Читать книгу Liquid Crystal Displays - Ernst Lueder - Страница 28

The cell operating with the deformation of aligned phases, called the DAP cell (Glueck, 1995), is the inverse of the Fréedericksz cell. In the field-free state the LC molecules are perpendicularly (or in other words, homeotropically) aligned to the surface of both substrates, as depicted in Figure 3.17. This cell is also called a Vertically Aligned (VA) LCD. In this situation, incoming linearly polarized light with a wave vector in parallel to the z-axis in Figure 3.17 does not encounter birefringence, and arrives at the second substrate with an unchanged state of its polarization. If the analyser is parallel to the polarizer, the full light can pass representing the normally white state. If the analyser is crossed with the polarizer, the light is blocked at the output for all wavelengths and independent of d. This is the normally black state. The cell exhibits an extremely good black state, since the blocking is again independent of λ. Further, the molecules on the orientation layer are also, contrary to the Fréedericksz cell, vertically aligned. A low black value in the denominator of the contrast in Equation (3.82) is most beneficial for a high contrast. The main attraction of the DAP cell is this extremely high contrast, reaching values of more than 500: 1.

Figure 3.17 The DAP cell or Vertically Aligned (VA) cell in the field-free state

If an electrical field is applied, the LC molecules orient themselves perpendicularly to the field as Δε < 0. This alignment corresponds to the same alignment of the Fréedericksz cell in the field-free state. Hence, all results in Equations (3.40) through (3.87) also apply to the DAP cell, which is exposed to an electric field. The DAP cell is as well suited for phase-only modulators, as the pertinent Equations (3.90) and (3.95) also hold if a voltage V is applied. However, for the voltage-dependent refractive index n(V), we obtain , but contrary to the Fréedericksz cell with n┴ for the lower voltage and n|| for the higher voltage. The homeotropic alignment of the molecules in the DAP cell requires special care. It is achieved by a spin-coated monomolecular silane-layer dissolved in ethyl alcohol, which is polymerized in the presence of humidity. The high polarity of silane thus generated anchors the polar LC molecules perpendicular to the surface. If a voltage is applied, all molecules are supposed to tilt in the same direction, since they have to end up all in parallel to each other and parallel to the plane of the substrates. This is realized by a small uniformly oriented pretilt of around 1 ° to 2° off the normal of the surface. A larger pretilt must be avoided, since it degrades the black state. The polymerized silane layer is uniformly rubbed with a carbon fibre brush to generate the grooves for the orientation of the molecules. As an alternative, this pretilted uniform orientation is produced with a very high manufacturing yield by an SiO₂ layer obliquely evaporated or sputtered under an angle of 2° off the normal. This alternative also achieves a very high contrast exceeding 500: 1. The sputtering of this SiO₂ layer is explained in Figure 3.18. The DAP cell is, like a Fréedericksz cell, designed as a λ/2-plate with a retardation Δnd = λ/2, and hence d = λ/2Δn. For most commercially available LC materials exhibiting Δn = 0.08, this leads for λ = 550 nm to a cell thickness of d= 3.4 μ. The reflective version is a λ/4-plate with a thickness of d= 1.7 μ, which is often too thin for a high yield fabrication because small particles could easily cause shorts. The search for electro-optical effects with a larger cell thickness leads to the HAN cells and the Twisted-Nematic cells (TN-cells), which are covered in the next subsection and in Chapter 4.

Figure 3.18 The sputtering of an SiO₂ orientation layer under an oblique angle of 70°

A reflective DAP cell with a thickness d/2 can be constructed in the same way as a Fréedericksz cell.