its center defect. The q-plate with topological charge q can generate a charge vortex based on the input beam polarization. An s-plate is a similar technology to a q-plate, using a high-intensity UV laser to permanently etch a birefringent pattern into silica glass with an azimuthal variation in the fast axis with topological charge of s. Unlike a q-plate, which may be wavelength tuned by adjusting the bias voltage on the liquid crystal, an s-plate only works for one wavelength of light. At radio frequencies it is trivial to produce a (non optical) electromagnetic vortex. Simply arrange a one wavelength or greater diameter ring of antennas such that the phase shift of the broadcast antennas varies an integral multiple of 2 around the ring. Nanophotonic metasurfaces can enable transverse phase modulation to create optical vortices. The vortex beams can be generated in either free space or on an integrated photonic chip. Detection An optical vortex, being fundamentally a phase structure, cannot be detected from its intensity profile alone. Furthermore, as vortex beams of the same order have roughly identical intensity profiles, they cannot be solely characterized from their intensity distributions. As a result, a wide range of interferometric techniques are employed. The simplest of the techniques is to interfere a vortex beam with an inclined plane wave, which results in a fork-like interferogram. By making a count of the number of forks in the pattern and their relative orientations, the vortex order and its corresponding sign can be precisely estimated. A vortex beam can be deformed into its characteristic lobe structure while passing through a tilted lens. This happens as a result of a self-interference between different phase points in a vortex. A vortex beam of order will be split into lobes, roughly around the depth of focus of a tilted convex lens. Furthermore, the orientation of lobes (right and left diagonal), determine the positive and negative orbital angular momentum orders. A vortex beam generates a lobe structure when interfered with a vortex of opposite sign. This technique offers no mechanism to characterize the signs, however. This technique can be employed by placing a Dove prism in one of the paths of a Mach–Zehnder interferometer, pumped with a vortex profile. Applications There are a broad variety of applications of optical vortices in diverse areas of communications and imaging. Extrasolar planets have only recently been directly detected, as their parent star