Spatial Structure and Propagation of Vortex Four-Wave Mixing Signal

The spatial structure and propagation of vortex Four-Wave mixing (FWM) generated in a cascade three-level atomic system when Laguerre-Gaussian (LG) beams are used as the incident pump fields are studied. Firstly, by solving the third-order density matrix elements in the steady state, the intensity expression of the vortex FWM signal is obtained. Next, their spatial image and phase during the propagation process are researched. It is shown that with the increase of propagation distance, its image of the intensity tends to a Gaussian beam, and the center of the spot gradually deviates from the original optical singularity. Furthermore, after superposing multiple topological charge (TC) numbers, the intensity and phase images present entirely different light spots at various positions. The TC number can rapidly increase the capacity of the signal for carrying information. This type of signal can be utilized in quantum communication and information, offering a scalable light source for high-dimensional quantum storage.