Modulation of spatial Raman gain induced by Laguerre–Gaussian vortex beams

Abstract

Electromagnetically induced transparency (EIT) provides a powerful platform for coherent control of optical properties in atomic media. Building upon this framework, we investigate a microwave-assisted closed-loop three-level atomic system driven by a Laguerre–Gaussian (LG) control beam and a Gaussian probe field. The present work emphasizes the spatial modulation mechanisms of Raman gain. Using the density matrix formalism, we numerically solve the steady-state equations to obtain two-dimensional maps and cross-sectional profiles of Raman gain.The results reveal that three key parameters—the orbital angular momentum (OAM) of the LG beam, the single-photon detuning, and the relative phase of the control fields—jointly determine the spatial morphology of the gain. Specifically, variations in OAM induce transitions from asymmetric single-lobe to symmetric multi-lobed gain structures; detuning modifies the gain intensity and spatial complexity; and relative phase produces rotational and mirror-symmetric transformations in the distribution.This study represents the first systematic analysis of spatial Raman gain modulation in a closed-loop three-level system, highlighting the strong capability of structured light to precisely tailor nonlinear gain processes. The results not only extend the theoretical framework of previous studies, but also provide a new theoretical basis for the design of spatial selective optical amplification, structured light field engineering and advanced nonlinear optical devices.