Polarizer

Polarizers by absorption

The simplest polarizer is the metal grid , consisted of long parallel wire of Métal. The waves electromagnetic S which can pass are those whose Electric field is perpendicular to metal wire. Indeed, for the electric fields presenting a different orientation, the electron S of metal are likely to oscillate (as at the time of the reflection of a light wave on a metal): the waves are reflected and thus do not pass.

Thus, this polarizer makes it possible to obtain a wave polarized rectilignement perpendicular to metal wire.

However, this result is valid only for waves whose Wavelength is large in front of the spacing between wire, i.e. for the Micro-onde S. It is possible to reduce this spacing thanks to advanced techniques of Lithographie, but one generally prefers to use other types of polarizers if one makes use of light wavelength more courte.

Some crystals present a Dichroïsme , i.e. an absorption of the different light according to its polarization. They can thus be used as polarizers. The crystal of this most known type is the Tourmaline, but it is seldom used because its dichroism too strongly depends on the wavelength: it appears coloured then. The Herapathite dichroic and is less strongly coloured, but more difficult to produce out of crystals of big size.

The film Polaroïd was, in its original version, an arrangement of many herapathite crystals. Its following version sheet H resembles the polarizer out of metal grid rather. It is made of plastic of polyvinyl Alcool (PVA) doped with the Iode. The PVA being long a Molecule, the stretching of the sheet makes it possible to align it in a particular direction. Its operation is then similar to the metal grid described above. This matter practices to use, not very fragile, not very expensive and relatively easy to produce, is the type of polarizer most largely widespread. One finds it in Photographie, in the posting with liquid crystals, like in some Sunglasses.

An important modern polarizer is the Polarcor , manufactured by Corning Incorporated . This material is a Verre containing money particles élonguées in a film close to its surface. It is more durable and polarizes the lumère best that the Polaroid, with a weak absorption for the correctly polarized light. It is largely used in the Télécommunication S by Fiberoptic.

Polarizers by separation of beam

The polarizers by separation of beam separate the incidental beam in two beams from different polarizations (most of the time, these polarizations are rectilinear and perpendiculars between them). They absorb the light very little, which makes of it an advantage compared to the polarizers by absorption. They are also useful if the two separate beams are necessary.

The way simplest to realize some consists of a series of blades of glasses directed with the Angle of Brewster compared to the beam. Of this angle, being worth approximately 57° for glass, the polarized light p (i.e. parallel to the Plane of incidence, comes from senkrecht , perpendicular German ) is not thought by glass and 16% of the light polarized S (perpendicular to the plan of incidence, comes from paralleler , parallel German ) are considered. Each blade of glass reflects the light twice (with the entry and the exit), with the result that 0.002% of the polarized light S are transmitted through three blades of glass.

Certain polarizers exploit the Biréfringence certain materials like the quartz, the Calcite and the Iceland spar. These crystals have the characteristic to divide a beam nonpolarized into two differently polarized beams: there exist two angles of Réfraction, from where the term of birefringence . One then speaks about a ordinary ray noted O and about a extraordinary ray noted E (in general, these two rays are not polarized rectilignement).

One of the first polarizers of this type was the Prisme of Nicol consisted of a crystal of Calcite crossed into two then restuck with Baume of Canada. The crystal is cut so that the rays O and E are orthogonal rectilinear polarizations between them. The ray O undergoes a total Réflexion with the interface between the two pieces of calcite. The ray E arises on other side of the prism parallel to its initial direction. This prism produces a polarization of high-quality very and was largely used in Microscopie although it was replaced, in the modern applications, by other tools like the Prisme of Glan-Thompson ( to also see : Prism of Glan-Foucault and Prism of Glan-Taylor).

the Prisme of Wollaston uses also the properties of birefringence of calcite and creates two beams slightly divergent of orthogonal polarizations between them. One finds other prisms similar like the Prisme of Rochon and the Prisme of Sénarmont.

By recovering a blade of glass with a special fine layer, the Interférence S inside the layer make it possible to obtain a polarizer by separation of faiceau.

Law of Malus and other properties

Let us consider a wave polarized rectilignement arriving on a perfect polarizer and whose polarization forms an angle $\ theta$ with the axis of this polarizer. The Law of Malus, of the name of Etienne Louis Malus, gives the fraction of the intensity of this wave passing through the polarizer. By noting $I_0$ the intensity of the incidental wave and $I$ the intensity of the transmitted wave, the law of Malus is written

I = I_0 \ cos^2 \ theta \,

For example, a not-polarized light like that of the Sun or the usual lamps will see its decreased intensity of half. Indeed, a not-polarized light is actually made up of all the directions of possible polarization. It is necessary of which to take the average of the law of Malus, i.e.

I = \ frac {I_0} {2}

In practice 50% of transmission are not obtained because the polarizers are not perfect: the Polaroids transmit 38% of the incidental radiation and certain birefringent prisms transmit 49.9% of them. Moreover, the polarizers let pass a little light of nondesired polarization : the relationship between the intensity of the not-desired component and the intensity of the correct component varies from 1/500 for the Polaroid with 1/1000000 for the Prisme of Glan-Taylor.

Uses

The polarizers have many applications in Optique, here are some:

While placing two polarizers on the way of an unspecified light, the law of Malus shows that by controlling the angle between their axes, one can adjust the luminous intensity which leaves.
the fields of Weiss in a material Paramagnétique can be visualized using a polarizer.
One can make interfere the two rays resulting from a Prisme of Wollaston thanks to a polarizer. This is useful, in Interférométrie, to visualize weak variations of optical Indice.

References

Collett, Edward. Field Guides to Polarization , SPIE Field Guides vol. FG05 , SPIE (2005) ISBN 0819458686.
Hecht, Eugene. Optics , 2nd ED., Addison Wesley (1990) ISBN 0-201-11609-X. Chapter 8.
Kliger, David S. Polarized Light in Optics and Spectroscopy , Academic Close (1990) ISBN 0124149758

See too

Filter (optical)

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