Spatially dependent Raman gain by vortex beam in a four-level N-typed atomic system

An efficient scheme for controlling the spatial Raman gain is proposed in a cold atomic ensemble with a four-level N -typed configuration. The cold atoms are driven by the Laguerre-Gaussian (LG) vortex beams. Considering the control and driving fields are vortex beams and using experimentally achievable parameters, we identify the condition under which the topological charges (TCs) allows us to manipulate the radial distribution of the Raman gain spectra. It is found that the spatial Raman gain can be effectively controlled via adjusting the relevant optical parameters. Furthermore, we show interesting optical properties of vortex-induced transparency(VIT) and vortex-induced gain (VIG) via controlling the spatial Raman-gain spectrum. Finally we show that the azimuthal modulation of the Raman gain profiles can be realized and controlled when the control field consists of a travelling-wave (TW) field and two optical vortex fields. Our scheme may provide a feasible approach for constructing novel vortex beams based on cold atomic ensemble.