The Orbital Angular Momentum of Light: An Introduction

Most physicists know that polarized light is associated with the spin angular momentum of the photon. The use of the flow of angular momentum flux across a surface, rather than angular momentum density, allows the separation of the spin and orbital angular momentum parts in a gauge invariant way. Simple comparisons of the behavior of spin and orbital angular momenta in different situations prove to be a fruitful way to demonstrate their properties. For spin angular momentum and circularly polarized light, the light source need not be either temporally or spatially coherent. For orbital angular momentum the situation is more complicated. Another aspect of clear distinction between spin and orbital angular momentum is the existence of a Fourier relationship for orbital angular momentum and angular position, and a related uncertainty relationship. Controlled Vocabulary Terms angular momentum; light