Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
1999-11-08
2003-04-15
Sikes, William L. (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
Reexamination Certificate
active
06549253
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to an optical device comprising two substantially parallel substrates with electrode layers on their facing sides for switching an optically active layer which is present between said substrates. Furthermore the present invention relates to a hybrid layer for use in such an optical device.
Optical devices as mentioned in the opening paragraph are known as such, for instance, in the form of optical shutters or liquid crystal displays.
A severe drawback of the known optical devices is their relatively high switching voltage.
Recently introduced display devices based on cholesteric liquid crystals (CLCs) are still considered limited in capabilities. The major roadblock for realization of the benefits of chiral liquid crystal optical devices and displays has been difficulty in creating sufficient long range alignment of the helical structure in the preferred orientation. Alignment attempts to date have used surface alignment layers on either side of the cell containing the liquid crystal to produce strong alignment at the surfaces. Unfortunately, it has been difficult to control alignment within the bulk of the cell where the liquid crystal prefers to form domain structures separated by unaligned dislocation regions. To achieve good contrast, cell thickness must be increased yet as it is, alignment, and optical properties, degrade.
The present invention aims among others at providing optical devices having a relatively low switching voltage.
In addition, the present invention aims at providing optical devices having sufficient long range alignment of the helical structure and structural control in three dimensions.
This object is achieved in optical device comprising two substantially parallel substrates with electrode layers on their facing sides for switching an optically active layer which is present between said substrates, these optical devices in accordance with the present invention being characterized in that the optically active layer is formed as a hybrid layer containing an optically anisotropic material as well as a porous columnar structure. Wherever in this Application the wording “columnar structure” is mentioned, different kinds of structures are meant, which can be obtained by the method to be described. Such shapes are, for example, helical structures, pillars, slanted pillars, zig-zag, chiral or sinusoidal structures. Especially when using liquid crystal material for the anisotropic material, the presence of said structure results in a lowering of the switching voltage of the optical device.
Here we demonstrate a technique where a liquid crystal material is embedded in an inorganic porous backbone structure to produce strong alignment and structural control in three dimensions. Cell thickness is limited only by difficulties in fabrication of thick films, with 50 &mgr;m thickness easily obtainable. In addition, liquid crystal alignment structures can be designed to engineer desired optical responses. For example, the narrow bandwidth of transmission/reflection typical of a CLC cell can be increased by producing a graded pitch structure. With the present method, pitch gradients or other structures can be accomplished with simple software modification to the deposition control system. The pitch can even be reversed within the layer, leading to substantially 100% reflection. Finally, because of the versatility of the present method, all polarization components needed for cell fabrication could be conducted with a small number of deposition steps, all based on the present method, to produce a complete device. While numerous particulars still remain to be investigated, the present method appears to be a promising technique for creating liquid crystal devices for display applications, but also for other components, both active and passive.
OBJECTS AND SUMMARY OF THE INVENTION
The invention also relates to a hybrid layer for use in an optical device, said layer containing an optically anisotropic material as well as a porous columnar structure. Said structure results in a lowering of the switching voltage of the optical device. Preferably the columnar structures comprise helical structures, in which the helixes may have, if wanted, square sides.
Chiral optical devices are used primarily for filtering of circularly polarized light, for example in liquid crystal (LC) displays. Various optical switching techniques based on chiral liquid crystals (CLCs) have been envisaged, with optical properties superior to linear polarization based devices, such as the twisted nematic cells used in the majority of commercial liquid crystal displays. In truth, twisted nematic cells are one type of chiral optical device where the chiral “twist” length is considerably longer than the wavelength of visible light. The chiral optical devices discussed here have twists or pitches comparable to the wavelengths of visible light and operate within the “resonance” regime, corresponding to the “zone of selective reflection” in the CLC literature. Switching with a chiral optical device is based on the phenomenon of circular Bragg reflection, where one of the left- or right-circularly polarized light components is selectively reflected by the helical structure of the chiral material. Circular Bragg reflection arises from constructive scattering of circular polarized light from helical structures, and is fundamentally very similar to constructive interference reflections of plane polarized light from high/low index multilayers, Circular Bragg reflection allows light to be polarized for switching (in display and other photonic applications) without the use of absorbing polarizers, such as those used in linear polarization devices, which reduce power efficiency by absorbing half of the light available for transmission through the device.
The most commonly used chiral optical materials, chiral nematic liquid crystals (CLCs), are composed of nematic (rodlike) molecules with a small asymmetry in shape, or a mix of nematic molecules with an asymmetric additive. The structure of a layer of these molecules can be described as being composed of many sheets with all the rodlike molecules aligned within a sheet, but with a small rotation in orientation from one sheet to the next. The orientation rotates in a helical fashion through the cell, with one full molecular rotation called the pitch, p. Note: the first liquid crystal which displayed this orientation were closely associated with cholesterol and this phase was originally named the “cholesteric” phase. The more accurate name for this phase is “chiral nematic”, however, and is the name that will be used in this description. The polarization selection property of CLCs, circular Brag reflection, occurs between &Sgr;
1
=pn
o
and &Sgr;
2
=pn
e
where n
o
and n
e
are the ordinary and extraordinary refractive indices of the locally uniaxial structure. Within this reflection band, right-handed light is reflected from a right-handed helix, and left-handed light is transmitted. Alternatively, left-handed light is reflected from a left-handed helix, and right-handed light is transmitted. Wavelengths outside of the reflection band are transmitted in all polarizations.
The present invention aims at providing a film forming system method that allows for the growth of complex microstructures with predetermined patterns of growth. In addition porosity and optical properties of the shadow sculpted thin film are enhanced by expanding the range of incidence angles of the vapor flux.
Further, the inventors have found to their surprise that rotation of the substrate while it is exposed to an oblique incident vapor flux at polar angles greater than about 80.degree. produces well defined microstructures.
Therefore, in accordance with one aspect of the invention, there is provided a method of sculpting vapor deposited thin films, the method comprising the steps of:
initially exposing a surface of a substrate to a vapor flux at an oblique incident angle to grow a columnar thin film; and
subsequently, and while conti
Brett Michael J.
Broer Dirk J.
Robbie Kevin J.
Sit Jeremy C.-H.
Koninklijke Philips Electronics , N.V.
Nguyen Dung
Sikes William L.
Waxler Aaron
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