Optical: systems and elements – Holographic system or element – Having particular recording medium
Reexamination Certificate
1999-09-16
2003-03-25
Chang, Audrey (Department: 2872)
Optical: systems and elements
Holographic system or element
Having particular recording medium
C359S015000, C359S022000, C359S024000, C359S618000, C359S634000, C349S074000, C349S086000, C349S092000, C349S115000, C349S201000, C353S030000, C353S031000
Reexamination Certificate
active
06538775
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to holographically-formed polymer dispersed liquid crystals (H-PDLCs). In particular, the invention relates to multiple grating reflective displays using H-PDLC technology. The invention also relates to H-PDLCs having multiple reflection and transmission gratings.
Polymer dispersed liquid crystals (PDLCs) in their conventional form consist of micrometer-sized liquid crystal (LC) droplets dispersed in a rigid polymer matrix. PDLCs are typically formed using phase-separation or emulsification methods. Photo-polymerization induced phase separation utilizes a mixture of a low molecular weight liquid crystal and a photo-curable monomer. Irradiation of the homogeneous pre-polymer mixture initiates polymerization, which in turn induces a phase separation between the polymer and liquid crystal. The result is a liquid crystal phase separated into droplets and immobilized in a rigid polymer matrix.
FIG. 1A
illustrates a conventional PDLC formed by phase separation of a liquid crystal phase from a matrix polymer phase. The entire LC-monomer film is photopolymerized and phase separation occurs randomly throughout the film and results in LC droplets on the order of microns. In the zero-voltage state, the symmetry axis of the droplets is randomly oriented and there is a mismatch of the index of refraction between the matrix polymer and the LC droplets. This condition results in a strongly light scattering (opaque) appearance. By matching the ordinary refractive index of the liquid crystal with that of the matrix polymer, a transparent appearance is achieved when sufficient voltage is applied to reorient the LC droplets. Thus, conventional PDLC displays are capable of switching between an opaque off-state and a transparent on-state, but do not have inherent ability to display color.
Reflective liquid crystal displays have been developed which rely on PDLC materials and, more recently, holographic or optical interference preparative techniques have been used to carry out polymerization to selectively positioned regions of liquid crystal and polymer. Planes of liquid crystal droplets are formed within the sample to modulate the LC droplet density on the order of the wavelength of light. On exposure to an optical interference pattern, typically formed by two coherent lasers, polymerization is initiated in the light fringes. A monomer diffusion gradient is established as the monomer is depleted in the dark fringes, causing migration of liquid crystal to the dark fringes. The result is LC-rich areas where the dark fringes were located and essentially pure polymer regions where the light fringes were located.
The resulting optical interference pattern reflects at the Bragg wavelength, &lgr;=2nd sin &thgr;, where n is the average index of refraction, &thgr; is the angle between the substrate and viewing direction, and d is the Bragg layer spacing. The interference pattern can be selected to form Bragg gratings which can reflect any visible light. In the “off state”, that is, with no applied voltage, the LC directors are misaligned and light of the Bragg wavelength is reflected back to the observer. Upon application of an applied voltage, the “on state”, the device becomes transparent. The reflection intensity is electrically controlled by changing the effective refractive index of the LC droplet planes with an applied voltage. If the refractive index of the LC droplet planes (n
LC
) is different from that of the polymer planes (n
p
), light of a specific wavelength is reflected by the periodic modulation in the refractive index. If n
LC
is equal to n
p
, the periodic refractive index modulation disappears and the incident light is transmitted. If the LC has a positive dielectric anisotropy and the ordinary refractive index n
o
is approximately equal to n
p
, the reflection intensity decreases with increasing applied voltage. This results in a material transparent at all wavelengths and all incident light is transmitted.
Displays incorporating these materials have been reported in “Holographically formed liquid crystal/polymer device for reflective color displays” by Tanaka et al. in
Journal of the Society for Informational Display
(
SID
), Volume 2, No. 1, 1994, pages 37-40; and also in “Optimization of Holographic PDLC of Reflective Color Display Applications” in SID '95
Digest,
pages 267-270 (1995). In each of the reported H-PDLC displays, however, reflection gratings capable of reflecting only a single wavelength of light were created. See, Tanaka et al. in U.S. Pat. No. 5,748,272.
A major interest in the display industry is the creation of full color reflective displays. U.S. Pat. No. 5,875,012 to Crawford et al. describes a full-color liquid crystal device incorporating three single-color stacked reflective H-PDLCs, which can be activated alone or in combination to provide a broad spectrum of color. Although this configuration results in high reflection efficiencies, it is complicated to fabricate and requires sophisticated electrical drive schemes.
Date et al. in “Three-Primary-Color Holographic Polymer Dispersed Liquid Crystal (H-PDLC) Devices for Reflective Displays” (Proceedings of the 15th International Display Research Conference, Hamamatsu, Japan, 1995; p. 603) report single exposure films of different color. A red, a green and a blue reflecting H-PDLC are reported formed using a single laser source, in which the different reflection gratings were obtained at different incident angles from different H-PDLC layers. Date also reported the use of prisms to obtain the appropriate cross angles for longer wavelengths of light. Using this technique, a full color reflective display can only be built by stacking three H-PDLC layers that individually reflect at red, green or blue wavelengths. There were no multiple grating films made from a single layer H-PDLC to reflect multiple colors.
There is a need to provide a single layer H-PDLC with multiple reflective gratings for constructing a reflective display device that can have a range of colors. Such displays are desirable due to their simplified configuration and because they are sufficiently reflective at low power and in normal operating environments.
Lastly, multiple Bragg gratings in display panels and other devices are desired because specular reflections off of multiple gratings within the layer would increase the operative viewing angle and improve the quality of the reflected image. There is currently no method which provides such capability in the prior art.
Creating a near infrared reflecting H-PDLC is a difficult task to accomplish due to the large wavelength shift required to create Bragg gratings in the near infrared band (~1000 nm) using light in the visible range. The use of visible light lasers to fabricate IR H-PDLCs is attractive for a variety of reasons. The beam is visible with the unaided eye which simplifies alignment and fabrication; and IR photoinitiators, needed for the polymer-initiated phase separation of the H-PDLC, are not readily available or are not developed to a point sufficient for use in this application.
Unfortunately, at the incident angles required to form the infrared interference pattern, the glass surface is highly reflective and very little of the light passes through the supporting glass into the LC-monomer layer. Furthermore, some of the light that does enter the layer is in the form of multiple reflections which wash out the interference pattern.
There is a need to provide an infrared reflective modulating device and a method for obtaining an infrared reflective device that addresses the problems and limitations of the prior art.
SUMMARY OF THE INVENTION
The present invention provides advances and improvements in the manufacture of H-PDLC compositions. The use of simultaneous, coherent multiple laser beam exposure has been exploited to provide multiple grating liquid crystal devices from a single layer H-PDLC.
In one aspect of the invention, a multicolored reflection liquid crystal display device is provided from a single layer con
Bowley Christopher C.
Crawford Gregory P.
Faris Sadeg M.
Fontecchio Adam K.
Li Le
Chang Audrey
Curtis Craig
Hale & Dorr LLP
Reveo Inc.
Scozzafava Mary Rose
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