A beam of light has radial polarization if at every position in the beam the polarization ( electric field) vector points towards the center of the beam. In practice, an array of waveplates may be used to provide an approximation to a radially polarized beam. In this case the beam is divided into segments (eight, for example), and the average polarization vector of each segment is directed towards the beam centre. ^[1]

Radial polarization can be produced in a variety of ways. It is possible to use so-called q-devices ^[2] to convert the polarization of a beam to a radial state. The simplest example of such devices is inhomogeneous anisotropic birefringent waveplate that performs transversally inhomogeneous polarization transformations of a wave with a uniform initial state of polarization. The other examples are liquid crystal, ^[3] and metasurface q-plates. In addition, a radially polarized beam can be produced by a laser, or any collimated light source, in which the Brewster window is replaced by a cone at Brewster's angle. Called a "Rotated Brewster Angle Polarizer," the latter was first proposed and put into practice (1986) to produce a radially-polarized annular pupil by Guerra ^[4] at Polaroid Corporation (Polaroid Optical Engineering Dept., Cambridge, Massachusetts) to achieve super-resolution in their Photon Tunneling Microscope. A metal bi-cone, formed by diamond-turning, was mounted inside a glass cylinder. Collimated light entering this device underwent two air-metal reflections at the bi-cone and one air-glass reflection at the Brewster angle inside the glass cylinder, so as to exit as radially-polarized light. A similar device was later proposed again by Kozawa. ^[5]

A related concept is azimuthal polarization, in which the polarization vector is tangential to the beam. If a laser is focused along the optic axis of a birefringent material, the radial and azimuthal polarizations focus at different planes. A spatial filter can be used to select the polarization of interest. ^[6] Beams with radial and azimuthal polarization are included in the class of cylindrical vector beams. ^[7]

A radially polarized beam can be used to produce a smaller focused spot than a more conventional linearly or circularly polarized beam, ^[8] and has uses in optical trapping. ^[9]

It has been shown that a radially polarized beam can be used to increase the information capacity of free space optical communication via mode division multiplexing, ^[10] and radial polarization can "self-heal" when obstructed. ^[11]

At extreme intensities, radially-polarized laser pulses with relativistic intensities and few-cycle pulse durations have been demonstrated via spectral broadening, polarization mode conversion and appropriate dispersion compensation. ^[12] The relativistic longitudinal electric field component has been proposed as a driver for particle acceleration in free space ^{[13] [14]} and demonstrated in proof-of-concept experiments. ^[15]

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