Spatial characterization of Fraunhofer diffraction in a four-level light-matter-coupling system

We explore the spatial features of various orders of Fraunhofer diffraction patterns in a four-level $N$-type atomic system. The system interacts with a weak probe light, a standing wave (SW) coupling field in the $x$ direction, and a cylindrical beam of composite optical vortex type. We derive the first-order linear and third-order cross-Kerr nonlinear parts of the probe susceptibility by expanding the probe susceptibility of the system into the second order of the SW beam. This allows us to solve the integral equation of Fraunhofer diffraction, decoding its varying degrees to specific degrees of Bessel functions containing the nonlinear susceptibility. Notably, the nonlinear susceptibility exhibits dependence on the orbital angular momentum (OAM) of the light beam, leading to spatial variations in the Bessel functions, and consequently, in the different orders of Fraunhofer diffraction. Leveraging the manipulation of OAM, we achieve precise control over the spatial mapping of diverse diffraction orders at various locations. Our research sheds light on the spatial behavior of Fraunhofer diffraction in complex atomic systems. It presents exciting prospects for harnessing the OAM characteristics of light in future optical technologies.

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