Refractive-index-selective longitudinal trapping and polarization conversion with a radially polarized Schell-model vortex beam

Partially coherent beams with tailored correlation structures offer unique opportunities for controlling focusing properties and optical trapping. Here, we propose a radially polarized Schell-model vortex beam and systematically investigate its focusing, optical trapping, polarization, and self-healing characteristics. Owing to the interplay between the radially polarized correlation structure and the vortex phase, the beam exhibits an unusual intensity evolution during propagation, with its focal-field profile tunable by the topological charge. The beam produces an on-axis intensity minimum at the focal plane and two tunable on-axis intensity maxima on either side of the focus, enabling the simultaneous and stable trapping of low-refractive-index particles near the focal-plane minimum and high-refractive-index particles near two off-focus axial positions, as confirmed by Rayleigh-particle calculations. In addition, the vortex phase enables controllable conversion between radial and azimuthal polarization in the central focal region. Experimental intensity measurements agree well with the theoretical predictions, validating the predicted propagation behavior and confirming the tunable focal-field modulation and robust self-healing capability. These findings establish a new strategy for the simultaneous trapping and longitudinal manipulation of particles with different refractive indices and provide new opportunities for the control of partially coherent vector beams.