Optical helicity of unpolarized light

Like energy and momentum, optical helicity is a fundamental dynamical property of light. In the prevalent plane wave and paraxial description of light the optical helicity is directly proportional to the degree of circular polarization, being zero for both linearly and unpolarized fields. Here it is shown that the nonparaxial fields generated by tightly focused optical vortices which have the phase factor exp(il phi) possess a contribution to the optical helicity density that is completely independent of the polarization state of the source paraxial field. In stark contrast to what is known in classical optics with plane waves and paraxial light, the physical consequence is that unpolarized light can exhibit optical activity and chiral light-matter interactions.

Automated summary

Optical helicity is a fundamental property of light, similar to energy and momentum. In the conventional description of light as plane waves, the optical helicity is proportional to the degree of circular polarization. However, this study shows that tightly focused optical vortices can generate nonparaxial fields with a contribution to the optical helicity density that is independent of the polarization state of the source paraxial field. This finding challenges the previous understanding and suggests that unpolarized light can exhibit optical activity and chiral light-matter interactions.