Spin angular momentum in planar and cylindrical waveguides induced by transverse confinement and intrinsic helicity of guided light

TL;DR

This paper analytically demonstrates that transverse confinement and intrinsic helicity of guided light in planar and cylindrical waveguides induce unique spin angular momentum components, enabling advanced control of spin-orbit interactions and optical forces.

Contribution

It reveals the mechanisms of transverse and longitudinal spin densities in waveguides, highlighting the role of mode confinement and intrinsic helicity in spin angular momentum generation.

Findings

Transverse spin density arises inside waveguides due to mode confinement.

Hybrid modes in cylindrical waveguides exhibit intrinsic helicity and longitudinal spin.

Results applicable to spin-orbit coupling and optical force manipulation in nanophotonics.

Abstract

In recent years, extraordinary spin angular momenta have been investigated in a variety of structured electromagnetic waves, being of especial interest in sub-wavelength evanescent fields. Here we demonstrate analytically that, in planar and cylindrical waveguides supporting transverse electric/magnetic modes, transverse spin density arises inside the waveguide (different from the spin induced in the evanescent region outside the waveguide), carrying indeed longitudinal extraordinary (so-called) Belinfante's spin momentum. Such contribution depends linearly on the mode transverse wave vector, and is thus induced by mode confinement. Cylindrical waveguides support in addition hybrid modes that exhibit a richer phenomenology with not only azimuthal (confinement-related) spin, but also an intrinsic helicity which leads to longitudinal spin density and transverse helicity-dependent spin momentum. Results are indeed presented for configurations relevant to spin-orbit coupling in nanophotonic waveguides and to manipulating optical forces in IR-to-microwave water-filled channels. Thus guided modes intrinsically carrying confinement-induced transverse spin, combined with intrinsic-helicity-induced longitudinal spin (when hybrid), hold promise of superb devices to control spin-orbit interaction and optical forces within confined geometries throughout the electromagnetic spectra.