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| Liquid Crystal twisted nematic polarization rotator (TN cell) is very useful when one wants to rotate the orientation of a linear polarization by a fix amount of typically 45° or 90°. When light is traversing LC twisted nematic cell its polarization follows the rotation of the molecules (see figure below). The screens of any laptop computer is based on the same effect (polarization rotator tutorial). In optical systems the polarization is often rotated by quartz retardation plates (l/2 or l/4 plates). Quartz plates shows high quality and good transmission performances especially in the UV region. However such plates present also some disadvantages: They are expensive, functions only for a narrow spectral bandwidth and have a small incidence angle acceptance (field of view less then 2°). The liquid crystal nematic cells have therefore a large acceptance angle, function over a very large spectral range from VIS to NIR (if they are thick enough) and are less expensive. Optionally, by applying a voltage on the TN cell, the polarization rotation can be “switched off”. Also when placing a 90° twisted cell between crossed polarizers it can be used as a shutter. |  |
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• l/2 plate for a very broadband range of wavelengths
• Optionally rotation effect can be electrically switched off
• In combination with two crossed polarizers it can be used as an optical shutter
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Polarization Rotators Type |
Specificities |
Applications |
Price |
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Industrial grade |
• Spacer (few microns) over the aperture• Large aperture (22mm)• Thin substrates• Phase distortions (spherical)• Low beam deviation |
• Polarization manag.• Polarization vision |
** |
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Scientific grade |
• Low phase distortions• No beam deviation• No spacers over the aperture• Aperture 10mm or 20mm• Thick substrates• Broadband AR coating |
• Interferometry• Metrology• Use in an imaging plane • colimated laser beam |
*** |
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Custom |
• Larger apertures.• High switching speeds.• Large quantities/low price.• Zero phase shift |
•Custom adapted cells |
* / **** |
A twisted nematic liquid crystal cell consists essentially of a liquid crystal layer placed between two treated glass substrates.
The inner-surfaces of the cell is composed of two layers: The first layer is a transparent electrode (mostly ITO). It permits to apply an electrical field across the cell and switch the cell between the OFF and the ON state. The second layer is responsible for the homogenous alignment of the LC. It is generally a rubbed polyimide layer of about 100nm.
The liquid crystal alignment at both sides of the cell is hence defined during cell manufacturing. By careful control, any twist-angle can therefore be induced in the helical structure across the liquid crystal layer. With a twist-angle of exactly 90°, the standard 90° twisted nematic (TN) cell is formed. Twist-angles of less than 90° form the low-twist (LT) cell whereas by definition, super-twist cells are cells that possess twist-angles exceeding 180°.The two glass substrates are separated by spacers with a well defined size (usually between 3mm and 20mm) and sealed with glue.
When the polarization rotator is in the off state, the helical structure formed by the LC molecules rotates the entrance polarization as shown in figure 1. In the ON state the polarization rotary power is suspended and the polarization state of the light entering normally to the entrance surface is not altered by the TN cell.100% efficient rotation of a linear entrance polarization can only be obtained in the limit of large cell thickness and in general the exiting light becomes elliptically polarized with components oscillating in directions lying both parallel and perpendicular to the exit liquid crystal molecules. Furthermore, it is the optical-path-difference in the liquid crystal cell that affects the overall magnitude of the polarization efficiency for the TN cell. The optical-path-difference is given by the Dnd parameter, where Dn is the anisotropic index of refraction for the liquid crystal material and d is the cell-gap. The following equation shows the transmission of a TN 90° cell as function of a normalized retardation parameter u. It assumes that the TN cell is placed between two parallel orientated ideal polarizers.
The best extinction (which means also the best rotation efficiency) is obtained with the highest optical path difference. So for optimal rotation of the entrance polarization over a broad spectral range it is better to use a TN cell with a high optical path difference (which means a large cell gap and a high anisotropy).
However one must be aware that higher cell gaps decrease drastically the switching time of the TN cell. So rapid switching times and high efficiency over a brad spectral range cannot be obtained. Notice that the curves shows some minimum and a custom made TN cell can be optimized to have a good rotatory efficiency (low transmission) and a rapid switching time (minimal cell gap) for a narrow range of wavelength .
In application where switching time does not matter it is better to choose a TN cell with a high optical path difference.
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Rotation angle |
90° or 45° (custom any angle possible for scientific and industrial grade) |
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wavelength range |
350-1700 nm |
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Active area |
scientific grade:10 mm or 20 mm (diam.) |
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Transmission |
About 85% (VIS) |
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Retarder material |
Nematic Liquid-Crystal Dn=0.28 |
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Substrates |
Glass |
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wavefront distortion |
scientific grade: |
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temperature range |
15°-35° |
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Rotation accuracy |
+/-1° (wavelength dependent) |
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Maximum modulation frequency of the phase shift |
< 10Hz |
| Save operating limit | 500 W/cm2 CW 300 mJ/cm2 10 ns, visible 200 mJ/cm2 10 ns, 1064 nm |
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Anti-reflection coating (scientific grade only) |
Broadband for VIS. |
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Total size (with housing) |
Scientific grade: 25 mm diameter, 15mm long |
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