Journal of Modern Physics, 2011, 2, 248-255
doi:10.4236/jmp.2011.24034 Published Online April 2011 (http://www.SciRP.org/journal/jmp)
Optical Properties of Aligned Nematic Liquid Crystals in
Electric Field
Suleyman Yilmaz1, Halide Melik1, Firat Angay1, Mehriban Emek2, Ahmet Yildirim3
1Department of Physics, Harran University, Osmanbey Camp us , Sanliurfa, Turkey
2Department of Physics, Adiyaman University, Adiyaman, Turkey
3Department of Physics, Siirt University, Siirt, Turkey
Received January 13, 2011; revised February 23, 2011; accepted F ebruary 25, 2011
Abstract
In this study, the optical transmittance of aligned nematic liquid crystals (ANLCs) was investigated in terms
of temperature variations through electrooptic effects under DC electric field. The optical transmittances of
the planar and homeotropically aligned liquid crystal cells, which were prepared by conventional rubbing
and photolithographic technique on the polyimide thin films for molecular alignment, were measured in their
phase transition region. The results of measurement for both orientations, the distribution curve of the optical
transmittance exhibits displacement toward to low level at the beginning and then to high level by the tem-
perature variations, while the electric field increases. It was also observed that the domain structure of the
materials were affected considerably by the applied electric field and phase transition region of the aligned
structures had broader range than by the pure crystalline structure and its phase transition temperature was
changed by the molecular anisotropy. Finally, in photolithographic method strong bonds between the mole-
cules and the orienting surface were observed in high contrast to rubbing method.
Keywords: Liquid Crystals, Thin Films, Photolithography, Optical Properties
1. Introduction
The nature of the liquid crystals (LC) is very complex
and their aligned surface is included the important fac-
tors such as van der Waals, dipolar and steric interactions,
hydrogen and chemical bonding and surface topography.
Their electrooptical properties also play important role
on optical processing systems and photonic devices such
as mobile phones, smart cards, integrated displays [1,2].
At the same time, NLCs are preferred in many different
applications due to their anisotropic structure, which
relates to molecular orientation and temperature variation
[3-7].
The treatise on the surface of glass substrate constrains
the LC and orients the surface director. The characteristic
small area and multi domain properties concerned with
molecular interactions and the other factors as well as the
molecular anisotropy, the electrooptical effects, the an-
choring energy, response time, contrast ratio etc.
There are several techniques for surface alignment for
LCs that depending on contact and non-contact proce-
dure. The rubbed polymer technique is widely used in
LCDs and the perfect electrooptic performance is pro-
vided by this method. However, the fiber residues, the
corresponding impurities, the static charges and the me-
chanical damage can also trouble for devices and result
in deterioration of the quality of LC switching, particu-
larly when active matrix elements are used for the pur-
pose [8]. Other alignment methods, such as photo- align-
ment, micron rubbing by microsphere, nanoim- printing
lithography, microcontact printing and ion beam bom-
bardment have also intensively studied.
In this study, the photolithography method was used to
providing the micropatterns on the polyimide surface that
includes the spin-coating, the UV exposure and the wet
chemical etching, respectively. By taking into account
the mentioned properties of NLCs, the optical and the
electrooptical properties of randomly [9] and aligned
NLCs are examined in the temperature range including
phase transitions.
2. Photolithographic Application
In these study, commercial NLCs 5CB was provided

S. YILMAZ ET AL.
249
from Sigma Aldrich, polyimide for planar alignment,
adhesive material and thinner were provided from HD
Microsystems, polyimide for homeotropic alignment and
solvent were provided from Brewer Science, NaOH de-
veloper and positive photo-resist were provided from
Rohm and Haas Electronic Materials.
The various materials used to aligned surface, but the
polyimide is a suitable material for coating on the surface
due to high cure temperatures (~350˚C). Because of their
excellent properties such as high refractive index, adhe-
sion the substrates, high resistivity, excellent transpar-
ency in the visible spectrum, high chemical and thermal
stability, they are usually preferred as the aligned sur-
faces.
On the experimental stage, firstly the adhesion pro-
moter is performed on the indium tin oxide (ITO) glass
substrates with resistance of 30 - 60  by using spin
coater, after the polyimide coating, the photoresist mate-
rial is coated on the surface of the polyimide film and
finally the coated multilayer is exposed under periodic
lined photomask by the UV light. Aligned multilayer
surface is subjected to development procedure to remov-
ing contaminated materials from the surface and it is so
called the wet chemical etching process. The alignment
of surface was completed by curing with suitable heating
and cooling procedure. The liquid crystal cells are
formed with two anti-parallel aligned substrates and in-
serted into microfibers as spacer.
Morphologic texture of the NLC samples is obtained
by using Leica polarization microscope with a CCD
camera. The interferometric measurement system is de-
signed by HeNe laser with 632.8 nm, optical chopper,
large area photoreceiver, lock in amplifier, other optical
apparatus such as polarizer and analyzer. Experimental
system is comprised of three units such as the heating,
the electric field and the optical measurement unit (Fig-
ure 1). The regular increments of the temperature were
provided by the temperature control unit. In progressing
system, the electric field was provided by the DC power
supply to affect the molecular orientation. For determin-
ing the molecular orientation and anisotropy, the angle of
polarizer was fixed in parallel position. The data ob-
tained by the experimental system were transferred to a
computer by the serial port and compiled by the Lab-
view8 program.
3. Optical Properties
The NLCs are insulating organic liquids and formation
of longitudinally oriented molecules. These structures
have molecular orientation at certain temperature inter-
vals as crystals have, but the gravity centers of the
molecules are in chaotic order as in liquids. While there
are no external alignment effects, there are randomly
oriented domains in the nematic phase. However, if the
domain walls of the liquid crystal are processed with
known methods, the nematic liqu id crystals will have the
homogeneous single alignment [9,10]. The electrooptical
effects in the LCs may be caused by two different physi-
cal processes: 1) the external electric field changes the
orientation of molecules causin g the absorption and scat-
tering spectrum intensity, 2) the electric field changes the
distances between energy levels of the molecule, shifts
the absorption bands and varies the transition intensities.
When the electric field is applied to such a system, the
interaction between liquid crystal and electric field can
be explained by means of at least two mechanisms [11].
The first one is the dependency of dielectric constant of
LCs to molecular anisotropy (//



nn
), this also implies
that material is birefringence (// or oe nn

). The
contribution of this anisotropy mechanism to the system
energy is as follows:
Figure 1. Schematic projection of the heating, the electrical field (DC) and the optical measurement units.

S. YILMAZ ET AL.
250

2
1
8π
ea
W   nE (1)
where, n is the director, E is the applied electric field and
//a



 dielectric anisotropy, //

and


denote
parallel to and perpendicu lar constants, respectively. The
second mechanism is similar to piezo-electric mecha-
nism in solid materials, which small deformations cause
formation of polarization. For a weak electric field, the
mechanical tension forces are dominant and the orienta-
tion of molecular director remains the same. However,
the orientation of director starts changing as the applied
electric field rises over the critical value (C
E
), which is
known as Frederick’s transition. To explain this process
better, a function known as the coherence length for
electric field is used;

12
4π1
a
K 



E
E
(2)
where, denotes the coherence length that ex-
presses the distance at which the director vector starts
getting affected by the electric field. Here,

E 
K
is a con-
stant denoting the mechanical tension. Thus, the critical
electric field (C
E
) is as follows:
12
14π
Ca
K
d 



E (3)
As seen from (Equation (3)), the critical electric field
depends on anisotropy (a

) of the material and the thick-
ness of the sample (d). The transmittance of the liquid
crystal cell is calculated as function of the applied elec-
tric field in two steps [12]: firstly, the reorientation of the
molecular director must be determined in regards to ap-
plied electric field variations and secondly, the orienta-
tion of molecules affecting the transmittance of the light
through the material must be calculated. These calcula-
tions are done by solving numerically the continuity
equations for LCs [13]. Thus, anisotropy of molecular
structure and the interaction of the incident light with the
material determine the polarization of the transmitted
light. If a polarization rotation is observed in material,
one can tell abou t dichroism of the optically active mate-
rial. The dichroism occurs in the LCs due to either the
optical anisotropy of the molecular structure or the pres-
ence of the impurities. The dichroism also refers to any
optical device which can split of light into beams with
differing wavelengths.
When the light passes through the sample, the polari-
zation state of the transmitted light is changed by mo-
lecular anisotrop y [14]. In order to calculate the intensity
of the light passing through a sample, Jones’ vector rep-
resentation is used [15]. The field components of the
incident light perpendicular to each other are the follow-
ing:


ˆ
cos
xx x
Atkz  
E
i (4)


ˆ
cos
yy y
Atkz  
Ej
(5)
The representation of the incident light components
with the Jones’ v ector is the following:
x
y
E
E



E (6)
By the interaction of the transmitted light with the
material, the components are the following:


ˆ
cos
xx x
Atkz 




E
i (7)


ˆ
cos
yy y
Atkz 




Ej
(8)
The representation of the transmitted light components
with the Jones’ vector is the following:
x
y
E
E





E (9)
Therefore, the optical transmittance of the LC cell sys-
tem is calculated by the following equation:
2
2
2
2
xy
xy
EE
TEE

 
 (10)
4. Results and Discussions
The most common applications of NLC are seen in the
electrooptic area depending on temperature. The optical
transmittance of NLC 5CB is analyzed under DC electric
field in terms of temperature variations. The relative
transmittance for both the planar and the homeotropic
orientation by the voltage values of 0-15 V is measured
in the temperature range from 298 K to 328 K in Figures
2-4. The temperature increment of 3 K per minute is em-
ployed during the all measurement.
The devised LC systems in this study contain not only
the conducting cell of the substrate glasses but also the
optical attachments such as polarizer and analyzer. The
variation of the anisotropy in the material due to the in-
teraction between the material and light under the elec-
tric field is monitored and new optical modes are derived
from these observations. An increase in the transparency
of the sample depending on the temperature is clearly
seen from the modulation spectrum graphics in Figures
5-7. The modulation spectrum data gives us detailed in-
formation about the optical properties of the material,
such as the phase transition, the transparency and the
molecular anisotropy.

S. YILMAZ ET AL.
251
Figure 2. The transmitted intensity versus the temperature
variations for the planar orientation by mechanical rub-
bing.
Figure 3. The transmitted intensity versus the temperature
variations for the planar orientation by photolithography.
Figure 4. The transmitted intensity versus the temperature
variations for the homeotropic orientation.
The analyzer angle is fixed 0˚ (parallel to polarizer)
and the LC cell is heated 3 K per minute in sample
holder. At the beginning, the transmitted intensity exhib-
its a decreasing to low level and by the critical limit it
reaches to high level on all of the molecular alignments
in Figures 2-5. In the measurements for planar orienta-
tion by mechanical rubbing, the curves of the transmitted
intensity exhibits firstly displacement toward to low level
and it inclines to high level by the critical limit, 6 V. At
the photolithography application for planar alignment,
the curves of the transmitted intensity exhibits firstly
drastically displacement toward to low level and it in-
clines to high level by critical limit, 2.5 V.
Finally, on the homeotropic alignment, the curves of
the transmitted intensity also exhibits displacement to-
ward to low level and it inclines to high level by the
critical limit, 6 V like as planar alignment by mechanical
rubbing.
The points at which the transmitted intensity reaches
the maximum or the minimum value are seen at the
modulation spectrum graphics in Figures 5-7. At the
spectrum modulation graphics for all of the alignment
application, the peak value of the transmitted intensity
displaces to high temperature point step by step by the
electric field variations. However, in Figure 6, the oscil-
lations of the spectrum modulation for the photolitho-
graphy application exhibit difference from the other pre-
vious alignments and by the critical point, they continue
to their progress as smoothly.
5. Conclusions
In this presented study, the rubbing and photolitho-
graphic technique have been treated for molecular
alignment and compared the results of photolithographic
application with the conventional rubbing. When the
electric field is applied, the electrooptical effects come
into existence with variation on the molecular orienta-
tions; the electric field changes the anisotropy and leads
to important effects on the processing of the optical ma-
terials.
It was observed that photolithography technique pro-
vides a controllable pretilt angle, strong anchoring
strength (polar and azimuthal orientations), high contrast
ratio of the electric field states (on and off) and response
time by the low voltage driving as well as having high
thermal and ultraviolet stability. It is interesting to note
that there are meaningful and comparable results in ex-
perimenting under the DC electric fields.
Finally, the measurements that were carried out de-
pending on the optical transmittance show that the phase
transition interval of the aligned molecular structures was
observed in larger temperature ranges.

S. YILMAZ ET AL.
252
Figure 5. The modulation spectrum of the optical transmittance versus the temperature variations for the planar orientation
by mechanical rubbing.

S. YILMAZ ET AL.
253
Figure 6. The modulation spectrum of the optical transmittance versus the temperature variations for the planar orientation
by photolithography.

254 S. YILMAZ ET AL.
Figure 7. The modulation spectrum of the optical transmittance versus the temperature variations for the homeotropic ori-
ntation. e

S. YILMAZ ET AL.
255
The results of this study, which is accomplished on ran-
domly orientated molecules in previous study [16], show
resemblance with the results of other works by devel-
oped in the past studies [17-24].
6. Acknowledgements
The authors thank to Harran University for financial
support. This study is supported by Harran University,
the Fund of Scientific Research Projects (SRP) with
grant no: 919.
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