Spin and orbital angular momenta of light have been a subject of fundamental interest since long ago, classically associated with circular polarization and wave vector. In recent years, extraordinary spin angular momenta in structured electromagnetic waves have been investigated, mostly in subwavelength evanescent fields at the nanoscale. Here we present an in-depth theoretical analysis of the transverse spin density and related momentum induced by mode confinement inside waveguides, with alternating spin layers governed by guided mode spatial symmetry, different from and indeed richer than that in the evanescent region outside. Furthermore, hybrid guided modes with intrinsic helicity exhibit in addition longitudinal spin density. Such fundamental features are manifested through fascinating phenomenology relevant to spin-orbit coupling in nanophotonic waveguides. Thus, guided light intrinsically carrying a wealth of spin momenta holds promise of superb devices to control spin-orbit interaction within confined geometries throughout the electromagnetic spectra.

This work has been supported by the Spanish Ministerio de Ciencia, Innovacion y Universidades (MCIU/AEI/FEDER, ́ UE) through Grant Nos. MELODIA PGC2018-095777-B-C21 and NANOTOPO FIS2017-91413-EXP (MCIU/AEI/ FEDER, UE), and through the FPU PhD Fellowship FPU15/03566.

12 pags., 6 figs.