Optical Properties of Aligned Nematic Liquid Crystals in Electric Field

doi:10.4236/jmp.2011.24034

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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

E-mail: [email protected]

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 (//





), this also implies

that material is birefringence (// or oe



). 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



8π



 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

), which is

known as Frederick’s transition. To explain this process

better, a function known as the coherence length for

electric field is used;



4π1









(2)

where, denotes the coherence length that ex-

presses the distance at which the director vector starts

getting affected by the electric field. Here,





is a con-

stant denoting the mechanical tension. Thus, the critical

electric field (C

) is as follows:

14π









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:





cos

xx x

Atkz





i (4)





cos

yy y

Atkz





(5)

The representation of the incident light components

with the Jones’ v ector is the following:







E (6)

By the interaction of the transmitted light with the

material, the components are the following:





cos

xx x

Atkz









i (7)





cos

yy y

Atkz









(8)

The representation of the transmitted light components

with the Jones’ vector is the following:











E (9)

Therefore, the optical transmittance of the LC cell sys-

tem is calculated by the following equation:

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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