A vortex phase plate is a type of optical element that imparts an optical vortex to an input beam. An optical vortex is the term used to describe a phase pattern that is characterized, in polar coordinates, by having a constant value in the radial coordinate while showing a linear change in the angular coordinate. Such a pattern resembles a vortex and hence the name given to this type of element. The remarkable feature of a beam with this type of phase distribution is not on the phase pattern itself but on the resulting irradiance distribution of the beam once it is brought to focus by a lens. Indeed, the focused beam will have a doughnut shape, that is, it will have zero energy at the centre bounded by a ring of light. Such a focused beam pattern could also be obtained by adding a couple of Gaussian modes but using a vortex phase plate is more practical.
A beam with these unique characteristics can be beneficial in many applications. One application can be found in state-of-the-art astronomical instruments. The ever-continuing search for exoplanets demands the use of coronagraphs whose function is to block the light from a distant star such that the orbiting bodies can be detected more easily. A vortex phase plate placed in the pupil of the astronomical telescope can accomplish this function.
Another application is in optical tweezers in which particles are first trapped and then move around solely by optical means. A conventional focal spot can do the trapping of the particles, but a vortex beam can add more functionalities, like a constant rotation of the particle.
Optical vortex phase plates are also used in microscopy to achieve super resolution. There is a technique called stimulated emission depletion microscopy, or STED, in which a sample is illuminated with a high numerical aperture objective lens and using two different wavelengths. The purpose is to induce fluorescence as in a normal fluorescence microscope but with the difference that one of the wavelength channels has an optical vortex phase plate that creates a doughnut shape spot at the focal plane. The central part of the doughnut is smaller than the diffraction limit and hence the fluorescence induced by the other wavelength is emitting from a smaller area than would be obtained if no vortex phase plate were present.
Another interesting application for optical vortex phase plates is quantum optics in which different angular momentums can be imparted to a beam by the use of different optical vortex phase plates.