Optical waveguides – Miscellaneous
Reexamination Certificate
2000-08-07
2002-10-01
Ullah, Akm E. (Department: 2874)
Optical waveguides
Miscellaneous
C349S115000
Reexamination Certificate
active
06459847
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a method of manufacturing a patterned layer of a cholesterically ordered polymer material, in which the axis of the molecular helix of the cholesterically ordered material extends transversely to the layer, and the layer is patterned in that it has at least an area in which the pitch of the molecular helix differs from that of another area.
SUMMARY OF THE INVENTION
The invention also relates to a layer of a cholesterically ordered polymer material.
The invention further relates to an information carrier provided with a patterned layer of a polymer material having a cholesteric order.
The invention further relates to a polarization conversion system provided with a patterned layer of a cholesterically ordered material.
The invention further relates to a cholesteric color filter having a patterned layer of a cholesterically ordered material.
The method of the type described in the opening paragraph is known per se. For example, United Kingdom patent specification GB 2,314,167 describes a patterned layer of a cholesterically ordered material. In accordance with this patent specification, such a layer may be manufactured by first providing a uniform layer of a cholesteric material on a substrate. By polymerizing areas of this layer at different temperatures, a patterned cholesteric layer is obtained. Use is made of the fact that the pitch of the molecular helix of the cholesterically ordered material is temperature-dependent. By polymerizing areas of the layer at a given temperature, the pitch associated with this temperature is, as it were, frozen in these areas.
The known method has drawbacks. For example, in practice it has been found that the known method is difficult to implement. This notably applies to the case where more than two areas having mutually different pitches must be provided in the layer. In that case, a relatively large number of masking steps is necessary and the precision with which the masks are adjusted is very critical. Moreover, the maximum difference in pitch which can be realized between the different areas by means of the known method appears to be relatively small. Patterning at different temperatures also appears to be difficult in practice.
It is an object of the invention to obviate the known drawbacks. More particularly, it is an object of the invention to provide a method in which the layer can be patterned at the same temperature and in which relatively large pitch differences between the different areas can be realized. It is a further object of the invention to provide a polarization conversion system having a patterned layer of a cholesterically ordered material, and a cholesteric color filter having a patterned layer of a cholesterically ordered material, manufactured by means of this method.
These and other objects of the invention are achieved by means of a method of the type described in the opening paragraph, wherein the method comprises the steps of:
a. providing a layer of a cholesterically ordered material comprising a quantity of a convertible compound which in its non-converted and in its converted state determines the pitch of the cholesterically ordered material to a different extent, in which the conversion of said compound may be induced by radiation,
b. irradiating the layer in accordance with a desired pattern so that at least a part of the convertible compound in the irradiated parts of the layer is converted,
c. polymerizing and/or crosslinking the cholesterically ordered material to form a three-dimensional polymer.
It has been found that, using the method according to the invention, that patterned layers of cholesterically ordered, liquid crystalline material can be manufactured in a simple way at the same temperature, with the maximum pitch difference between the areas being relatively large. By (partially) converting the convertible compound in the irradiated areas of the layer, the pitch of the molecular helix in the layer is altered in these areas. The conversion of the convertible compound is effected by irradiation with energy in the form of, for example, electromagnetic radiation, nuclear radiation or an electron beam. Preferably said conversion is effected by means of UV radiation. The pitch of the molecular helix in the irradiated parts differs from the pitch of the molecular helix in the non-irradiated parts of the layer. By polymerizing and/or crosslinking the patterned layer thus obtained, the pitch in the different layer parts is frozen, as it were, and said pitch remains fixed during further process steps, storage and use of the patterned layer. In this way, a patterned layer of cholesterically ordered material can be manufactured in a simple manner.
The degree of conversion of the convertible compound in a certain area is determined by the irradiation dose in said area. Consequently, the pitch of the molecular helix is determined by the local irradiation dose. Said pitch of the molecular helix determines the local optical properties.
It is to be noted that, preferably, the cholesteric layer has a low absorbance for the radiation used in step b, and the radiation intensity along the axis of the helix (i.e. transverse to the layer) is relatively constant in each area. Consequently, the irradiation dose transverse to the layer is relatively constant, and therefore the value of the pitch, viewed along the axis of the helix, is relatively constant in each area. However, this value may differ for the different areas obtained by patterning. Viewed in the plane of the layer, the different areas are adjacent to each other, not subjacent.
When the cholesteric layer has a high absorbance for the radiation used in step b, the radiation intensity will show a gradient transverse to the layer according to Beer-Lambert's law. Consequently, the top of the layer will receive more radiation than the bottom of the layer. This will lead to the formation of a gradient in the pitch, viewed along the axis of the helix (i.e. transverse to the layer). The presence of an absorbing material in a non-absorbing cholesteric layer yields also a gradient in the pitch.
A variation in the pitch, transverse to the cholesteric layer, can be obtained by a method of manufacturing a layer of a cholesterically ordered polymer material, in which the material is oriented in such a way that the axis of the molecular helix of the cholesterically ordered material extends transversely to the layer, wherein the method comprises the following steps:
a. providing a layer comprising a cholesterically ordered material, which material comprises a quantity of a convertible compound which in its non-converted and in its converted state determines the pitch of the cholesterically ordered material to a different extent, the conversion of said compound being inducible by radiation, and the layer substantially absorbs said radiation.
b. irradiating the layer so that at least a part of the convertible compound in the irradiated parts of the layer is converted,
c. polymerizing and/or crosslinking the cholesterically ordered material to form a three-dimensional polymer.
According to the prior art, a gradient in the pitch transverse to the layer could be obtained by a method described in the U.S. Pat. No. 5,793,456 (PHN 14.629), which discloses a method of manufacturing a cholesteric polarizer by providing a mixture of chiral and nematogenic monomers, each having a different reactivity, in the form of a layer. The pitch of the molecular helix is governed to an important degree by the ratio between the chiral and the mesogenic monomer in the polymer material. Owing to the difference in reactivity between both monomers, the capture probability of the most reactive monomer is greater than that of the least reactive monomer. If during the polymerization of the mixture, which is initiated by actinic radiation, a variation in the radiation intensity is realized across the optically active layer to be formed, the most reactive monomer is preferably incorporated in the polymer at the locations of the highest radiation i
Lub Johan
Van De Witte Peter
Koninklijke Philips Electronics , N.V.
Ullah Akm E.
Waxler Aaron
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