Polarization can be rotated eefectively and achromatically with Liquid Crystal twisted nematic Polarization rotator (TN cell). These TN cells are very useful when one wants to rotate the orientation of a linear Polarization by a fixed amount of typically 45° or 90°. When light is crossing LC twisted nematic cell its Polarization follows the rotation of the molecules (see figure below). The screen of any laptop computer is based on the same effect.

In optical systems, the Polarization is often rotated by quartz retardation plates (l/2 or l/4 plates). Quartz plate shows high quality and good transmission performances especially in the UV region. However, such plates present also some disadvantages: they are expensive, function only for a narrow spectral bandwidth and have a small incidence angle acceptance (field of view less than 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.

For more information, please download the product description PDF file in the tabs below or contact ARCoptix: info@arcoptix.com

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 polyamide 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 curve 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.

For further information contact ARCoptix: info@arcoptix.com