Orbital angular momentum of light

Light can have orbital angular momentum, in addition to its intrinsic spin angular momentum. Light beams with orbital angular momentum have a helical structure due to an azimuthal phase dependence in the form exp(-imφ), where m is the "quantum" number characterizing the projection of the orbital angular momentum on the light propagation direction and φ is the azimuthal angle. For m ≠ 0, the light intensity on the axis of the beam has to be zero. The plot shows surfaces of constant phase. For m = 0, the surfaces are a set of planes separated by the wavelength of the light in the direction of light propagation. For helical (m ≠ 0) beams the surfaces are |m| helices, rotated around the beam axis by 2π/m with respect to each other. Note that the pitch (the advance per turn) of an individual branch goes as m.