Liquid Crystal Photoalignment
Vladimir G. Chigrinov
Hong Kong University of Science and Technology
CWB, Kowloon, Hong Kong,
Email: eechigr@ust.hk
ABSTRACT
Photoalignment possesses obvious advantages in comparison with the usually
“rubbing” treatment of the substrates of liquid crystal display (LCD) cells. The liquid
crystal photoalignment is nano-technology, as the thickness of the alignment layer is
about 2-15 nm. The photoalignment materials can be very useful for the new generation
of the liquid crystals displays. The physical mechanisms and characterization of the
photoaligning technique, as well as the application for liquid crystal devices, including
displays and photonic devices will be reviewed.
INTRODUCTION
Photoalignment has been proposed and studied for a long time. In fact, the subject
of light-molecule interactions has been a fascinating subject of research for a long time
and is still capturing the imagination of many people. Light is responsible for the delivery
of energy as well as phase and polarization information to materials systems. In this
particular case, the alignment of the molecules takes place due to a partial ordering of the
molecular fragments after a topochemical reaction of a photoselection (Weigert`s effect).
While the first photo-patterned optical elements, based on polyvinyl-cinnamate films,
appeared in 1977, the technology became a subject of interest for LCD only in the
beginning of the 90’s. It was soon shown that these materials could provide high quality
alignment of molecules in a LC cell. Over the past twenty years, a lot of improvements
and variations have been made for photoalignment. Commercial photoalignment
materials are now readily available. Many new applications, in addition to the alignment
of LCD, have been proposed and demonstrated. In particular, the application of
photoalignment to active optical elements in optical signal processing and
communications is currently a hot topic in photonics research.
Photoalignment possesses obvious advantages in comparison with the usually
“rubbing” treatment of the substrates of liquid crystal display (LCD) cells. Possible
benefits for using this technique include:
(i)
Elimination of electrostatic charges and impurities as well as mechanical damage
of the surface;
(ii)
A controllable pretilt angle and anchoring energy of the liquid crystal cell, as well
as its high thermo and UV stability and ionic purity;
(iii) Possibility to produce the structures with the required LC director alignment
within the selected areas of the cell, thus allowing pixel dividing to enable new
special LC device configurations for transflective, multi-domain, 3D and other
new display types;
(iv)
Potential increase of manufacturing yield, especially in LCDs with active matrix
addressing, where fine tiny pixels of a high resolution LCD screen are driven by
thin film transistors on a silicone substrate;
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(v)
New advanced applications of LC in fiber communications, optical data
processing, holography and other fields, where the traditional rubbing LC
alignment is not possible due to the sophisticated geometry of LC cell and/or high
spatial resolution of the processing system;
(vi)
Ability for efficient LC alignment on curved and flexible substrates;
(vii) Manufacturing of new optical elements for LC technology, such as patterned
polarizers and phase retarders, tunable optical filters, polarization non-sensitive
optical lenses, with voltage controllable focal distance etc.
The technique of azo-dye photoalignment does not involve any photochemical or
structural transformations of the molecules. As well, the new photoaligning films are
robust and possess rather good aligning properties such as anchoring energies and
voltage holding ratios. They can be very useful for the new generation of the liquid
crystals devices as well as in new photovoltaic, optoelectronic and photonic devices
based on highly ordered thin organic layers. Examples of such applications are light
emitted diodes (OLED), solar cells, optical data storage and holographic memory
devices. The novel and highly ordered layer structures of organic molecules may exhibit
certain physical properties, which are similar to the aligned LC layers. The brief
summary of our research results is given below. The results have been published (see the
Reference List).
Superthin polarizers, new types of phase retarders, patterned polarizers
and retarders
We demonstrated excellent photoalignment capability of the sulfonic azo-dye SD1
for polymerizable liquid crystals (PLC) whose anisotropic films are extensively studied
as basic elements of many passive optical devices (retardation films, polarizers, color
filters, etc.). These dyes exhibit a high affinity to various substrates and insolubility in
polymer liquid crystals (PLC) and organic solvents commonly used for their dilution.
Extremely low exposure dose (less than 50 mJ/cm2) is needed to induce an excellent
planar alignment of PLC. No sign of alignment deterioration is observed after a
photopolymerization action. This altogether implies that new photoalignment materials
may be effectively used for the industrial production of optical films based on PLC. They
eminently suit for the continuous manufacturing process of such films. Parallel and
twisted positive A plates as well as splayed O plates can be produced for the needs of TN
and STN LCD compensation. The SD1 layers can also be effectively used to align dyed
PLC particularly needed for dichroic polarizers. The alignment of neat and dyed PLC can
be easily patterned that is topical problem of patterned compensation films and polarizers
requested by modern developments of LCD (multidomain wide viewing angle LCD,
transflective displays, 3D imaging, etc.). A simple preparation of photoalignment layers
and PLC films suggests to realize manufacturing of optical films in the form of assembly
belt. It can be further simplified for flexible substrates, when continuous roll to roll
process can be easily arranged. It can be basically used to manufacture any functional
film containing aligned PLC layer or a stack of them.
We fabricated a superthin polarizer using LCP mixture UCL017 (DIC, Japan) and
the dichroic azo-dye dye AD1, using rubbed polyimide (PI) layer. The dichroic ratio was
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15 at =490 nm. Patterned polarizers and phase retarders using polymerized LC (UCL011) from DIC (Japan) were also fabricated.
We have demonstrated a photoalignment method for generating a wide range of
liquid crystal pretilt angles (20-900) as well as anchoring energy of the pretilt alignment
(0.2-1.610-3 J/m2). This method has the advantages of simplicity, wide process window
and low exposure dose of deep UV light. It is also time-saving because the baking
process is carried out in-situ with the deep UV alignment process. As well it is a noncontact method in which rubbing is not required. Based on this method, we attained
reactive mesogene (RM) alignment with pretilt angle controllable in, practically, full
range from 0o to 90o. Optically these RM layers act as positive O films, which transform
to positive A film and positive C film at pretilt angles 0o and 90o, respectively. The
structure of RM layer is preserved in a solidification process. This allowed us to obtain
solid O films with controllable tilt angle of optic axis. The prepared O films are confined
between two aligning substrates or coated on one aligning substrate. In the first case, RM
alignment is uniform along the film normal, while in the second case splayed structure is
formed, which depends on the content of RM mixture. In spite of the use of glass
substrates in our experiments, the approach is applicable to plastic substrates. Moreover,
RM film can be extracted to use separately from the alignment substrates. We believe
that the developed method will be useful for optimization of LCD angular characteristics
with O compensation films.
We have demonstrated a method for generating a wide range of liquid crystal pretilt
angles. This method has the advantages of simplicity, a wide process window, and a low
exposure dose of UV light. It is also a nonrubbing method and is based on
photoalignment. The process time is short because the baking process is carried out in
situ with the UV alignment process.
Photopatterned phase retarder was made for transflective LCD using SD1
photoalignment layer. We have optimized a photo-alignment layer to suit for making incell retarders and achieved high thermal stability of the layer comparable to that of
conventional rubbed PI alignment layer. Patterning resolution using an optimized photoalignment was about 5 m. Thermal stability is substantially improved and was
comparable to polyimide (PI). Sensitivity was also improved (≦50mJ/cm2). Adhesion
and resistance to solvent were good. Basic processes were not changed, so roll-to-roll
process of patterned retarders production for transflective LCD is under way.
Vacuum deposition provides optically much more uniform films than spin coating
with a higher dichroic ratio. Thick (500-1000 nm) uniform films can be obtained. High
CR (35-40) can be achieved in thin films (d<200 nm), while high CR in thick films is still
a problem. Superthin internal polarizers based on solid photoaligned dichroic films are
envisaged.
Thus we provided a (i) method of manufacturing a photoalignment layer, which
comprises coating a material for the photoalignment layer, containing dichroic azodye
with one or more sulfo groups promoting good adhesion, on a substrate, and exposing the
coated layer to polarized light, thereby imparting the photoalignment function; (ii)
photoalignment layer of good quality on different kinds of substrates commonly used in
LCD industry; (iii) alignment patterns with desirable shape and size of patterns and
desirable orientation of PLC alignment in the film plane; (iv) a photoalignment layer
insoluble in PLC and solvents conventionally used to form PLC films; (v) a
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photoalignment layer providing high quality alignment of PLC with sufficient thermal
and photo stability to preserve alignment of PLC during polymerization; (vi) a
photoalignment layer with low exposure dose needed to impart alignment function. The
anisotropic optical films based on PLC films aligned by the said photoalignment layers
may be used as optical retarders and compensators, polarization converters, interference
filters, dichroic polarizers and information storage media.
The in-cell retarders were fabricated by in-situ photo polymerization of liquid
crystalline monomers. For this, a photoalignment layer was used to align the monomers.
We have optimized
materials of photo-alignment layers to improve the thermal stability. Liquid crystalline
monomers (UV-curable liquid crystals (UCLs)) were also improved. In addition, the
patterned retarders using the optimized photo-alignment layer were developed. The
application in a high performance transflective LCD is envisaged.
We extended extension of ion/plasma beam technique for the patterned alignment of
reactive mesogens (RM). We showed that this technique can be successfully used for the
alignment of the patterning RM films. A variety of prototypes of patterned retarders and
dichroic polarizers was prepared.
New materials for photoalignment, photoaligned polarizers and phase
retarders. Mechanism of photoalignment
Various new methods were applied to investigate the mechanism of the
photoalignment, like spectral ellipsometry and IR spectroscopy. In spectral ellipsometry
the Lorentz oscillator model was applied to isotropic and photo-induced anisotropic
layers. The resonances were close to absorption ones. Obtained results confirm proposed
model of pure reorientation of azo dye molecules under the action of the linearly
polarized UV light. The wavelength dispersions of refractive indices of the investigated
azo dye mixture as well as the thicknesses of the layers have been accurately extracted
and reported. Obtained optical properties of the azo dye SD1/SDA2 mixture can be used
for simulation of the optimal parameters of the LC devices. The method of IR
spectroscopy helped to explain the photoalignment process from the molecular point of
view through hydrogen bonds associates and photoinduced mesophase of azo-dye
molecules. The novel considerations on the molecular level comment on the importance
of steric factor and linear geometry of the molecule and bring light to the functional role
of the specified radical groups at the molecular ends. On the basic of new considerations
the photoalignment properties of new azo-dye CD1 were predicted and confirmed in the
experiment.
We have discovered the azo-dye CD1, which has very similar structure to SD1 and
exhibits excellent photo alignment quality both for nematic (TN, STN, VAN, IPS) and
ferroelectric LCs. The order parameter S of CD1 is equal to -0.24 at  = 394 nm. The
azimuthal anchoring energy determined by measuring deviation angle in TN cell filled
with LC MLC-12100-000 was about 0.989  10-4 J/m2, which is nearly equal to the
anchoring energy obtained for SD1 (1.6  10-4 J/m2). Power of the UV radiation was 3
mW/cm2 and exposure time - 30 minutes. The cells were possessed a high thermal
stability of LC alignment. We will have a possibility to tune the alignment properties like
(anchoring energy, ability to align LC materials, image sticking, light sensitivity, photo
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stability, reorientation speed etc.) by preparing the proper weight ratio of the azo-dye
compound, if we have the required azo-dye materials, differing in the position and the
orientation of the limited number of groups like: –COOH, –NaSO3 and –CF3 to tune the
intermolecular interactions. Thus a molecular engineering of the photoalignment layers
becomes possible.
We have verified various azo-dye structure and showed the field of the highly
effective sulfuric azo dyes is broader than we conceived. We have investigated different
azo-dyes that covered almost all the wavelengths starting from 389 nm (SD-1) to 600 nm
(Acid Blue). Dyes named BY, CBY and DR and others show excellent photoalignment
properties. The detailed investigations on BY is a strong evidence for good alignment on
various liquid crystals. Taking into account of high photo and thermal alignment stability
one can offer it for various applications. The evidence of these dyes open up the path for
using different dyes for effective photoalignment.
Several new azo-dyes were verified aiming at the application in superthin polarizers
and phase retarders. The dyes were synthesized by several groups in Ukraine and
Belarus. The azo-dyes are capable to cis-trans transformations in dye doped LC
mixtures. A new class of LCD is envisaged based on this principle. We plan also to
continue our cooperation wit the chemists aiming at new substances with -OH and COOH terminal groups for a higher stabilization of the photosensitive film for LCD
optical elements.
We have developed the photo-alignment layer possessing high thermal stability
comparable to that of the conventional polyimide (PI) alignment layer. We have recently
found that the performances of the photo-alignment layer on the plastic substrates are
different from those on glass substrates. Since the application of the alignment layer to a
plastic substrate is important from the practical point of view, the photo-alignment layer
and the manufacturing processes have been investigated and optimized to improve
uniformity, thermal stability and adhesion.
A new effect of the combined action of an electric field and light on the slow
surface dynamics in a layer of nematic liquid crystal (NLC) contacted with a layer of dye
pre-treated by UV irradiation was described. After turning off both the electric field and
light, extremely slow relaxation of the system to the initial state was observed. This effect
depends on a number of control parameters (applied voltage, intensity of light, time of
application, dose of a preliminary UV irradiation). The critical slowing down of this
process (up to some weeks) via a proper choice of control parameters was established.
The physical processes responsible for the combined effect were considered and applied
to modify a previously proposed phenomenological model for the electrically induced
slow azimuthal rotation of the easy axis of an NLC. The modified model was found to be
in a qualitative agreement with the main experimental results. The effect is important as a
proper control of anchoring energy of a photoaligned layer is very important for LC
device.
We studied both theoretically and experimentally kinetics of photoinduced
ordering in azo-dye films. From the absorption-vs-incidence angle curves measured for
the probe light linearly polarized parallel and perpendicular to the plane of incidence we
obtain dependence of the principal extinction coefficients on the irradiation dose. The
transient photoinduced structures were found to be biaxial. The measured irradiation time
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dependence of the absorption order parameters is interpreted by using the generalized
diffusion model.
We studied both theoretically and experimentally switching dynamics in surface
stabilized ferroelectric liquid crystal (SSFLC) cells with asymmetric boundary conditions
created by a photoaligned layer on only one of the substrates. In these cells the bounding
surfaces were treated differently to produce asymmetry in their anchoring properties. It
was found that the asymmetry effects were dominated by the polar contribution to the
anchoring energy. The polar anchoring parameter was estimated by making a comparison
between the results of modeling and the experimental data for various switching regimes.
New types of LC devicess based on photoalignment technology, such as
e-paper, transflective and 3D displays and others
A new transflective LCD with single cell gap consisting (OCB) and low twist nematic
modes has been studied. Such a configuration does not include double cell gap structure
and only one more step UV exposure is needed during photoalignment process to
produce domains, thus, the fabrication process is easy. Based on this new configuration, a
transflective LCD with high brightness and high contrast could be obtained. Moreover,
the TV and RV curves are very close, which means similar gray scale can be obtained
under same operating voltage. Very effective transflective LCDs with the same cell gap
and optical birefringence, but different twist angles made by photoalignment in
transmissive and reflective regions with very high contrast ratio were also proposed.
Tranflective LCDs with the single cell gap, same LCD mode, but patterned retarders
were also investigated.
Twisted nematic (TN) liquid crystal display (LCD) is optimized for transflective
liquid crystal displays. This new configuration does not separate the pixel into two subpixels of transmissive and reflective mode. Each pixel is partially transmissive and
reflective with the help of a semi transparent mirror. The electro-optical characteristics in
the transmissive mode and reflective mode match each other, which indicate that the
transmissive and reflective functions can be controlled using the same voltage in a
transflective LCD. TN LCD configuration exhibits a high contrast ratio, wide viewing
angles, fast response time, and suitable for high quality transflective thin film transistor
(TFT) LCDs.
Transflective liquid crystal display (LCD) with two different modes low twist
nematic (MTN) and electricity controlled birefringence (ECB) has been proposed. The
structure contains only a single cell gap in transmissive and reflective part, therefore the
fabrication process is easy and only one more step UV exposure is needed during
photoalignment process to produce domains. This configuration has a good contrast,
viewing angle, response time and transmittance / reflectance. As a result it is suitable for
high quality transflective thin-film transistor (TFT) LCDs.
A ferroelectric liquid-crystal (FLC) display was optimized as a transflective
liquid-crystal display (LCD). In this configuration, the single-cell-gap approach was
considered. The optimized configuration exhibits a high contrast ratio, wide viewing
angles, and achromatic (black/white) switching in both the transmissive and reflective
modes. Because no double-cell-gap structure, no subpixel separation, and no patterning
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polarizers and retarders are included in the configuration, the configuration is easy to
fabricate and also possess a perfect dark state. This configuration is also suitable for
bistable applications.
We developed the Optical Rewritable liquid crystal alignment technology (new
e-paper) that provides controllable patterned, reversible planar or vertical, liquid crystal
alignment. The image is truly stable, can be written to grey level with saturation and
rewritten a large number of times with high reproducibility of properties. We come out
with low power consuming high efficiency ORW-device that consists of three major parts
developed by us: optically rewritable azo-dye photoalignment and ORW LCD with
polarizer-substrates, LED-exposure light source and phase-mask LCD polarization
rotator. ORW technology is completely compatible with standard photolithography and
enables a patterned LC alignment within one-mask process. The new e-paper is light
weight thin, paper-like image carrier with good brightness, high contrast and full viewing
angle and is capable to display either 2D or both 2D&3D images. The e-paper has
continuous grey levels, requires zero power to show the image and is free from conductor
bonding and cracking problems. The unique property of being cell-gap non-sensitive
allows to maintain proper performance even when the device is bent, which is necessary
for flexible and robust image display. Utilizing the developed light printing device, we
successfully tested in experiment the principle of creation of two independent images,
which are separated by polarization using the optical equivalent structure of the double
size printable ORW electronic paper. We developed a light printer based on polarization
rotation to change ORW image. The designs based on twist nematic (TN) or ferroelectric
liquid crystals (FLC) operational element were suggested.
The application of fluorescent dyes was shown to provide significant
improvement for the ORW e-paper both dark and bright state without any influence on
image writing and rewriting capabilities. Moreover, it was possible to optimize the
reflection spectra of the bright state by changing the concentration percent of the
fluorescent dye dopant. At the same time, dye doping was shown to be a cheap and
simple way of obtaining coloured images. Still implementation of ORW e-paper has large
paper replacement potential in labels, electronic newspapers and even regular office
printing, as ORW e-paper saves both paper and ink. The possible but not limiting
applications of e-paper concept are light printable rewritable paper, labels and plastic
card displays, as well as rewritable 3D paper for security applications and overhead
projection including building design, various 3D outline sketches and maps.
The optical parameters of the photoaligned bistable FLCD with monochromatic
switching have been optimized using the optical retardation of the liquid crystal layer, the
cone angle of molecules, orientations of the polarizers and the retardation films, if any.
Several types of low power consumption nonexpensive bistable photoaligned
monochromatic FLC with memorized colors were demonstrated. Similar birefringent
color deformed helix ferroelectric (DHF) FLC were obtained, if high quality
photoaligned azo-dye films were used. The change of the color was made by the variation
of the driving voltage amplitude from 0 to 5 V at the frequency 5 ÷ 500 Hz for 5µm
photoaligned DHF-FLC display cells with a helix pitch less than 1 µm. This principle of
efficient color generation looks promising for a field sequential FLCD without color
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filters. Passively addressed chromatic bistable displays become attractive in many
applications, like e-book, PDA and mobile phones with a low power consumption.
Multistable electro-optical modes were shown to exist under certain conditions in
photoaligned Ferroelectric liquid crystal (FLC) cells, which means that any lighttransmission level can be memorized after the driving voltage is switched off. The
multistability is responsible for three new electro-optical modes with different shapes of
the gray-scale curve that can be either S-shaped (double or single dependent upon the
applied-voltage pulse sequence and boundary conditions) or V-shaped dependent upon
boundary conditions and FLC cell parameters. The origin of these modes was described.
The application in FLC full color E-paper is envisaged.
We studied and experimentally investigated liquid crystal (LC) alignment on
silicon surfaces with submicrometer-sized straight and curved waveguide profiles. Our
analysis shows that surface profiles can strongly affect LC alignment near the waveguide
edges, and thus the resulting LC-clad waveguide-based devices. LC alignment on this
topological device surface is primarily affected by the alignment material. At zero
applied electrical potential, the liquid crystal film is oriented by the bottom
photoalignment layer and the top rubbed planar or homeotropic polyimide layer. Under
an applied electric field, the liquid crystal molecules are re-oriented with a tilt angle that
depends on the applied field. The liquid crystal cladding refractive index is then varied
according to the applied voltage, and subsequently the microresonator resonance
wavelengths are tuned.
We have considered theoretically the possibility to create liquid crystal lenses
with a voltage controllable focal distance. The LC deflectors and LC prisms are also
envisaged. Non-uniform spatial distribution of anchoring energy, made by
photoalignment technology is a new principle proposed by us.
A complete methodology using matrix representations for describing light
transmission and reflection at an interface between an isotropic medium with high
refractive index and a uniaxial birefringent material where total internal reflection (TIR)
could happen was described systematically. A new TIR-based liquid-crystal (LC) switch
system was proposed and investigated in detail by using this analyzing method. The
criteria of selection of critical parameters such as LC mixture, waveguide, and operation
mode of the LC layer, etc., was developed. Dependence of transmission on incident angle
and dynamic characteristics under an electric field was given for different cell gaps. The
results give detailed and useful guidance in the fabrication of the LC switch system.
We have also proposed and checked experimentally the simple method to control
light beams in a plane waveguide using a sharp boundary between the regions of a
different LC orientations, regulated by electric field. Using different ITO templates, it was
possible to create LC switch and other different optical processing data elements, e.g.
attenuators.
We calculated the smooth collimating refractive interface that can be written by
light in front of the waveguide immerged into liquid crystal. Such passive LC structure is
stabilized by the photoalignment layers and does not need applied voltage to operate. It
can out-couple the s-polarized light, coming from the waveguide, into a collimated beam
inside LC bulk for further processing, while the p-polarized light can be guided by
matching polarization maintaining LC waveguide. We suggest the polarization
independent design of the LC Photonic device that can convert both polarization
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components out coupled from polarization independent waveguide to one polarization for
further process of light by the polarization dependent LC structure for routing or other
purposes. The novel design consists of polarization maintaining LC waveguides, LC
polarization dependent passive lens and active half wave plate (HWP).
The brief List of our journal publications on photoalignment is provided below.
The support of HKUST grants CERG 612409 and CERG 612208 is gratefully
acknowledged.
Reference
[1] V.G. Chigrinov, V.M. Kozenkov, H.S. Kwok, Photoalignment of Liquid Crystalline
Materials: Physics and Applications, 248 pp., Wiley, August 2008.
[2] W.C. Yip, H.S. Kwok, V.M. Kozenkov, and V.G. Chigrinov, Photo-patterned e-wave
polarizer, Displays, vol.22, pp.27-32 (2001).
[3] V. Chigrinov, E. Prudnikova, V. Kozenkov, H. Kwok, H. Akiyama, T. Kawara, H.
Takada, H. Takatsu, Synthesis and properties of azo dye aligning layers for liquid crystal
cells, Liquid Crystals, Vol. 29, no.10, pp. 1321-1327 (2002).
[4] V.M. Kozenkov, V.G. Chigrinov and H.S. Kwok, Photoanisotropic Effects in
Poly(vinyl-cinnamate) Derivatives and Their Applications, Mol. Cryst. Liq. Cryst., Vol.
409, pp. 251-267 (2004).
[5] Eugene Pozhidaev, Vladimir Chigrinov, Danding Huang, Andrei Zhukov, Jacob Ho,
Hoi Sing Kwok, Photo-alignment of ferroelectric liquid crystals by azodye layers, Jpn. J.
Appl. Phys., 43, No.8A, pp. 5440-5446 (2004).
[6] Vladimir Chigrinov, Anatoli Muravski, Hoi Sing Kwok, Hirokazu Takada, Hidenari
Akiyama and Haruyoshi Takatsu, Anchoring properties of photo-aligned azo-dye
materials, Physical Review E, 68, pp.061702-1-061702-5 (2003).
[7]V.A.Konovalov, V.G.Chigrinov, H.S.Kwok, H.Takada and H. Takatsu, Photoaligned
Vertical Aligned Nematic Mode in Liquid Crystals, Jpn. J. Appl. Phys., 43, No.1, pp.
261-266 (2004).
[8]D.D. Huang, E.P. Pozhidaev, V.G. Chigrinov, H.L. Cheung, Y.L. Ho, H.S. Kwok,
Photo-aligned ferroelectric liquid crystal displays based on azo-dye layers, Displays, Vol.
25, issue 1, pp.21-29 (2004).
[9] Vladimir Chigrinov, Sergey Pikin, Andrey Verevochnikov, Vladimir Kozenkov,
Maxim Khazimullin, Jacob Ho, Dan Ding Huang, and Hoi-Sing Kwok, Diffusion model
of photoaligning in azo-dye layers, Physical Review E, 69, pp.061713-1-061713-10
(2004).
[10] Al. Muravsky, An. Murauski, X. Li, V. Chigrinov, H. S. Kwok, Optical rewritable
liquid-crystal-alignment technology, J. SID, 15/4, pp. 267-273 (2007).
[11] Oleg Yaroshchuk, Jacob Ho, Vladimir Chigrinov, and Hoi-Sing Kwok, Azodyes as
Photoalignment Materials for Polymerizable Liquid Crystals, Jpn. J. Appl. Phys., 46,
No.5A, pp. 2995-2998 (2007).
[12] J. Ho, V. Chigrinov, H. S. Kwok, Variable liquid crystal pretilt angles generated by
photoalignment of a mixed polyimide alignment layer, Appl. Phys. Lett., 90, 2435061243506-3 (2007).
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[13] Iryna Valyukh, Hans Arwin, Vladimir Chigrinov, and Sergiy Valyukh, UV-induced
in-plane anisotropy in layers of mixture of the azo-dyes SD-1/SDA-2 characterized by
spectroscopic ellipsometry, Physica Status Solidi (c), 5,(No.5, pp.1274-1277 (2008).
[14] Alexander Muravsky, Anatoli Murauski, Vladimir Chigrinov, and Hoi-Sing Kwok,
Light Printing of Grayscale Pixel Images on Optical Rewritable Electronic Paper, Jpn. J.
Appl. Phys., 47, No.8, pp. 6347-6353 (2008).
[15] Vladimir G. Chigrinov, Hoi-Sing Kwok, Hiroshi Hasebe, Haruyoshi Takatsu,
Hirokazu Takada, Liquid-crystal photoaligning by azo dyes, J. SID, 16/9, pp. 897-904
(2008).
[16] Oleg Yaroshchuk, Leonid Dolgov, Jacob Ho, Hoi-Sing Kwok, Vladimir G.
Chigrinov, Haruyoshi Takatsu, Hiroshi Hasebe, O films with adjustable tilt of optical axis
for LCD compensation, J. SID, 16/9, pp. 933-938 (2008).
[17] Alexander Muravsky, Anatoli Murauski, Vladimir Chigrinov, and Hoi-Sing Kwok,
New properties and applications of rewritable azo-dye photoalignment, J. SID, 16/9, pp.
927-931 (2008).
[18] Hin Yu Mak, Xihua Li, Peizhi Xu, Tao Du, and Vladimir G. Chigrinov,
Transflective ferroelectric liquid-crystal display with a single-cell gap, J. SID, 16/9, pp.
953-956 (2008).
[19] Alexander Muravsky, Anatoli Murauski, Vladimir Chigrinov, Hoi-Sing Kwok,
Optical Rewritable Electronic Paper, IEICE Trans. Electron., vol.E91–C, N.10, pp. 15761580 (2008).
[20] Xiaojin Zhao, Farid Boussaid, Amine Bermak, and Vladimir G. Chigrinov, Thin
Photo-Patterned Micropolarizer Array for CMOS Image Sensors, IEEE Phot. Techn.
Lett., 21, No. 12, pp.805-807 (2009).
[21] O. Yaroshchuk, V. Kyrychenko, Du Tao, V. Chigrinov, H. S. Kwok, H. Hasebe, and
H. Takatsu, Stabilization of liquid crystal photoaligning layers by reactive mesogens,
Appl. Phys. Lett., 95, 021902-1-021902-3 (2009).
[22] Alexei D. Kiselev, Vladimir G. Chigrinov, and Hoi-Sing Kwok, Kinetics of
photoinduced ordering in azo-dye films: Two-state and diffusion models, Phys. Rev. E,
80, 011706 (1-16), 2009.
[23] Gurumurthy Hegde, V. M. Kozenkov, V. G. Chigrinov, and H. S. Kwok,
Photoisomerization in Photoaligned Azo Dyes Exhibiting Photostability, Mol. Cryst. Liq.
Cryst., Vol. 507, pp. 41-50 (2009).
[24] Xiaojin Zhao, Amine Bermak, Farid Boussaid, Tao Du, and Vladimir G. Chigrinov,
High-resolution photoaligned liquid-crystal micropolarizer array for polarization imaging
in visible spectrum, Opt. Letts., Vol. 34, N.23, pp. 3619-3621 (2009).
[25] Sarik R. Nersisyan, Nelson V. Tabiryan, Diane M. Steeves, Brian R. Kimball,
Vladimir G. Chigrinov, and Hoi Sing Kwok, Study of azo dye surface command
photoalignment material for photonics applications, Appl. Opt. , Vol. 49, No. 10, pp.
1720-1727 (2010).
[26] Tao Du, Lishuang Yao, Vladimir G. Chigrinov, Hoi-sing Kwok, Single-cell-gap
transflective liquid- crystal display and the use of photoalignment technology, J. SID,
18/5, pp. 391-396 (2010).
2-33