Liquid Crystal Displays. Ernst Lueder
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Figure 3.13 An LCD used as an SLM operating as a multiplier
The explanation of the SLM for phase-shifts starts with the most general Equations (3.40) and (3.41) of the Freedericksz cell containing the arbitrary angle a of the polarizer at the input and the pertinent angle γ of the analyser (Figures 3.4(a) and 3.8).
Equations (3.40) and (3.41) yield, for γ = α (that is, for parallel polarizers), the Jones vectors Jzξ and Jzη measured in the ξ−η coordinates in Figure 3.4(a)
(3.89)
The Jones vector Jz% of the light passing through the analyser parallel to the polarizer is, for a = 0, n and a = n/2 and for no voltage applied,
and
(3.91)
The magnitude is constant, whereas the phase changes with the distance z from the input.
|Jzξ| from Equation (3.88) is plotted in Figure 3.14 versus α and z. The constant magnitude 1 for α = 0, (π/2) and π independent of z is visible as well as the maximum amplitude modulation for α = π/4. However, we want arc Jzξ to change with the voltage V across the cell. To this aim, we consider the Fréedericksz cell in Figure 3.15, where a voltage has been applied to tilt the molecules by an angle φ. The linearly polarized light E0 stemming from the polarizer with angle α = 0 to the x-axis has to meet the boundary condition at the transition from the polarizer into the cell. The tangential components have to be equal on both sides, which means they are E0 in Figure 3.15. This indicates that the light wave has the wave vector
Figure 3.14 |Jzξ| in Equation (3.88) plotted versus α and z
Figure 3.15 The linearly polarized light in parallel (α = 0) to the projection of the long axis of the LC molecules into the x-y plane
With a large enough voltage V, all molecules are perpendicular to the surface or the x-axis, yielding
From Equations (3.92) and (3.93), we detect the voltage-induced change of the refraction index
(3.94)
with
How n changes with V cannot yet be determined. For that we need the propagation of light obliquely to the LC molecules, which will be discussed in Chapter 6. So far, arc Jzξ(V) is determined by measurements.
Figure 3.16 Measured phase-shift curves of a Fréedericksz cell
For λ0 = 0.5