Twisted photons: what is the orbital angular momentum of light?

In this introductory article, we explain the concept of orbital angular momentum (OAM) of light, discussing its physical meaning and its relationship with the more familiar spin angular momentum of circularly polarized waves. We address both classical and quantum aspects, emphasizing the distinction between OAM eigenstates – helical waves – and the general case. Finally, we briefly touch upon the main optical techniques to generate these optical states and emerging applications in the field.