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(-*im*φ), 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*.