Optical pulling force on Janus particles via azimuthally-polarized Bessel beams

Janus particles have aroused a great deal of interest over the years due to their intriguing properties and potential applications, including optical manipulation, biomedical sensing, and imaging. However, controlled manipulation of composite Janus particles with optical fields is still a challenging task. Here, we study optical pulling forces experienced by Janus nanoparticles, specifically polystyrene microspheres that are half-coated with gold, when illuminated by an azimuthally polarized zero-order vector Bessel beam. We demonstrate that the pulling forces generated by such a vector Bessel beam are stronger than those produced by its linearly and radially polarized counterparts. The asymmetric structure of the Janus particles facilitates the generation of optical pulling forces. The size of the Janus particles and the orientation of the gold layer are the key factors regulating the sign and amplitude of the optical forces they experience. Our study illustrates a new possibility for optical manipulation of Janus particles, offering potential insights that could enhance their nanophotonic and biophotonic applications.