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.
| Polarization Rotators Type | Specificities | Applications | Price |
| Industrial grade | - Spacer (few microns) over the aperture
- Large aperture (22 mm)
- Thin substrates
- Phase distortions (spherical)
- Low beam deviation
| - Polarization manager
- Polarization vision
| ** |
| Scientific grade | - Low phase distortions
- No beam deviation
- No spacers over the aperture
- Aperture 10 mm or 20 mm
- Thick substrates
- Broadband AR coating
| - Interferometry
- Metrology
- Use in an imaging plane
- collimated laser beam
| *** |
| Custom | - Larger apertures
- High switching speeds
- Large quantities/low price
- Zero phase shift
| - Custom adapted cells for industrial applications
- Specific scientific applications
| * / **** |