Optical vortex bullets with large angular momenta
We investigate three-dimensional spatiotemporal optical vortex solitons (light bullets with a toroidal shape) carrying higher-order orbital angular momentum in Kerr media with anomalous group-velocity dispersion and a transversely localized Gaussian profile of the linear refractive index. Stationary vortex bullets with topological charges (winding numbers) $1\leq \ell \leq 4$ are obtained as numerical solutions of the three-dimensional nonlinear Schrödinger equation, and their existence, structure and stability are systematically analyzed. In terms of atomic Bose-Einstein condensates, these solutions represent matter-wave solitons with embedded vorticity. We examine the dependence of the solution families on the propagation constant, the depth and width of the guiding profile, and $\ell $. It is found that the increase of $\ell $ requires one to use a stronger or wider waveguiding potential to sustain localized states satisfying the Vakhitov-Kolokolov (VK) stability criterion. VK-complying and non-complying branches of vortex-soliton families are separated by a degenerate Townes branch. Direct simulations reveal that, although the guiding profile substantially suppresses the collapse and extends the propagation distance of vortex bullets, their robustness is ultimately limited by azimuthal instabilities, whose growth is enhanced for values of $\ell $. These results demonstrate that engineered refractive-index landscapes significantly enlarge the stability domain of spatiotemporal vortex bullets and provide guidelines for supporting optical bullets carrying large orbital angular momenta.