Spiral Bandwidth of Four-Wave Mixing in Rubidium Vapour

Atomic vapours are an experimentally simple and efficient system in which to study nonlinear wave mixing. We study a four-wave mixing (FWM) process in rubidium vapour which efficiently converts 780 and 776 nm light to 420 nm and 5.2 μm fields. In particular, we quantitatively investigate the transfer of orbital angular momentum (OAM) between the FWM fields. OAM, and structured light more generally, is an important research tool for optical manipulation, imaging and communication. Phase-matched nonlinear processes are both longitudinally and transversely phase coherent, and therefore OAM, which is associated with spiral phase fronts, must be conserved between the pump and generated fields. This makes wave mixing an ideal tool for frequency conversion and generation of a variety of OAM states for use in both classical and quantum communication.