  A general orbital angular momentum (OAM) mode selection principle is put
forward involving the rotationally symmetric superposition of chiral states.
This principle is not only capable of explaining the operation of spiral zone
plate holograms and suggesting that naturally occurring rotationally symmetric
patterns could be inadvertent sources of vortex beams, but more importantly, it
enables the systematic and flexible generation of structured OAM waves in
general. This is demonstrated both experimentally and theoretically in the
context of electron vortex beams using rotationally symmetric binary amplitude
chiral sieve masks.
