A beam of light has radial polarization if at every position in the beam the polarization (

electric field An electric field (sometimes E-field) is the physical field that surrounds electrically charged particles and exerts force on all other charged particles in the field, either attracting or repelling them. It also refers to the physical field fo ...

) vector points towards the centre of the beam. In practice, an array of

waveplate A waveplate or retarder is an optics, optical device that alters the Polarization (waves), polarization state of a light wave travelling through it. Two common types of waveplates are the ''half-wave plate'', which shifts the polarization directio ...

s 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. Radial and Azimuthal Polarisation

Radial polarization can be produced in a variety of ways. It is possible to use so-called q-devices 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 Liquid crystal (LC) is a state of matter whose properties are between those of conventional liquids and those of solid crystals. For example, a liquid crystal may flow like a liquid, but its molecules may be oriented in a crystal-like way. T ...

, and metasurface

q-plate A q-plate is an optical device that can form a light beam with orbital angular momentum (OAM) from a beam with well-defined spin angular momentum (SAM). Q-plates are based on the SAM-OAM coupling that may occur in media that are both anisotropic ...

s. In addition, a radially polarized beam can be produced by a

laser A laser is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation. The word "laser" is an acronym for "light amplification by stimulated emission of radiation". The fir ...

, or any collimated light source, in which the

Brewster window Brewster's angle (also known as the polarization angle) is an angle of incidence at which light with a particular polarization is perfectly transmitted through a transparent dielectric surface, with ''no reflection''. When ''unpolarized'' light ...

is replaced by a cone at

Brewster's angle Brewster's angle (also known as the polarization angle) is an angle of incidence at which light with a particular polarization is perfectly transmitted through a transparent dielectric surface, with ''no reflection''. When ''unpolarized'' light ...

. 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 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. A related concept is ''azimuthal polarization'', in which the polarisation vector is tangential to the beam. If a laser is focused along the optic axis of a

birefringent Birefringence is the optics, optical property of a material having a refractive index that depends on the Polarization (waves), polarization and propagation direction of light. These optically anisotropic materials are said to be birefringent (or ...

material, the radial and azimuthal polarizations focus at different planes. A

spatial filter A spatial filter is an optical device which uses the principles of Fourier optics to alter the structure of a beam of light or other electromagnetic radiation, typically coherent laser light. Spatial filtering is commonly used to "clean up" the ou ...

can be used to select the polarization of interest. Beams with radial and azimuthal polarization are included in the class of ''cylindrical vector beams''. A radially polarized beam can be used to produce a smaller focused spot than a more conventional linearly or circularly polarized beam, and has uses in

optical trapping Optical tweezers (originally called single-beam gradient force trap) are scientific instruments that use a highly focused laser beam to hold and move microscopic and sub-microscopic objects like atoms, nanoparticles and droplets, in a manner simila ...

. 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, and radial polarization can "self-heal" when obstructed. 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. The relativistic longitudinal electric field component has been proposed as a driver for particle acceleration in free space and demonstrated in proof-of-concept experiments.

References

{{reflist Polarization (waves)