  We show that a homogeneous and isotropic slab, illuminated by a circularly
polarized beam with no topological charge, produces vortices of order two in
the opposite circularly polarized components of the reflected and transmitted
fields, as a consequence of the difference between transverse magnetic and
transverse electric dynamics. In the epsilon-near-zero regime, we find that
vortex generation is remarkably efficient in sub-wavelength thick slabs up to
the paraxial regime. This physically stems from the fact that a vacuum paraxial
field can excite a nonparaxial field inside an epsilon-near-zero slab since it
hosts slowly varying fields over physically large portion of the bulk. Our
theoretical predictions indicate that epsilon-near-zero media hold great
potential as nanophotonic elements for manipulating the angular momentum of the
radiation, since they are available without resorting to complicated micro/nano
fabrication processes and can operate even at very small (ultraviolet)
wavelengths.
