Liquid crystal cells – elements and systems – With specified nonchemical characteristic of liquid crystal... – Within smectic phase
Reexamination Certificate
1997-06-23
2001-01-30
Parker, Kenneth (Department: 2871)
Liquid crystal cells, elements and systems
With specified nonchemical characteristic of liquid crystal...
Within smectic phase
C349S171000, C349S183000, C349S184000, C252S299010, C252S299650, C428S001400
Reexamination Certificate
active
06181407
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to an electro-optic material for modulating light, whose optical properties are able to be changed by applying an external electric field.
RELATED TECHNOLOGY
Ferroelectric liquid crystals used as electro-optic material for manufacturing optically active elements for information processing are known. Optically active elements of this kind are, for example, switches, fiber optic channels, dimmers operated at low voltages or modulators in the microsecond range, quick-reacting, optical delay elements, etc.
U.S. Pat. Nos. 4,367,924 and 4,563,059 describe a ferroelectric, liquid crystalline material having a chiral smectic C-phase, which has been used to produce a surface-stabilized, ferroelectric liquid crystal, abbreviated SSFLC, component. The optical state of the described liquid crystalline material is changed by applying an external electric field. For that, the longitudinal axes of the tilted liquid crystal molecules orient themselves in a preferred direction to the polarity of the applied electric field, collectively on a conical surface having an opening or cone angle, which corresponds to double the angle of inclination of the smectic C-phase of the liquid crystal. As a result of the collective reorientation of the longitudinal molecule axes in the smectic C-phase, the reaction time, that is the optical switching time of the liquid crystal, is relatively long in the SSFLC component. It may be that the reaction time is able to be shortened, as a general principle, by increasing the drive voltage; however, higher voltages are precluded because of the necessary small layer thickness of the material of about 1.5 &mgr;m.
U.S. Pat. No. 4,838.663 describes a ferroelectric, liquid crystalline material having a chiral, orthogonal, smectic A-phase which has been used in what is known as a “bookshelf” arrangement. The reaction time of this material in the micro- and submicro-second range in response to an applied electric field is, in fact, shorter and is only minimally dependent on the applied voltage. The optical response is small, though. When the electric field is applied, the longitudinal axes of the molecules, such as the longitudinal axes of the molecules of the liquid crystal having a chiral smectic C-phase, orient themselves to the polarity of the electric field, collectively in a preferred direction. However, the change in the optical state takes place in this case as a result of the collective tilting of the longitudinal molecule axes normal to the direction of the electric field. This is also known as electroclinic effect, and was described by S. Garoff and R. B. Meyer in Phys. Rev. Let., vol. 38, p. 848 (1977).
SUMMARY OF THE INVENTION
An object of the present invention is to create an electro-optic material whose optical switching time is lower than or is in the range of the fastest conventional ferroelectric liquid crystals, but requires clearly lower drive voltages.
The present invention therefore provides an electro-optic material, whose optical properties are able to be changed by applying an external electric field, characterized by a lamellar liquid crystal (
10
), which contains one or more components and into which chiral molecules (
20
), whose longitudinal axes (m) are longer than the longitudinal axes of the molecules (
10
) forming the lamellar liquid crystal, are introduced at a predefined concentration, so that the longitudinal axes (m) of the chiral molecules (
20
) are tilted, without an external electric field, statistically by a predefined angle (&thgr;
m
) toward the normal (z) of the layer of the lamellar liquid crystal.
The present invention further may provide that each chiral molecule (
20
) has at least one chiral center with a dipole element (
30
), which is disposed transversely to the layer normal (z) and which assumes a preferred direction in response to the application of an external electric field. Moreover, the lamellar liquid crystal (
10
) may be in a thermotropic smectic phase, in an orthogonal smectic phase, and in achiral or chiral smectic C-phase.
The inherent drawback of the two ferroelectric, liquid crystalline materials mentioned is that the change in their optical properties in response to an applied electric field has a relationship with a collective reorientation of the longitudinal axes of the liquid crystal molecules, and with a spontaneous polarization. The present invention creates an electro-optic material, whose electro-optic properties are characterized more by a reorientation of individual molecules than by a collective reorientation. For that, the electro-optic material comprises a lamellar liquid crystal, which contains one or more components and into which chiral molecules are introduced at a predefined concentration. The chiral molecules are dissolved in the lamellar liquid crystal. The longitudinal axes of the chiral molecules used as dopants are longer than the longitudinal axes of the molecules forming the lamellar liquid crystal, so that they are tilted, without an external electric field, statistically by a predefined angle, toward the normal of the layer of the lamellar liquid crystal.
The expression “lamellar liquid crystal” found in the claims and throughout the specification is understood to be a liquid crystal that is built up in a layered structure and that can be in an achiral or chiral, orthogonal smectic A-phase, a tilted smectic C-phase or in a lyotropic phase. Moreover, the lamellar liquid crystal can have a low-molecular or polymer structure. The lamellar liquid crystal can also consist of chiral compounds.
Each chiral molecule has at least one chiral center with a dipole element, which is disposed transversely to the layer normal and which assumes a preferred direction in response to the application of an external electric field. Due to this measure, the liquid-crystal molecules are no longer collectively reoriented in response to an applied external electric field, rather, for the most part, it is the chiral molecules introduced as dopant into the lamellar liquid crystal that are reoriented.
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Garoff et al., “Electroclinic Effect at the A-C Phase Change in a Chiral Smectic Liquid Crystal,” Physical Review Letters, vol. 38, No. 15, pp. 848-851, Apr. 11, 1977.
Anderson et al., “Smectic A Materials with 11.25 Degrees Induced Tilt Angle for Full Gray Scale Generation,” Ferroelectrics, 1991, vol. 114, pp. 137-150.
Davey et al., “Potential and Limitations of the Electroclinic Effect in Device Applications,” Ferroelectrics, 1991, vol. 114, pp. 101-112.
Miyasato et al., “Direct Method with Triangular Waves for Measuring Spontaneous Polarization in Ferroelectric Liquid Crystals,” Japanese Journal of Applied Physics, vol. 22, No. 10, Oct., 1983, pp. L661-L663.
Baikalov et al., “Measures of the Molecular Tilt Angle and Optical Anisotropy in Ferroelectric Liquid Crystals,” Mol. Cryst. Liq. Cryst., 1984, vol. 127, pp. 397-406.
Beresnev Leonid
Bizadar Ashok
Dultz Wolfgang
Haase Wolfgang
Deutsche Telekom AG
Kenyon & Kenyon
Ngo Julie
Parker Kenneth
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