Generation of Optical Vortices by Diffraction from Circular Apertures

In optics, to generate vortex beams, we normally need specific man-made structures such as spiral phase plates and computer-generated holograms. On the other hand, wave diffraction is the cornerstone of physical optics, and the effect of diffraction from a circular aperture is of importance for many optical instrumentation. Here, we propose and demonstrate that when a circularly polarized Gaussian beam is diffracted by a circular aperture, optical vortices are generated in the Fresnel region. The generation of an optical vortex can be attributed to spin-to-orbital angular momentum conversion. Moreover, we show that the orbital angular momentum can be transferred to the trapped particles and make orbital rotation. Counterintuitive, we find that the sense of rotation is consistent with the spin (Belinfante’s) momentum, contrary to the time-averaged Poynting momentum.