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SSFLC Cell Parameters |
By means of the Clark-Lagerwall effect, the birefringence of the SSFLCs can be used to create black and white display states. This is accomplished by creating a switchable half wave plate involving the bistable states of the SSFLC and crossed polarizers as illustrated in the animation below. To understand this process, note that with the director parallel to the surface plates in the SSFLC (smectic planes perpendicular to the glass surface), the azimuthal angle in the cell will be constant and the director will be uniform in a defect free sample. Although the SmC* phase is actually weakly biaxial, it can, to a good approximation, be treated as a uniaxial material, just like the nematic phase which has optical anisotropy (birefringence) Dn. Thus, the optical phase can be adjusted through the choice of an appropriate FLC material, which determines Dn, and cell thickness, producing a half wave plate which is effective over the central region of the visible spectrum. The function of the half wave plate, which is to retard the phase of one of two orthogonal components of light by 180 degrees in passing through the cell, can be reviewed in the Birefringence Section as well as the Birefringence Lab. This characteristic is used by putting the cell between linear crossed polarizers with one polarizer parallel to one of the two possible directions of the director. The state with the director parallel to the polarizer is non transmissive while the other allows light to cross the cell. This is shown in the animation below where the non transmissive state is the down state, named so because the polarization vector is pointing down. Note how the director is parallel to the polarizer on the bottom, which is where the light is coming from.
In the up state, light is transmitted because the director is no longer parallel to the polarizer and one component of the polarization has been retarded by 180 degrees, thereby allowing light to pass through the analyzer on the top. The intensity of the light across the cell is dependent on the cone angle, delta n, cell thickness, and wavelength of the light. This is shown in the following simulation where you can explore these parameters through an analytic expression for the intensity.
The Clark-Lagerwall effect in SSFLCs shows a much more rapid response to the externally applied electric field than in nematic liquid crystals because it is interacting with the sizeable spontaneous polarization rather than with an induced polarization.
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FLC Structure |  |
Spontaneous Polarization |  |