Circular polarization controlling optical element and method...

Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only

Reexamination Certificate

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C349S096000, C349S187000

Reexamination Certificate

active

06816215

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a circular polarization controlling optical element for extracting right- or left-handed circularly polarized light from non-polarized light, and to a method of producing the same.
BACKGROUND ART
There has conventionally been known, as a circular polarization controlling optical element having the above-described function, an optical element that includes a liquid crystal layer having a cholesteric order (cholesteric regularity) and that reflects, in a specific reflection wave rage, either right- or left-handed circularly polarized light having a wavelength equivalent to the pitch (helical pitch) in the helical structure of the liquid crystal layer and transmits the other circularly polarized light. The term “liquid crystal layer” herein used means a layer having the properties of liquid crystal in the optical sense, and includes not only a layer of liquid crystal phase having flowability but also a layer of solid phase obtained by solidifying liquid crystal phase while retaining the alignment of molecules characteristic of the liquid crystal phase.
Such circular polarization controlling optical elements are extensively used in liquid crystal display panels or the like, and are often required to have reflection wave ranges as wide as the entire visible light range.
In the circular polarization controlling optical elements, there has conventionally been known, as a technique for broadening the reflection wave range, such a method that a plurality of liquid crystal layers having reflection wave ranges centered at different wavelengths are laminated. Another known method is such that a cholesteric liquid crystalline material whose helical pitch can be varied stepwise (continuously) is used so that the helical pitch is varied in the direction of thickness (U.S. Pat. No. 5,691,789 and Japanese Patent Laid-Open Publication No. 281814/1994). Further, Japanese Patent Laid-Open Publications No. 319235/1998 and No. 44816/1999 disclose such a method that, after two cholesteric liquid crystalline polymer layers are subjected to contact bonding, the helical pitch is varied stepwise through heat treatment.
In the above-described conventional method in which a plurality of liquid crystal layers having reflection wave ranges centered at different wavelengths are laminated, the entire reflection wave range of the laminate is simply the sum of the reflection wave ranges of the respective liquid crystal layers. It is therefore necessary to laminate a great number of liquid crystal layers if it is desired to obtain a laminate having a reflection wave range that covers the entire visible light range. In this case, however, the influence of reflection of light caused at the interface between each two liquid crystal layers laminated is not negligible, and the laminate is to have poor optical properties.
The above-described method using a cholesteric liquid crystalline material whose helical pitch can be varied stepwise (continuously) is advantageous in that the degree of reflectance of circularly polarized light can be made constant to some extent because the broadening of reflection wave range can be attained by the use of a single liquid crystal layer. In this method, however, it is necessary to incorporate non-crosslinkable liquid crystalline molecules (U.S. Pat. No. 5,691,789) or coloring materials (Japanese Patent Laid-Open Publication No.281814/1994) into the liquid crystalline material. The liquid crystalline material containing such molecules or coloring materials is poor in heat resistance. Moreover, the resulting liquid crystal layer is colored, so that it is poor in optical properties.
Further, in the above-described conventional method in which two cholesteric liquid crystalline polymer layers are subjected to contact bonding and then to heat treatment, the liquid crystalline materials are required to have heat resistance because the heat treatment is carried out at high temperatures. Therefore, types of liquid crystalline materials useful in this method are inevitably limited. In addition, the bonded surfaces of the liquid crystalline polymer layers have been polymerized; this means that the optical interface cannot fully disappear. If such interfaces remain to a large extent, the resulting liquid crystal layer shows poor optical properties.
Furthermore, those liquid crystals that are used in the aforementioned conventional methods are not reactive. It is therefore difficult to fix the structure of the liquid crystal layers after their reflection wave ranges are broadened; if the liquid crystal layers are heated again, they undergo structural changes.
SUMMARY OF THE INVENTION
The inventor has made earnest studies in order to overcome the forgoing problems, and, as a result, finally found that it is possible to make the transition from a cholesteric phase in which the helical pitch is intrinsically uniform throughout a liquid crystal layer to such a cholesteric phase in which the helical pitch continuously varies in a liquid crystal layer, by a simple method imparting different degrees of curing to the two surfaces of a liquid crystal layer having a cholesteric order.
The present invention has been accomplished on the basis of the above finding. An object of the present invention is therefore to provide a circular polarization controlling optical element having a broadened reflection wave range without experiencing deterioration of optical properties by interfacial reflection or the like, and to provide a method of producing such an optical element.
Another object of the present invention is to provide a circular polarization controlling optical element showing heat resistance, having optical properties that have been fixed and that will not change even when heated, and to provide a method of producing such an optical element.
A first aspect of the present invention is a circular polarization controlling optical element that includes a cured liquid crystal layer having a cholesteric order in planar alignment, the liquid crystal layer including liquid crystalline molecules and a chiral agent for controlling the helical pitch in the helical structure of the liquid crystalline molecules; wherein the concentration of the chiral agent in the liquid crystal layer linearly varies in the direction of thickness of the liquid crystal layer.
In the first aspect of the present invention, it is preferable that the optical element further includes a substrate for supporting the liquid crystal layer, the substrate having an aligning surface facing the liquid crystal layer, the aligning surface having an aligning power for aligning the liquid crystalline molecules contained in the liquid crystal layer. Further, it is preferable that the liquid crystal layer has a first main surface facing the substrate and a second main surface opposite to the first main surface; and the helical pitch in a portion of the liquid crystal layer, placed on a side of the first main surface, is shorter than that in a portion of the same, placed on a side of the second main surface. On the other hand, it is also preferable that the helical pitch in a portion of the liquid crystal layer, placed on a side of the first main surface, is longer than that in a portion of the same, placed on a side of the second main surface. In addition, it is preferable that the liquid crystal layer further includes a photopolymerization initiator and that the liquid crystalline molecules contained in the liquid crystal layer are at least either one of polymerizable liquid crystalline monomer molecules and polymerizable liquid crystalline oligomer molecules.
A second aspect of the present invention is a method of producing a circular polarization controlling optical element, that includes the steps of: applying a cholesteric liquid crystal solution to a first substrate so as to form an uncured liquid crystal layer, the cholesteric liquid crystal solution having a photopolymerization initiator; and applying ultraviolet light to the uncured liquid crystal layer formed on the first substrate so

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