Absorptive polarizersThe simplest polarizer in concept is the wire-grid polarizer, which consists of a regular array of fine parallel metallic wires, placed in a plane perpendicular to the incident beam. Electromagnetic waves which have a component of their electric fields aligned parallel to the wires induce the movement of electrons along the length of the wires. Since the electrons are free to move, the polarizer behaves in a similar manner as the surface of a metal when reflecting light; some energy is lost due to Joule heating in the wires, and the rest of the wave is reflected backwards along the incident beam.
For waves with electric fields perpendicular to the wires, the electrons cannot move very far across the width of each wire; therefore, little energy is lost or reflected, and the incident wave is able to travel through the grid. Since electric field components parallel to the wires are absorbed or reflected, the transmitted wave has an electric field purely in the direction perpendicular to the wires, and is thus linearly polarized. Note that the polarization direction is perpendicular to the wires; the notion that waves "slip through" the gaps between the wires is incorrect.
For practical use, the separation distance between the wires must be less than the wavelength of the radiation, and the wire width should be a small fraction of this distance. This means that wire-grid polarizers are generally only used for microwaves and for far- and mid-infrared light. Using advanced lithographic techniques, very tight pitch metallic grids can be made which polarize visible light. Since the degree of polarization depends little on wavelength and angle of incidence, they are used for broad-band applications such as projection.
It is interesting to consider why there is a reflected beam, but no transmitted beam, when the symmetry of the problem suggests that the electrons in the wires should re-radiate in all directions. In simple terms the transmitted beam does "exist", but is in exact antiphase with the continuing incident beam, and so "cancels out". This, in turn, seems to contradict the idea that the incoming wave is "driving" the electrons in the wires, and so is "used up" (leaving no continued beam to cancel out the transmitted wave). In fact, if we assume that there is no heating, then no energy is used to drive the electrons — a better mental image is to think of them as "riding" on the waves that result from the interaction.
Certain crystals, due to the effects described by crystal optics, show dichroism, a preferential absorption of light which is polarized in a particular direction. They can therefore be used as polarizers. The best known crystal of this type is tourmaline. However, this crystal is seldom used as a polarizer, since the dichroic effect is strongly wavelength dependent and the crystal appears coloured. Herapathite is also dichroic, and is not strongly coloured, but is difficult to grow in large crystals.
Polaroid film was in its original form an arrangement of many microscopic herapathite crystals. Its later H-sheet form is rather similar to the wire-grid polarizer. It is made from polyvinyl alcohol (PVA) plastic with an iodine doping. Stretching of the sheet during manufacture ensures that the PVA chains are aligned in one particular direction. Electrons from the iodine dopant are able to travel along the chains, ensuring that light polarized parallel to the chains is absorbed by the sheet; light polarized perpendicularly to the chains is transmitted. The durability and practicality of Polaroid makes it the most common type of polarizer in use, for example for sunglasses, photographic filters, and liquid crystal displays. It is also much cheaper than other types of polarizer.
An important[citation needed] modern type of absorptive polarizer is made of elongated silver nanoparticles embedded in thin (≤0.5 mm) glass plates. These polarizers, are more durable and can polarize light much better than Polaroid film, achieving polarization ratios as high as 100,000:1 and absorption of correctly-polarized light as low as 1.5%.[2] Such glass polarizers perform best for short-wavelength infrared light, and are widely used in optical fiber communications.
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Polarizer
July 30, 2018, 03:29:07 pm
