation fields produces a distinct spatial periodicity and allows the spin extrema to coincide with the field-intensity maxima. A simple interference model reproduces these contrasting spin textures.