Optical properties of radially-polarised twisted light

TL;DR

This paper analyzes the angular momentum and helicity of radially-polarized twisted light modes, showing they carry only orbital angular momentum and exhibit chirality, with properties depending on mode parameters.

Contribution

It provides a theoretical framework for understanding the angular momentum and helicity of radially-polarized paraxial twisted light modes, highlighting their exclusive orbital angular momentum and chirality.

Findings

Radially-polarized modes carry only orbital angular momentum, no spin.

Helicity is proportional to the mode's winding number, indicating chirality.

Helicity magnitude decreases with larger beam waist.

Abstract

We show that, in general, any type of radially-polarised paraxial twisted optical mode carries only an axial total optical angular momentum (AM) ${\bar {\cal J}_z}=\ell{\cal L}_0$ where $\ell$ is the winding number and ${\cal L}_0$ is a constant. This mode, however, is shown to have zero spin angular momentum (SAM), so it is endowed only with orbital angular momentum (OAM) and no SAM. The helicity is found to be proportional to $\ell$, hence radially-polarised modes display chirality. When applied to a Laguerre-Gaussian (LG) mode our treatment leads to a total helicity equal to $(\ell/|\ell|){\cal Q}$, where ${\cal Q}$ is the action constant. The factor $(\ell/|\ell|)=\pm 1$, depends on the sign, not the magnitude of $\ell$ and so the result holds for any radially-polarised LG mode however large the magnitude of its winding number $\ell$ is. The magnitude of the action constant ${\cal Q}$ and hence the helicity are diminished for all such LG modes of large beam waist $w_0$.