Single-frequency Brillouin vortex beam generation via Raman-mediated excitation

In this work, we exploit two third-order nonlinear effects, stimulated Raman scattering (SRS) and stimulated Brillouin scattering (SBS), within an optical cavity for Brillouin vortex beam generation, using diamond as the nonlinear medium. The SRS process is employed to generate a first-order Stokes field, which serves as an intermediate pump to drive the generation of SBS in diamond. A theoretical model is developed to illustrate how variations in cavity length can be used to manipulate the transverse mode profiles of the diamond Brillouin fields under this indirect pumping scheme. Experimentally, by precisely tuning the diamond cavity length, we achieve Brillouin laser emission at ∼1240 nm, with output modes including both the fundamental Gaussian mode and Hermite-Gaussian modes with transverse orders ranging from 1 to 9. Furthermore, by implementing an extra-cavity astigmatic mode converter, we generate free-space diamond Brillouin vortex beams with topological charges from 1 to 4. These beams exhibit linewidths of 4.5-4.9 kHz, corresponding to a linewidth compression factor greater than 1.3 relative to the fundamental frequency. To the best of our knowledge, this is the first demonstration of free-space high-order Brillouin vortex beam generation, offering a new pathway for structured light generation in advanced optical systems.