Advances in Holographic Polymer Dispersed Liquid Crystal Technology
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Mat. Res. Soc. Symp. Proc. Vol. 559 01999 Materials Research Society
Ambient "OFF" State
Transmitted
0P
'
--
Reflection Band-
="'''
Ambient Ligtl
"ON" State
All Light
-
Transmitted-
Figure 1. Schematic showing operation of H-PDLC. The top figure shows the field-off, Bragg-reflectingstate. The bottom figure shows thefield-on, transparentstate. Sutherland et al. 9 reported switchable transmission gratings formed using these techniques in 1993, and the group at Science Applications International Corporation has been active in this field ever since. Tanaka et al., the first to recognize the potential of these materials in reflective displays, reported reflection gratings formed by orienting the droplet planes parallel to the glass substrates in 1994.10 The interference of successive reflections from the multilayer structure results in strongly peaked reflection at the Bragg wavelength, A = 2nd, where d is the modulation wavelength. All other wavelengths are transmitted. Application of an electric field aligns the droplet directors and erases the index modulation, resulting in a transparent material at all wavelengths. The schematic in Figure 1 shows the operation of such a device. H-PDLC FORMATION The pre-polymer mixture used in H-PDLC formation can be prepared from commercially available constituents. The monomer used is typically a multi-functional acrylate, such as SR399* from Sartomer Inc. E7 or BL038, both available from EM Industries, are suitable liquid crystals. BL038 has a large birefringence (An - 0.28). A photoinitiator, sensitive to the laser wavelength used in curing, is necessary to sensitize the monomer to the light. We use a modestabilized Ar+ laser at 514mn. Rose Bengal is a suitable photoinitiator for this wavelength. NPhenyl Glycine is used as a co-initiator, as is the chain-terminator N-Vinyl Pyrrolidinone. All these materials are commercially available from Sigma-Aldrich Inc. The pre-polymer needs to be
98
1931 CIE Chromaticity Diagram
y --
-,
; 0..
I
-Q
00
o,0•
.. i..
.
...
OD al01 02 0 .4 0U 0
X
..
i..
01 0.0
Figure 2. 1931 CIE Chromaticity Diagram showing x and y coordinatesfor red, green, and blue H-PDLCs. The dashed lines indicate the color gamut of such a stack prepared under darkroom conditions, as exposure to ambient room light may result in unwanted polymerization. Our cells were prepared by drop-filling the pre-polymer between 2 l"x 1" ITO-coated glass substrates. 5pm fiber spacers were used to control the cell gap. The materials were exposed immediately after filling. The exposed area was -1" diameter. Exposure was typically for 20-30 seconds, with ~50mW in each beam. The spacing of the Bragg planes in H-PDLCs are governed by the grating equation:
A=
A
2njsin 01
(M).
Here, A is the laser wavelength (514nm in our case), n is the average index of refraction of the holographic medium, and 0 is half the angle between the interfering beams. We note the geometric dependence of the grating spacing. Theoretically, this means that for the same materials a
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