Photo-alignment control of topological defects in nematic
liquid-crystal cells
Golub P. V. and Vasnetsov M. V.
Institute of Physics of the National Academy of Sciences of Ukraine, Prospect
Nauky 46, 03028 Kyiv, Ukraine, pvgolub@live.com
Received: 17.02.2021
Abstract. We report the experiments aimed at controlling the position of linear
topological defect (disclination) in a so-called -cell filled with a nematic liquid
crystal sandwiched between circularly rubbed polyimide (Kapton) layer and photoaligning PVCN material oriented unidirectionally. To satisfy surface-alignment
conditions, the director within this cell experiences a twist. The topological defect
appears in a plane where the twist angle of the director becomes indeterminate (90º).
In an achiral liquid crystal, orientation of the disclination coincides with the alignment
occurring on the unidirectionally oriented substrate. The idea of this work is to control
the alignment direction on the substrate by a polarized ultraviolet light. Photo-induced
director reorientation on the substrate changes the twist angle in the bulk and so varies
the azimuthal angle of the disclination. We monitor reorientation of the disclination as
a function of light-polarization angle and irradiation dose.
Keywords: liquid crystals, photo-alignment, topological defects, disclinations
UDC: 535.21+539.2
1. Introduction
Application of photo-alignment to liquid crystals (LCs) is one of the hottest topics in the modern
applied physics of LCs. It can result in widely ranging applications, some of which could become
good alternatives to traditional surface-rubbing techniques, while the others could not be
implemented with the latter technique [1–3]. In most cases, photosensitive aligning layers are
irradiated with a polarized light before a cell is filled with a LC. However, photo-alignment of LCs is
also frequently used after a cell is filled, i.e. in a freshly filled LC cell [4]. Moreover, some
mechanisms of LC orientation are based on the polarization-sensitive absorbance of LC molecules,
e.g. in a bulk-mediated photo-alignment technique [5–10].
The photo-alignment performed at a substrate of a LC cell dictates the boundary orientation of
LC in a cell. Then the variation of its orientation on a substrate surface should also influence the LC
director in the bulk [4, 11, 12]. This effect can be applied to control topological defects in the cell
volume, which are created due to 90º uncertainty in the angle of twist of the director occurring
between the substrates.
In this work we suggest a technique for controlling linear topological defects formed in socalled -cells. These cells are composed of two substrates, one of which is circularly rubbed. A
(unidirectional) alignment of the director at the other substrate can be controlled due to irradiation of
the cell by a linearly polarized light.
2. Materials and experimental setup
In our experiments, we used -cells of LCs of a special design. A typical construction of the cell
is shown in Fig. 1. The cell was composed of two substrates. One of them, a reference substrate,
was covered with a polyimide (Kapton) layer, whereas a control substrate was covered with
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Golub P. V. et al
fluoro-polyvinyl-cinnamate (PVCN) film [13]. The polyimide film on the reference substrate was
circularly rubbed, which provided strong circular alignment of the LC [4]. The gap between the
substrates formed by calibrated spacers corresponded to the cell thickness d = 25 μm. The cell was
filled with a nematic LC 5CB (4-Cyano-4'-pentylbiphenyl) in its isotropic phase (at the
temperature 40oC), using capillary forces. Unidirectional orientation of the nematic director at the
substrate covered by PVCN can be controlled by linearly polarized light. To preset the alignment
on the control substrate, it was irradiated with the polarized light from an ultraviolet diode laser
(the wavelength of 405 nm) before the cell was filled with the LC.
(a)
(b)
(c)
Fig. 1. Construction of our LC cell: (a) LC alignment on the control substrate; (b) sandwich of glass substrates
(1), photosensitive PVCN (2), LC (3) and circularly rubbed polyimide layer (4); (c) LC alignment on the reference
substrate.
The construction of the cell is schematically depicted in Fig. 2. The orientation of the LC
director in the cell between the substrates is gradually twisted from unidirectional one (at the
control substrate − see Fig. 2a) to the final state which satisfies the alignment determined by the
circular rubbing of the reference substrate (see Fig. 2c). In a plane where the angle between the
alignments provided on the control and reference substrates makes the twist 90º, a topological
defect occurs, which has the shape of a disclination line [14]. It is known that the disclination line
in the θ-cells filled with achiral nematic liquid crystals is parallel to the unidirectional alignment
[14] (see Fig. 2b).
(a)
(b)
(c)
Fig. 2. (a) Sketch of our experiment, (b) θ-cell before irradiation (the orientation angle φ = 0), and (c)
after irradiation (the reorientation angle φ = 15).
88
-cell
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Photo-alignment control
The cell prepared and filled as explained above was irradiated by the ultraviolet diode laser
which emitted linearly polarized light. The time of irradiation varied from 10 to 150 min (i.e.,
2.5 h). The appropriate intensity was measured by a powermeter.
In most of our experiments, the LC cell faced the input ultraviolet beam by its control
surface, i.e. the PVCN film was exposed at first (see Fig. 2a). After the exposure, the irradiated
areas were examined between crossed polarizers, using a microscope Olympus BX53 equipped
with a CCD camera connected to a computer. Note that reorientation of the disclination line was
unambiguously detected, as seen from Fig. 2b and Fig. 2c.
3. Experimental results
It is known that the light-induced azimuth angle φ depends on the intensity I of ultraviolet beam
and the exposure time texp. The twist angle φ increases with increasing intensity, with further
saturation to a steady state determined by the total light dose [4].
When the cell is irradiated by the polarized light with the polarization plane producing some
angle φ0 with respect to the unidirectional pre-alignment at the control face, the angle of the
disclination in the cell differs from the initial one (see Fig. 2c). It has been found that reorientation
of the alignment on the control surface causes changes in the whole LC volume and the
disclination line follows the azimuth φ, in accordance with orientation taking place on the control
substrate. The experimental dependence of reorientation angle of the disclination on the irradiation
dose is shown in Fig. 3.
0
10
0
20
Irradiation dose J/m2
20
30
40
50
60
70
80
60
80
100
120
140
160
Reorientation angle, deg
25
20
15
10
5
0
40
Irradiation time, min
Fig. 3. Dependences of reorientation angle φ of the disclination on the irradiation dose and time.
One can see from Fig. 3 that the characteristic time necessary for achieving the stationary
state described by the angle φ ≈ 25° is equal to texp ≈ 150 min. This time characterizes a lightinduced development of a new ‘easy axis’ on the PVCN surface. According to Ref. [15], this time
is associated with the time needed for settling equilibrium for the angular distribution of adsorbed
LC molecules on the photosensitive surface during the irradiation process.
Hence, variations occurring in the boundary angular distribution eventually cause the changes
in the director distribution in the overall LC volume. As a result, the topological defect changes its
position according to orientation acquired on the photosensitive surface. In this way we are able to
move the disclination azimuth in both angular directions.
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Golub P. V. et al
4. Conclusion
We have demonstrated the possibilities for controlling the position of the topological defect in the
LC bulk using the photo-alignment occurring on the border surface. It has been shown that the
disclination direction in achiral LCs follows its orientation direction on the control substrate. Then
the light-induced reorientation is possible. This technique can be used for precise control and
deposition of particles trapped at the topological defect, as well as in irradiation dosimetry or in
some other fields.
Acknowledgement
This study was performed within the Project “Photophysics of the processes of interaction of
optical radiation with photorefractive solid-state and bio-organic media” by the National Academy
of Sciences of Ukraine.
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Golub P. V. and Vasnetsov M. V. 2021. Photo-alignment control of topological defects in nematic
liquid-crystal cells. Ukr.J.Phys.Opt. 22: 87 – 91. doi: 10.3116/16091833/22/2/87/2021
Анотація. Ми повідомляємо про експерименти, спрямовані на керування рухом лінійного
топологічного дефекту (дисклинації) у т. зв. -комірці, заповненій нематичним рідким
кристалом, який затиснуто між циркулярно натертим поліімідним (каптонівським)
шаром та односпрямовано орієнтованим фотовирівнюючим матеріалом PVCN. Щоб
задовольнити умови вирівнювання на поверхні, директор у цій комірці зазнає скручування.
Топологічний дефект з’являється в площині, де кут скручування директора стає
невизначеним (90º). В ахіральному рідкому кристалі орієнтація дисклинації збігається з
напрямком вирівнювання, що має місце на односпрямовано орієнтованій підкладці. Ідея цієї
роботи полягає в керуванні напрямком вирівнювання на підкладці за допомогою
поляризованого ультрафіолетового світла. Фотоіндукована переорієнтація директора на
підкладці змінює кут скручування в об’ємі рідкого кристала, а тому й азимутальний кут
дисклинації. Ми також відстежили переорієнтацію дисклинації як функцію кута
поляризації світла та дози опромінення.
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