Radial polarisation
Encyclopedia
A beam of light has radial polarization if at every position in the beam the polarization (waves) (electric field
) vector points towards the centre 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.
Radial polarization can be produced in a variety of ways. It is possible to use a liquid crystal
device to convert the polarization of a beam to a radial state, or a radially polarized beam can be produced by a laser
in which the Brewster window is replaced by a cone at Brewster's angle
.. A related concept is azimuthal polarisation, in which the polarisation vector is tangential to the beam. If a laser is focussed 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
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
.
Electric field
In physics, an electric field surrounds electrically charged particles and time-varying magnetic fields. The electric field depicts the force exerted on other electrically charged objects by the electrically charged particle the field is surrounding...
) vector points towards the centre 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.
Radial polarization can be produced in a variety of ways. It is possible to use a liquid crystal
Liquid crystal
Liquid crystals are a state of matter that have properties between those of a conventional liquid and those of a solid crystal. For instance, an LC may flow like a liquid, but its molecules may be oriented in a crystal-like way. There are many different types of LC phases, which can be...
device to convert the polarization of a beam to a radial state, or a radially polarized beam can be produced by a laser
Laser
A laser is a device that emits light through a process of optical amplification based on the stimulated emission of photons. The term "laser" originated as an acronym for Light Amplification by Stimulated Emission of Radiation...
in which the Brewster window is replaced by a cone at Brewster's angle
Brewster's angle
Brewster's 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 incident at this angle, the light that is reflected from the surface is therefore perfectly...
.. A related concept is azimuthal polarisation, in which the polarisation vector is tangential to the beam. If a laser is focussed along the optic axis of a birefringent material, the radial and azimuthal polarizations focus at different planes. A spatial filter
Spatial filter
A spatial filter is an optical device which uses the principles of Fourier optics to alter the structure of a beam of coherent light or other electromagnetic radiation. Spatial filtering is commonly used to "clean up" the output of lasers, removing aberrations in the beam due to imperfect, dirty,...
can be used to select the polarization of interest
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
Optical tweezers are scientific instruments that use a highly focused laser beam to provide an attractive or repulsive force , depending on the refractive index mismatch to physically hold and move microscopic dielectric objects...
.