Liquid Crystals. Iam-Choon Khoo

Чтение книги онлайн.

СКАЧАТЬ (i.e. on the plane of the cell wall). As a matter of fact, there is a commercial so‐called rubbing machine for preparing planar cell windows. Another method is to deposit silicon oxide obliquely onto the glass slide.

      For preparing a PVA‐coated planar sample in the laboratory, the following technique has proven to be quite reliable. Dissolve chemically pure PVA (which is solid at room temperature) in distilled deionized water at an elevated temperature (near the boiling point) at a concentration of about 0.2%. Dip the cleaned glass slide into the PVA solution at room temperature and slowly withdraw it, thus leaving a film of the solution on the slide. (Alternatively, one could place a small amount of the PVA solution on the slide and spread it into a thin coating.) The coated slide is then dried in an oven, followed by unidirectional rubbing of its surfaces with a lens tissue. The rest of the procedure for cell assembly is the same as that for homeotropic alignment.

Ideally, of course, these cell preparation processes should be performed in a clean room and preferably in an enclosure free of humidity or other chemicals (e.g. a nitrogen‐filled enclosure) in order to prolong the lifetime of the sample. Nevertheless, the liquid crystal cells prepared with the techniques outlined above have been shown to last several months and can withstand many temperature cycling through the nematic–isotropic phase transition point, provided the liquid crystals used are chemically stable. Generally, nematics such as 5CB and E7 are quite stable, whereas MBAA (p‐methoxybenzylidene‐p′‐n‐butylaniline) tends to degrade in a few days.

      Besides these two standard cell alignments, there are many other variations such as hybrid, twisted, supertwisted, and fingerprint; multi‐domain vertically aligned; and so on. In recent years [17, 27], photo‐alignment of dye‐doped cell window surface has also been shown to be highly effective for imparting the desired liquid crystal director axis arrangement.

      For smectic‐A, the preparation method is similar to that for a homeotropic nematic cell. In this case, however, it helps to have an externally applied field to help maintain the homeotropic alignment as the sample (slowly) cools down from the nematic to the smectic phase. The cell preparation methods for surfaced‐stabilized FLC (SSFLC) operation is more complicated as it involves surface stabilization [28, 29]. On the other hand, Sm‐A^* cells for soft‐mode (SM‐FLC) operation are easier to prepare using the above methods [30].

      1.5.2. Cholesteric Liquid Crystal Cell Assembly

Cholesteric liquid crystals occupy a special niche in fundamental studies as well as optical applications owing to their photonic crystal properties. They are prepared in a similar fashion to the planar nematic sample, except that a small amount of chiral dopant is added to the starting nematic material [31]. Owing to the strong anchoring condition on the two cell windows, and the rotation ability of the chiral agent, the director axis of the CLC evolves as a spiral helix from one cell wall to the other, cf. Figure 1.11. In general, such helical arrangement of the director axis can be maintained by cell surfaces’ anchoring for sample thicknesses up to tens of microns; thicker cells invariably exhibit highly scattering focal conic texture associated with the helical axis becoming randomly oriented in bulk, cf. Figure 1.20a.

      Recent studies [31–33] have shown that the so‐called field assisted self‐assembly (FASA) technique could produce very stable and well‐aligned CLC cells with thicknesses approaching 1 mm, with a period number N as large as 3000. The technique works by using nematic liquid crystals with negative dielectric anisotropy and a strong AC field during the sample preparation stage to enforce the required planar alignment of the LC molecules and therefore prevent the director axis rotation helix from deviation into a focal conic structure, as illustrated in Figure 1.20b and c. The empty cell is first filled with CLC mixture in the isotropic phase (see Figure 1.20b); then, as the sample is allowed to cool down slowly to room temperature, an AC electric (~2500 V; 1 kHz) is applied across the cell window for an extended period while the mixture sits at room temperature (Figure 1.20c and d).

Figure 1.20. (a) CLC with focal conic structure as a result of director axis deviating from planar alignment. (b) Random orientation of director axis in the isotropic liquid phase; AC voltage on. (c) Sample cooling down to ordered phase; strong AC field enforces planar alignment. (d) AC field removed after an extended period.

Figure 1.21. Left: Image of logo obstructed by a thin CLC cell with focal conic alignment. Right: Clear image viewed through a well‐aligned sample following the FASA treatment.

The fabricated cells are found to exhibit uniform planar alignment with low scattering loss, cf. Figure 1.21, and well‐defined photonic bandgaps with long‐ and short‐wavelength band‐edges that can be tailored to fit any wavelength in the visible to near‐rIR spectrum (400–2000 nm). The dispersion and other chiral photonic properties of these extraordinarily thick CLC’s have enabled advanced photonic processes such as ultrafast modulation of femtosecond laser pulses and polarization rotation/switching that are impossible with their conventional thin counterparts [31–33].

      1.5.3. Blue‐phase Liquid Crystal Cell Assembly

Cholesteric blue phase liquid crystals, commonly called blue‐phase liquid crystals (BPLC) [25, 31,34–39], are a special class of LC that may be regarded as the 3‐D variant of CLC. When first discovered, they existed only in a very narrow temperature range (<1 °C), but following the work by several groups, room temperature BPLCs with moderate to large temperature range became available in recent years; room temperature pristine BPLC [34] with a sizeable blue‐phase range (~9 °C) can now be routinely synthesized using a mixture that contains a chiral dopant S‐811 (Merck) and two nematic hosts (E48 and 5CB) in the ratio: S811 (36%):E48 (32%):5CB (32%). Owing to the increased concentration of the chiral agent in the BPLC, the molecules self‐assembled into tightly wound double‐twist cylinders, cf., instead of a simple helical arrangement as in CLC. The disclination lines among the cylinders form a network that exhibits cubic crystalline structures, cf. Figure 1.22.

      In order to extend the working temperature range, a standard practice is polymer stabilization of the BPLC lattice [34–38]. Polymer‐stabilized blue‐phase liquid crystals (PS‐BPLC) are obtained by blending the chiral nematic material used to synthesize BPLC with photocurable prepolymers and polymerize the blend with UV or visible light depending on the type of photocurable prepolymer used. Typical phase sequence of the resulting PS‐BPLC in [34] is: Iso‐(56.2 °C)‐BP‐(<0 °C)‐N, i.e. the temperature range is ~56 °C.

      As a function of the temperature, pristine BPLC with the constituents described in [34] typically exhibits the following phase sequence: transparent СКАЧАТЬ