Upper Bound to the Orbital Angular Momentum Carried by an Ultrashort Pulse.

Photons in a ring-shaped vortex light beam can have an arbitrarily high orbital angular momentum (OAM) lℏ, in addition to the spin angular momentum ±ℏ. For a pulsed vortex beam, there is, however, an upper bound to the integer units l of OAM, or topological charge of the vortex, and a lower bound to the pulse duration to carry OAM. These limits have implications in experiments with ultrashort vortices, e.g., in the generation of twisted attosecond bursts in the extreme ultraviolet, in the temporal resolution in ultrafast spectroscopy, or in the performance of OAM-based optical communications or cryptographic systems, as well as in other areas of physics as acoustics or electron waves.