Ultrathin metasurfaces consisting of a monolayer of subwavelength
plasmonic resonators are capable of generating local abrupt phase
changes and can be used for controlling the wavefront of electromagnetic
waves. The phase change occurs for transmitted or reflected wave components
whose polarization is orthogonal to that of a linearly polarized (LP)
incident wave. As the phase shift relies on the resonant features
of the plasmonic structures, it is in general wavelength-dependent.
Here, we investigate the interaction of circularly polarized (CP)
light at an interface composed of a dipole antenna array to create
spatially varying abrupt phase discontinuities. The phase discontinuity
is dispersionless, that is, it solely depends on the orientation of
dipole antennas, but not their spectral response and the wavelength
of incident light. By arranging the antennas in an array with a constant
phase gradient along the interface, the phenomenon of broadband anomalous
refraction is observed ranging from visible to near-infrared wavelengths.
We further design and experimentally demonstrate an ultrathin phase
gradient interface to generate a broadband optical vortex beam based
on the above principle.