Necklace beam generation in negative index materials

Abstract In this work, we study the propagation of the laser beam in a composite structure comprising two metamaterial waveguides arranged in series, each exhibiting distinct nonlinear characteristics. The beam evolution is modeled using the nonlinear Schrödinger equation, and numerical simulations based on the split-step Fourier method are performed to study its evolution. Our results demonstrate that, when both waveguides are purely Kerr nonlinear, the laser beam with a two-dimensional Gaussian intensity profile undergoes catastrophic collapse. The inclusion of both cubic and quintic nonlinearities can further lead to supercritical collapse when the two nonlinear effects act cooperatively. In contrast, when a balance is established between defocusing cubic and focusing quintic nonlinearities, both beam collapse and diffraction-induced spreading are effectively suppressed, resulting in the formation of a quasi-stable spatial ring. Furthermore, we propose a metamaterial-based configuration for the controlled generation of necklace beams, providing a promising approach for beam shaping and structured-light manipulation.