Liquid crystal cells – elements and systems – Cell containing liquid crystal of specific composition – In nematic phase
Reexamination Certificate
2001-12-19
2004-02-24
Ngo, Julie (Department: 2871)
Liquid crystal cells, elements and systems
Cell containing liquid crystal of specific composition
In nematic phase
C349S115000, C349S193000, C349S194000, C349S117000
Reexamination Certificate
active
06697144
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to switchable optical devices, and, more particularly, to switchable optical components such as switchable mirrors and variable retarders and their fabrication.
BACKGROUND OF THE INVENTION
The art of producing useful optical components based on porous thin films fabricated via the method of vacuum deposition of inorganic materials at oblique angles has steadily evolved over the latter half of the 20
th
century. In a seminal article appearing in Nature 183, 104 (1959), Young and Kowal describe the remarkable observation of optical activity in obliquely deposited films made using a material which is found otherwise to be optically isotropic. They achieved this result by depositing the material at an oblique incidence angle on a rotating substrate. Hodgkinson and Wilson review subsequent work that elucidates the relationship between the structure and properties of obliquely deposited thin films in an article entitled “Microstructural-induced anisotropy in thin films for optical applications,” appearing in CRC Critical Reviews in Solid State and Materials Sciences 15 (1), 27 (1988).
There are several noteworthy technologies based on obliquely deposited thin films. First is the optical retardation plate, as described by Motohiro and Taga in Applied Optics 28 (13), 2466 (1989). Motohiro and Taga demonstrated a quarter-wave retarder at a wavelength of 546 nm using an obliquely deposited Ta
2
O
5
thin film structure. In that work, the anisotropy of the Ta
2
O
5
film is attributed to form birefringence of inclined Ta
2
O
5
nano-columns formed on the substrate during the deposition process.
A second technology is a high spatial frequency chiral thin film structure, proposed in a theoretical work by Azzam in Applied Physics Letters 61 (26), 3118 (1992). This structure bears similarity to the films fabricated by Young and Kowal; however, Azzam suggested higher spatial frequency structures that would act as polarization sensitive Bragg reflectors. Such films reflect light within a band of wavelengths that has the same polarization handedness as the chiral film, and pass light of the orthogonal polarization. Examples of these structures were reported by Fan et al. in Optics Communications 147, 265 (1998), as well as by Hodgkinson and Wu in Applied Physics Letters 74(13), 1794 (1999). It is useful to note that these structures are artificial analogs of naturally occurring cholesteric liquid crystal materials. The optical properties of cholesterics are discussed by DeGennes and Prost in
The Physics of Liquid Crystals
(2
nd
ed., Clarendon Press, Oxford, 1993), chapter 6.
Similarly, Robbie and Brett in U.S. Pat. No. 5,866,204 describe methodologies for growing helical columns using the technique of oblique vapor deposition on a rotating substrate.
A third proposed application for porous films formed by oblique vapor deposition is as a micro-sieve. Robbie and Brett in the aforementioned patent point out that the volume density of obliquely deposited films can be made to vary between 10-90%. Further, the characteristic channel size in the films is in the nanometer regime. Both of these characteristics make them suitable for use as micro-sieves.
A fourth application of obliquely deposited films is as an alignment layer for liquid crystal displays. John L. Janning was the first to report on the “sympathetic alignment” of nematic liquid crystals on an obliquely deposited film in Applied Physics Letters 21(4), 173 (1972). In that work, an 8-&mgr;m-thick twisted nematic display was aligned using 7-nm-thick obliquely evaporated SiO films on the top and bottom display substrates. Lawrence A. Goodman et al. in IEEE Transactions on Electron Devices ED-24, 795 (1977) and Julian Cheng et al. in Applied Physics Letters 37(8), 716 (1980) provide physical descriptions of nematic alignment using porous columnar structures obtained via oblique depositions.
Akhlesh Lakhtakia et al. in Innovations in Materials Research 1(2), 165 (1996) and in Microwave and Optical Technology Letters 17(2), 135 (1998) mention the possibility of imbibing porous obliquely evaporated films with “suitable fluids” to obtain devices aimed at “some desired goal.” However, this prior art lacks sufficient information to effectively fabricate switchable optical components.
It is therefore an object of this invention to utilize liquid crystal-imbibed obliquely evaporated films as switchable optical components.
It is another object of this invention to utilize liquid crystal-imbibed obliquely evaporated films to fabricate variable retarders.
It is yet another object of this invention to utilize liquid crystal-imbibed obliquely evaporated films to fabricate mirrors with electrically switchable reflectance.
SUMMARY OF THE INVENTION
The objects set forth above as well as further and other objects and advantages of the present invention are achieved by the embodiments of the invention described herinbelow.
The present invention relates to switchable optical components that are produced using a unique class of materials. These materials are thin film composites comprising of porous inorganic films imbibed with liquid crystal materials. The porous inorganic thin films are fabricated using the technique of vacuum deposition at an oblique angle of incidence. The liquid crystal materials possess an inherent large optical anisotropy that can be altered using an applied electric field or thermally, by heating or cooling the materials. When combined according to the present invention, the resulting composite materials exhibit large tunable optical indices. This enables the construction of variable retarders and mirrors with switchable reflectance. The composite materials described in the present invention will be referred to as dielectric confined liquid crystals, or DCLCs, in analogy with polymer dispersed liquid crystal materials, or PDLCs.
For a better understanding of the present invention, together with other and further objects thereof, reference is made to the accompanying drawings and detailed description.
REFERENCES:
patent: 5799231 (1998-08-01), Gates et al.
patent: 5812233 (1998-09-01), Walsh et al.
patent: 5866204 (1999-02-01), Robbie et al.
patent: 5928819 (1999-07-01), Crawford et al.
patent: 6072629 (2000-06-01), Fan et al.
patent: 6252710 (2001-06-01), Fan et al.
“New technique of aligning liquid crystals on surfaces”,Applied Physics Letters, vol. 29, No. 11, Dec. 1, 1976, p. 691.
I.J. Hodgkinson and Peter W. Wilson, “Microstructural-Induced Anisotropy in Thin Films for Optical Applications”,CRC Critical Reviews in Solid State and Materials Sciences, vol. 15 Issue 1 (1988), pp. 27-61.
Tomoyoshi Motohiro and Y. Taga, “Thin film retardation plate by oblique deposition”,Applied Optics, vol. 28, No. 13, Jul. 1, 1989, pp. 2466-2482.
R.M.A. Azzam, “Chiral thin solid films: Method of deposition and applications”, American Institute of Physics,Appl. Phys. Lett., vol. 61, No. 26, Dec. 28, 1992, pp. 3118-3120.
B. Fan, et al., “Optical circular dichroism of vacuum-deposited film stacks”,Optics Communications, 147 (1998), pp. 265-268.
Ian Hodgkinson and Qi Hong Wu, “Birefringent thin-film polarizers for use at normal incidence and with planar technologies”,Applied Physics Letters, vol. 74, No. 13, Mar. 29, 1999, pp. 1794-1796.
P.G. deGennes and J. Prost, “The physics of Liquid Crystals”,Clarendon Press, Second Edition (Table of Contents).
John L. Janning, “Thin film surface orientation for liquid crystals”,Appl. Phys. Lett., vol. 21, No. 14, Aug. 15, 1972, pp. 174-174.
Lawrence A. Goodman, et al. “Topography of Obliquely Evaporated Silicon Oxide Films and Its Effect on Liquid-Crystal Orientation”,IEEE Transactions on Electron Devices, vol. ED-24, No. 7, Jul., 1977, pp. 795-804.
Julian Cheng, et al., “A scanning electron microscope study of columnar topography and liquid-crystal alignment on obliquely deposited oxide surfaces at low rates”,Appl. Phys. Lett, Oct. 15, 1990, pp. 716-719.
A. Lakhtakia, et al., “Sculptured Thin Films (STFS) For Optical, Chemical and Biological Applicat
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