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Journal of the Korean Physical Society, Vol. 51, No. 4, October 2007, pp. 1492∼1496
Fabrication of a Reverse Twisted Nematic Liquid-Crystal
Grating via Holographic Writing
Hyunhee Choi and J. W. Wu∗
Department of Physics, Ewha Womans University, Seoul 120-750
Byoungchoo Park
Department of Electrophysics, Kwangwoon University, Seoul 139-701
(Received 7 September 2006)
By use of a photo-isomerization process occurring inside an azo-polymer alignment layer, a
polarization modulation is recorded on an azo-benzene-containing liquid crystal cell in a reflection
holographic configuration. A polarization analysis measurement of diffracted orders shows that the
resulting structure is a reverse twisted liquid crystal grating. The electro-optic tunability of the
diffraction efficiency is also demonstrated.
PACS numbers: 42.25.Hz, 42.25.Ja, 42.40.Eq, 42.40.Ht
Keywords: Reflection hologram, Photo-isomerization, Liquid crystal, Azo-molecule
I. INTRODUCTION multi-domain structures can be fabricated by controlling
the exposure of writing beams [12,13].
In this work, we adopted a slanted reflection configura-
The liquid-crystal (LC) devices have received a signif- tion to write a reflection hologram. In this configuration,
icant amount of attention for their wide number of op- an optical attenuation takes place through absorption
tical applications in the display industry [1]. When LC and scattering inside the LC cell. When two orthogonally
devices are fabricated, an initial LC alignment should linearly polarized beams are counter-propagating inside
be carried out to induce a collective orientational order- the LC cell, the polarization interference pattern formed
ing, and the alignment is usually achieved by mechanical inside the cell gives rise to micro-sized polarization do-
contact treatments of the LC alignment layer, such as a mains, inducing twisted nematic LC domains. The re-
rubbing technique [2, 3] that produces fine grooves and sulting structure was optically analyzed and was found
electro-statical changes on the LC alignment layers. to be a reverse twisted nematic liquid crystal(RTNLC)
Recently, there have been many reports on the fabri- grating. The polarization dependence of the diffraction
cation of multi-domain LC gratings with display appli- efficiency was measured, and the electro-optic tunability
cations [2–6]. Among the methods to fabricate multi- of the diffraction efficiency of the RTNLC grating was
domain LC gratings, the holographic recording method demonstrated.
has an advantage over other methods, because the po-
larization and intensity modulation is recorded by two
interfering writing beams on a photo-sensitive LC align-
ment layer holographically [7–10]. In most works em- II. EXPERIMENTS
ploying holographic methods, a transmission holographic
configuration in which two writing beams are incident The azo-polymer is commercially available from
on a recording material such as azo-polymer along the Sigma-Aldrich and is an azo-benzene side-chain polymer,
same direction is adopted [7–11]. The polarization and poly [(methylmethacrylate)-co-(disperse red 1 methacry-
intensity modulations induce an orthogonal orientation late)] (PDR1-MMA). PDR1-MMA was dissolved in
of azo-moieties inside the azo-polymer via a photo- tetrahydrofurane by 3 – 4 wt%. A photo-alignment layer
isomerization process. A subsequent LC alignment is consisting of a 300 nm-thick polymer film was readily ob-
achieved by a macroscopic motion of LCs following the tained by spin coating onto an indium-tin-oxide (ITO)
orientation of the azo-moieties in the azo-polymer. The substrate and subsequent drying for one hour at 100
◦
C. The UV-visible spectrum of the PDR1-MMA film
∗ E-mail: jwwu@ewha.ac.kr; Fax: +82-2-3277-2372 showed an absorption peak at 490 nm, and the opti-
-1492-
Fabrication of a Reverse Twisted Nematic Liquid-Crystal· · · – Hyunhee Choi et al. -1493-
Fig. 1. Slanted reflection holographic configuration to in-
scribe the S- and P-polarization modulation in a nematic LC
cell with a magnified view of the LC cell with the azo photo-
alignment layer (PL1 and PL2: azo photo-alignment layers 1
and 2, θs : the slant grating angle).
cal density at 514.5 nm, the wavelength of the writing Fig. 2. (a) Schematic illustration of the formation of polar-
ization modulation in an interfering field and (b) the polariza-
beams, was about 0.8. An empty cell, separated by
tion modulation from the interference of S- and P- polarized
spacers, was fabricated by gluing two PDR1-MMA spin- writing beams for three different intensity ratios.
coated ITO substrates with a UV-curable epoxy. Fil-
tered mixtures of a nematic LC (ZLI-2293, ∆n = 0.1322,
Merck) and 0.3 wt% DR1 were introduced into the empty polarization state and the eletro-optic tunability of the
cell by a capillary action at a temperature of 55 ◦ C. The diffracted orders were examined at the He-Ne laser wave-
resulting LC cell at room temperature did not exhibit length of 632.8 nm. All experimental measurements were
any optical birefringence. We adopted the reflection performed at room temperature.
holographic configuration shown in Figure 1. Initially,
a photo-alignment of the filled LC cell was carried out
by using normal-incident illumination with the 514.5 nm
line of an Ar+ laser with its linear polarization along III. RESULTS AND DISCUSSION
the y-axis, called S-polarization, at an intensity of 100
mW/cm2 on the film with a typical illumination time The grating structure was examined by observing the
of 30 min. The photo-isomerization process occurring in textures of the LC cells under a polarizing optical micro-
the azo-benzene moiety inside the two photo-alignment scope. The light path was from the polarizer to P L1
layers, P L1 and P L2, resulted in an optical birefringence to P L2 to the analyzer. The upper left, the upper
in the nematic LC cell with the optical axis along the x- right, the lower left, and the lower right ones correspond
axis, as shown in Figure 1. Next, in order to inscribe the to the textures for the sample with rotation angles of
polarization grating inside the birefringent nematic LC φ = 45◦ , 90◦ , −45◦ and 0◦ texture in Figure 3(a) of the
cell, we prepared two counter-propagating beams with grating vector relative to the polarization axis of the po-
orthogonal S- and P- linear polarizations from the 514.5 larizer, respectively. As seen in Figure 3(a), the micro-
nm line of an Ar+ laser as writing beams by use of a scopic textures, characteristic of a phase grating, confirm
beam splitter and half-wave plates, as shown in Figure the presence of TN array by a white wave-guiding mode,
1. The light intensity was 200 – 300 mW/cm2 and the where the reddish color in the pictures originates from
illumination time was 10 – 20 s. the optical absorption in the azo alignment layers and the
In the case of normal incidence, θs = 0, the polar- doped azo dyes located inside the LC cell. To examine
ization state, corresponding to one of the polarization the twist angle of the LC cell, we rotated the analyzer to
states described in the top or bottom rows of Figure 2(b), +45◦ and –45◦ with respect to the polarization direction
is uniform on the entire x-y plane of the P L1 and the of the polarizer shown in Figure 4(a). The dark domain
P L2 layers, which is the photo-alignment polymeric sur- in Figure 4(a) changes to a bright color in Figure 4(b),
face. However, when the birefringent nematic LC cell which indicates that the adjacent TN domains are in a
is slightly slanted with a slant grating angle θs of 0.1 – reversed twist direction to each other.
2.0◦ in a reflection holographic configuration, as shown Next, we examined what kind of polarization modula-
in Figure 1, a periodic modulation of the polarization tion resulted when two counter-propagating beams inter-
state emerges along the x-axis on the photo-alignment fered. In the S/P polarization reflection hologram, the
polymeric surface with the period Λ = λ/2 sin θs de- polarization modulation is given as
termined by the slant grating angle θs and the wave-
length of the writing beam λ, as shown in Figure 2. The Es + + Ep − = Eos cos(ωt − kz + ρ)y
-1494- Journal of the Korean Physical Society, Vol. 51, No. 4, October 2007
Fig. 4. (a) Microscopic texture of the TN domains for
examining the reverse twist angle, as taken with an optical
polarizing microscope and (b) schematic front view illustra-
tion of the LC grating formed by the polarization modulation
and micro-textures (θt : the twist angle of the TN domain).
Fig. 3. (a) Microscopic texture of the TN domains as taken
with an optical polarizing microscope (upper left: φ = 45◦ ,
upper right: φ = 90◦ , lower left: φ = −45◦ , lower right: φ = tering in the LC cell. Two counter-propagating S- and
0◦ ) and (b) schematic illustration of the LC grating formed P-polarized light beams with initially equal intensities,
(1) (2)
by the polarization modulation and micro-textures (θof f : the Eos = Eop , get attenuated by a different amount, re-
off-set angle of the polarization direction with respect to the (1) (1) (2) (2)
sulting in Eos > Eop on the P L1 layer and Eos < Eop
x-axis). on the P L2 layer. The resulting polarization phase mod-
ulation inscribed on the P L1 and the P L2 layers will be
as in the top and the bottom imaged shown in Figure
+Eop cos(ωt + kz)x (1)
2(b), respectively.
≈ Eop {cos kz cos ωt[Eos /Eop y + x] In the current experimental configuration, Eos > Eop
+ sin kz sin ωt[Eos /Eop y − x]}, (2) on the P L1 layer, Eos < Eop on the P L2 layer, and
θs = 0; hence, the major axes of the two modulated el-
where Es + and Ep denote S- and P-polarization electric
−
liptical polarizations on the PL1 and the PL2 layer do
fields propagating along the +z and the −z axis, respec- not coincide. Now, on each PL1 and PL2 layer the pres-
tively, and ρ is the phase difference between the two. ence of a modulated polarization state gives rise to a
When Eos = Eop , Eq. (1) describes two standing waves modulated orientation of azo moieties along the x-axis
ˆ y
with wave planes of x+ˆ + z and −ˆ+ˆ + z and a polar-
√
2
ˆ x
√ y
2
ˆ on the surface of the LC alignment layer via a photo-
ization modulation results from the optical interference, isomerization process, and the relative orientational di-
as shown in the middle row of Figure 2(b). We find that rections of the azo moieties inside two facing alignment
the polarization state goes through a periodic modula- layers P L1 and P L2 are determined by the values of
Eos
tion from right-circular to linear to left-circular to linear the ratio Eop at each layer, as shown in Figure 3 and
polarization along the z-axis, which corresponds to the Figure 4. A non-parallel orientation of azo moieties in-
sample depth direction. However, when Eos = Eop , the side two photo-alignments gives rise to a TN structure
wave planes of the two standing waves deviate from the inside the birefringent LC cell, which is twisted in x-y
planes of x+ˆ + z and −ˆ+ˆ + z , and at the same time,
ˆ y
√
2
ˆ x
√ y
2
ˆ plane with the twist axis along the z-axis. Furthermore,
a polarization modulation from right-elliptical to linear at the surface of each alignment layer, the alignment of
to left-elliptical to linear polarization is produced along LCs is achieved by an orthogonal photo-alignment fol-
the z-axis, as described in the top and the bottom rows lowing the zig-zag-shaped polarization modulation pat-
(Eos > Eop or Eos < Eop ) of Figure 2(b). When the bire- tern along the x-axis with a non-zero off-set angle θof f
fringent nematic LC cell is positioned inside the polariza- with respect to the x-axis, and the off-set angles are dif-
tion modulation produced, writing beams go through an ferent in P L1 and P L2. See Figure 4(b). This leads to
optical attenuation owing to the absorption and the scat- the formation of two different kinds of TN domains, one
Fabrication of a Reverse Twisted Nematic Liquid-Crystal· · · – Hyunhee Choi et al. -1495-
Fig. 5. (a) CCD picture of the diffraction orders and plot Fig. 6. (a) Diffraction efficiencies of the +1st order and
of the polarization state of the diffracted orders and (b) the (b) -1st order as a function of the applied voltage.
diffraction efficiency as a function of the polarization.
Next, the diffraction property of the fabricated LC
twisted in the clockwise sense and the other twisted in grating was investigated. In the inset of Figure 5(a) is
the counter-clockwise sense. The resulting LC grating is shown a CCD picture of the diffraction pattern. The
in the structure of a reverse TN LC grating (Figures 3 polarization state of the out-going diffracted order was
& 4) [3]. examined and is plotted in Figure 5(b). The polariza-
From the measurement of the dichroic absorption of tion direction of the light propagating from P L1 to P L2
each layer of the fabricated birefringent cell, the ratios is y-polarized, perpendicular to the grating vector, with
Eos /Eop at PL1 and PL2 are determined as 3.0 and 1.0, the angle 0◦ corresponding to the y-axis. The ±1 orders
respectively. From the relation of the offset angle θof f go through a 90◦ rotation in the polarization state while
with the ratio Eos /Eop , i.e., tan(θof f ) = Eos /Eop , we there is no change for the 0th order [14]. Figure 5(b)
find that the axes of the linear polarization state appear- shows the polarization independence of the diffraction
ing in the polarization modulation are rotated by +75 ◦ efficiency η for the 0th and ±1st orders, which is the
at the PL1 layer and +35 ◦ at the P L2 layer relative to characteristic feature of a reverse TN LC grating. We
the +x axis. We found that θt depends on the LC cell also looked into the electro-optic tunability of the diffrac-
thickness and could be increased up to 90◦ for a thick tion efficiency by measuring the diffraction efficiency of
cell. When the twist angle was close to 90◦ and the cell the ±1st order as a function of the applied voltage. As
thickness satisfied the interference minimum condition, shown in Figure 6, below the Freedericksz transition, η
the incident light was completely diffracted to a high or- is constant and polarization-independent while different
der, that is, the 0th order intensity I0th = 0 [14]. changes in η appear, depending on the polarization when
-1496- Journal of the Korean Physical Society, Vol. 51, No. 4, October 2007
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