An optical vortex (also known as a photonic quantum vortex, screw dislocation or phase singularity) is a zero of an optical field; a point of zero intensity. The term is also used to describe a beam of light that has such a zero in it. The study of these phenomena is known as singular optics.

Explanation

In an optical vortex, light is twisted like a corkscrew around its axis of travel. Because of the twisting, the light waves at the axis itself cancel each other out. When projected onto a flat surface, an optical vortex looks like a ring of light, with a dark hole in the center. The vortex is given a number, called the topological charge, according to how many twists the light does in one wavelength. The number is always an integer, and can be positive or negative, depending on the direction of the twist. The higher the number of the twist, the faster the light is spinning around the axis. This spinning carries orbital angular momentum with the wave train, and will induce torque on an electric dipole. Orbital angular momentum is distinct from the more commonly encountered spin angular momentum, which produces circular polarization. Orbital angular momentum of light can be observed in the orbiting motion of trapped particles. Interfering an optical vortex with a plane wave of light reveals the spiral phase as concentric spirals. The number of arms in the spiral equals the topological charge. Optical vortices are studied by creating them in the lab in various ways. They can be generated directly in a laser, or a laser beam can be twisted into vortex using any of several methods, such as computer-generated holograms, spiral-phase delay structures, or birefringent vortices in materials.

Properties

An optical singularity is a zero of an optical field. The phase in the field circulates around these points of zero intensity (giving rise to the name vortex). Vortices are points in 2D fields and lines in 3D fields (as they have codimension two). Integrating the phase of the field around a path enclosing a vortex yields an integer multiple of 2π. This integer is known as the topological charge, or strength, of the vortex. A hypergeometric-Gaussian mode (HyGG) has an optical vortex in its center. The beam, which has the form is a solution to the paraxial wave equation (see paraxial approximation, and the Fourier optics article for the actual equation) consisting of the Bessel function. Photons in a hypergeometric-Gaussian beam have an orbital angular momentum of mħ. The integer m also gives the strength of the vortex at the beam's centre. Spin angular momentum of circularly polarized light can be converted into orbital angular momentum.

Creation

Several methods exist to create hypergeometric-Gaussian modes, including with a spiral phase plate, computer-generated holograms, mode conversion, a q-plate, or a spatial light modulator.

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. n = l + 1 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.

Applications

There are a broad variety of applications of optical vortices in diverse areas of communications and imaging.