Opt. Lett. 38, 3414–3417 (2013)

Optical vortices — light beams with twisted wavefronts — are employed in numerous applications, including quantum entanglement and communications. Research into vortex beams is now focusing on how to convert and manipulate their wavefront topology. Now, Yaroslav Kartashov and co-workers from Spain and Russia have numerically demonstrated that spiralling shallow refractive index landscapes induce coupling and periodic energy exchange between different topological light states. The scientists considered resonant excitation of vortex modes in a waveguide that has a helical refractive index profile. They investigated propagation of light beams with a wavelength of 633 nm in fused silica with a refractive index contrast of 7 × 10−4. Numerical simulations of the intensity and phase distributions revealed an evolution from the m = 0 mode to the m = 1 mode, where m is the topological charge. The energy exchange between these two modes varied sinusoidally as a function of the propagation distance, whereas its period was determined by the difference of the propagation constants of the two modes.