2.1 Excitation configuration

The schematic diagram of a four-level atomic system is shown in Figure 1. We denote |0 > to be the atomic ground state and |i > with i = 1,2,3 to be the respective first, second, and third excited states separated by energy Ei-Ei-1. The atom is cooled down to the recoil temperature and localized at the intersecting point of the excitation laser beams in the transversal plane z = 0 as sketched in Figure 2(a). The wave vectors ki, i = 1, 2, 3, lie in x-z surface with the z axis directed along the LG-Rydberg excitation laser beams wave vector, k3. While the first and second Gaussian excitation laser beams wave vectors, k<sup>1</sup> and k2, have the respective incidence angles θ<sup>1</sup> and
