Angular momenta, helicity, and other properties of dielectric-fiber and metallic-wire modes

TL;DR

This paper provides the first comprehensive analytical and numerical analysis of spin and orbital angular momenta, helicity, and related properties of guided modes in dielectric and metallic nanowire waveguides, revealing fundamental quantization and phase relations.

Contribution

It introduces accurate calculations of angular momenta and helicity for cylindrical waveguide modes, uncovering fundamental quantization and phase effects in nonparaxial guided light.

Findings

Quantization of the canonical total angular momentum in nonparaxial modes

Noninteger values of spin and orbital angular momenta due to geometric phases

Spin angular momentum in metallic wires linked to surface plasmon-polariton trajectories

Abstract

Spin and orbital angular momenta (AM) of light are well studied for free-space electromagnetic fields, even nonparaxial. One of the important applications of these concepts is the information transfer using AM modes, often via optical fibers and other guiding systems. However, the self-consistent description of the spin and orbital AM of light in optical media (including dispersive and metallic cases) was provided only recently [K.Y. Bliokh et al., Phys. Rev. Lett. 119, 073901 (2017)]. Here we present the first accurate calculations, both analytical and numerical, of the spin and orbital AM, as well as the helicity and other properties, for the full-vector eigenmodes of cylindrical dielectric and metallic (nanowire) waveguides. We find remarkable fundamental relations, such as the quantization of the canonical total AM of cylindrical guided modes in the general nonparaxial case. This quantization, as well as the noninteger values of the spin and orbital AM, are determined by the generalized geometric and dynamical phases in the mode fields. Moreover, we show that the spin AM of metallic-wire modes is determined, in the geometrical-optics approximation, by the transverse spin of surface plasmon-polaritons propagating along helical trajectories on the wire surface. Our work provides a solid platform for future studies and applications of the AM and helicity properties of guided optical and plasmonic waves.