Role of the Cholesteric Phase in the Formation of Twisted Smectic Structures
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an effort to improve the optical quality of these devices through a fundamental understanding of the alignment process in smectic liquid crystals. FERROELECTRIC TWISTED SEMCTIC STRUCTURE (FETS) The structure described in figure 1, is obtained by rubbing the cell surfaces at an angle of 90 degrees with respect to each other. In this case, a mono-domain sample is obtained, with the smectic layers at a 45 degree angle to the rubbing axis. The smectic layers are arranged in a book shelf geometry. Within each layer, the projection of the molecules on the cell surface rotates by 90 degrees from one surface to the other. The molecules have to be tilted by an angle equal to the tilt angle with respect to the layer normal since it is a fundamental thermodynamic constant. Thus, the molecules lie in the plane of the substrate at cell surfaces, but they deviate from this plane in the middle of the cell. Figure 1 schematically shows that the molecules are
constrained on the cone's surface. The local ferroelectric polarization P of this sample undergoes a 180 degree rotation while moving from one surface to the other. This is because the polarization direction is constrained to lie in the plane of the smectic layers and normal to n. This coupling between n and P is important because it allows the molecular position to be changed by reorienting P using an externally applied electric field.
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Figure 1 : Operating principle of a twisted ferroelectric structure The construction and the operating principle of the device is described elsewhere [1,2,3]. The ferroelectric materials needed for constructing these device requires a 45 degree tilt angle in the smectic C phase, which most often results in materials with a higher temperature cholesteric phase. The role of the cholesteric phase in providing an improved twisted smectic structure is the focus of this work.
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This study has been motivated by our observations, although treated almost identically, often produce quite different smectic structures. This was believed to be due to a subtle variation of the cholesteric structures during the cooling process. EXPERIMENTS To develop a more detailed understanding of the relationship between the twisted smectic and the cholesteric liquid crystalline phase, we need to accurately measure the pitch of liquid crystlas in the cholesteric phase. In addition, the surface boundary conditions imposed by rubbing process affects the liquid crystal structure in both the cholesteric phase and the smectic phase. The CS2004 liquid crystal obtained from Chisso Corporation is the material used in the present study. Chiral Pitch Measurement Using a method developed at Penn State, we use a sample in which the cell is constructed with two glass plates, which have been rubbed circularly on one surface and uniform parallel on the other [4]. This method allows us to measure both the amount and direction of cholesteric pitch simultaneously. The measurement of the pitch was made by observing the position of a defect with respect to the plates' rubbi
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