Trans-spectral orbital angular momentum transfer via 4WM in Rb vapor

We report the transfer of phase structure, and in particular of orbital angular momentum (OAM), from near-infrared pump light to blue light generated in a four-wave-mixing process in ⁸⁵Rb vapour. The intensity and phase profile of the two pump lasers at 780nm and 776nm, shaped by a spatial light modulator (SLM), influences the phase and intensity profile of light at 420nm which is generated in a subsequent coherent cascade. In particular we observe that the phase profile associated with orbital angular momentum is transferred entirely from the pump light to the blue. Pumping with more complicated light profiles results in the excitation of spatial modes in the blue that depend strongly on phase matching, thus demonstrating the parametric nature of the mode transfer. These results have implications on the inscription and storage of phase information in atomic gases.