Journal of Nanophotonics • Open AccessThe q-plate and its future
A thin light beam, such as that emitted by a laser, may possess a hidden rotational structure, invisible to the naked eye. This structure is rooted in the electromagnetic wave nature of light and it takes two distinct forms, which may be dubbed spin and twist. Spin is associated with the rotation of the electric and magnetic fields oscillating within the optical wave—i.e., the circular polarization of light. Twist instead occurs in light waves having a helical-shaped (or twisted) wavefront and an optical vortex located at the beam axis. When a free material particle absorbs light having spin or twist, it is itself made to spin—in other words, the light exerts a rotational form of radiation pressure, showing that this kind of light carries angular momentum. More specifically, the angular momentum of light having circular polarization is named as spin angular momentum (SAM), while that associated with a spiral wavefront is called orbital angular momentum (OAM). The OAM of light has recently been attracting much attention for its possible technological applications in the areas of particle manipulation, optical sensing, and classical and quantum optical communication.