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Study of Cholesteric Liquid Crystal – Rhodamine 6G doped
polymethyl methacrylate thin film - Cholesteric Liquid Crystal system
R. B. Alaverdyan
1
, T.K. Dadalyan
1
, N. A. Hayrapetyan
1
Yerevan State University, Yerevan, Alex Manoogyan 1, 0025
ABSTRACT
In this work we have experimentally investigated optical properties of multilayered structure, consisting of two
right handed Cholesteric Liquid Crystal layers with the same pitch and Rhodamine 6G (R6G) doped polymethyl
metacrylate (PMMA) thin film sandwiched between them. Particularly transmission spectrum dependence on
temperature is studied. The 10 nm of CLC selective reflection band tuning is achieved. The photoluminescence of R6G
is registered.
Keywords: multilayer system, Cholesteric Liquid Crystals, Rhodamine 6G doped film, luminescence,
polymethyl metacrylate, temperature tuning of photonic band gap
1. INTRODUCTION
Creation of thin layer structured, low threshold, mobile and miniscule optical elements that provide control of
light radiation and parameters, i.e. state of polarization, propagation direction and spectrum of laser emission is very
important problem of contemporary science. In this regard a growing attention is paid to photonic crystals and various
photonic structures. Because of their optical properties Photonic Crystals (PC) have attracted a great deal of interest from
both fundamental and practical points of view. In the recent years a considerable number of works is devoted to
investigation of Photonic Band Gap materials or PC-s. These are structures with periodic modulation of refractive index.
The key property of these structures is existence of photonic band gaps i. e. the light wavelength region the propagation
of which is forbidden
1
.
The investigation of PC materials have started in 1970-s with the pioneering work of Bykov
2
. There have been
proposed many practical applications for photonic band gap (PBG) materials since prediction of existence of PBG in the
medium with periodical modulation of refractive index by Yablonovich
3
and John
4
. One of the most exciting properties
of PBG materials is ability to inhibit spontaneous emission within band gap, which may lead to creation of low threshold
lasers
5
. In one dimensional periodic structure, the laser action is predicted at the photonic band edge, where photon group
velocity approaches zero
1
.
Particularly, great accent has been put on studying photonic crystals that assume control of their properties by
external influence. This enables to control the state of polarization, transmission and reflection spectra of the light that
propagates through a crystal. This can be realized by means of anisotropic media which can be tuned with external
factors (pressure, temperature, electric and magnetic fields, light radiation etc.) and are easy and not expensive in
fabrication and synthesis process. All these requirements can be easily met with both liquid crystals and their dye-doped
polymeric film containing structures.
Cholesteric Liquid Crystals have self organized helical structure, with periodical modulation of refractive index
and are able to reflect circularly polarized light with the same handedness of helix (selective reflection of the light); this
allows us to consider CLC-s as 1D photonic crystals
6, 7
. The unique optical and physical properties of CLC-s are making
Liquid Crystals XV, edited by Iam Choon Khoo, Proc. of SPIE Vol. 8114, 811418
© 2011 SPIE · CCC code: 0277-786X/11/$18 · doi: 10.1117/12.897282
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them very interesting object of intensive studies and predicts many interesting practical applications, such as optical
diodes, filters and optical windows and other compact optical components
8-10, 15,
.
But the most exciting of their applications CLC-s have found in combination with laser dyes, which led to
creation of tunable cholesteric liquid crystalline lasers
11
. Sensitivity to the external factors such as temperature, pressure,
electromagnetic fields provides opportunity to tune selective reflection band of CLC, which provides unique opportunity
for creation of easy processible, compact, low threshold and mirror less tunable dye lasers
11-14
. In these systems as a solid
state matrix various types of polymers such as polymethyl metacrylate (PMMA), epoxide resins and films obtained by
means of sol gel technique could be used
16-19
. Compared with liquid state dye lasers these types of solid state dye lasers
have several advantages of small size of active medium and use of inflammable and safe for environment materials. The
dye molecules are immobilized within the structure of polymeric matrices, and thus thermo optical distortions, Dye
degradation caused by strong laser irradiation could be avoided. The problem of laser beam big divergence because of
CLC resonator shortness could be solved for example by means of external resonator
20
, and the stability of light
irradiation could be enhanced by CLC cell rotation
21
. From viewpoint of easy processability and other physical
properties polymethyl metacrylate provides great opportunity to conduct experiments on multilayer systems consisting of
Liquid Crystalline materials and polymer films.
In the present work we are investigating spectral properties of the system consisting of Rhodamine 6G doped
polymethyl metacrylate thin film sandwiched between two right handed CLC layers with the same handedness of helical
pitch. We have experimentally studied thermal dependence of the transmission spectrum and the photoluminescence
spectrum of this system.
1.1 Cholesteric Liquid Crystals
Liquid Crystals (LC) are fluids that consist of elongated, rod-like molecules. Intermolecular forces tend to align
LC molecules parallel to each other which cause anisotropy of LC properties. The molecules are directionally correlated;
they are aligned in a general direction defined by a unit vector (
), the so-called director axis. Due to the fact that LC-s
are fluids, their anisotropic properties can be easily controlled by external factors. Particularly they are very sensitive to
external electric, magnetic, optical, acoustic fields and temperature
6
.
Recently increasing interest is represented by NLC-s with chiral dopants and Cholesteric Liquid Crystals
(CLC), which under certain conditions attain the property of selective reflection of light.
Thin films of CLC in the planes parallel to the glass substrates confining it (monomolecular layers) by their
alignment are alike with NLC-s, i.e. in those planes molecules are aligned along same axis parallel to the substrates. The
molecules of CLC have helical structure that can be both left-handed and right handed. Because of its helical structure
the director of CLC continiously rotates at α≈0.5° angle when going through one monomoelcular layer to another
forming helical structure. After each 180/α-th layer the alignment of molecules will be repeated. The distance on which
the director of CLC rotates at 360° angle is termed the pitch of CLC (p= (360/α)*a, where a-is the distance between
adjacent monomolecular layers and also describes the periodicity of CLC
22
.
As their twisted helical structure CLC-s are able to rotate the plane of polarization of the light, by means of
optical activity; the angle of the plane of polarization rotation is given by φ=ρ*d, where ρ is the specific rotation, d is the
thickness of the CLC layer (cell). For CLC-s ρ has a very big value. For instance, for quartz it is about 15.5°/mm
whereas for CLC-s it is about 60000-70000°/mm. Such a great specific rotation of the plane of polarization for CLC-s
and their high sensibility to external fields makes possible to control the state of polarization of light propagating
through
22
.
Due to their periodical structure CLC-s provide property of Selective Reflection of light, which makes them
more interesting and promising for applications in photonic structures, for certain polarizations selective reflection from
thin CLC layer leads to appearance of photonic band gaps in reflection spectra. This enables us to regard to CLC-s as to
1D photonic crystal.
In the helical structure of CLC molecular alignment repeats at a distance equal to the pitch p of CLC helix.
Because of equivalency of physical properties of CLC molecules on
and -
directions, the period of CLC structure
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therefore becomes p/2. Thus CLC-s can be regarded as diffraction grating which permits dynamic and smoothly control
the grating constant by means of external fields including light
7
.
The positions of maxima and minima of diffraction pattern of these diffraction gratings are given by Wolf-
Bragg condition sin, where is the angle of incident light with the plane of diffraction grating, is the
wavelength of incident light in the medium, m is integer number. In the simplest case, when incident light is
perpendicular to the cell (/2), the condition of Wolf-Bragg becomes p=m*λ i.e. CLC will reflect light with
wavelength equal to its pitch (selective reflection will occur)
6
.
The reflection coeficent for circularly polarized light with same handedness of polarization as CLC helix
having the wavelength from the n
o
p<λ< n
e
p range incident normal to the cell is 1 while for opposite polarized light it is
0. Here n
o
and n
e
are ordinary and extraordinary refractive indices of CLC. Circularly polarized light reflected from CLC
layer has the same handedness of polarization as incident light, whereas reflected from other reflecting surfaces
polarization of the light changes to the opposite
6
.
1.2 Rhodamine 6G properties
Xanthene dyes are very efficient laser dyes and cover the wavelength region from 500 to 700 nm. Most of
commercial dye lasers are from this class. Rhodamine 6G is typical representative of xanthene laser dyes. In the Fig. 1
the chemical structure of Rhodamine 6G is represented. Rhodamine 6G is known to have efficient laser action in the 590
nm region and the quantum yield is about 0.95
23
.
Fig. 1 Rhodamine 6G chemical structure
This laser dye has remarkable photostability and solubility in the most known solvents
24
. It is one of the most
often used and studied laser dyes. Rhodamine 6G laser action is observed from 555 to 585 nm range with maximum on
560 nm. Usually Rhodamine 6G gain medium is pumped by the 2-nd harmonic of Nd: YAG laser (532 nm).
1.3 Polymethyl Metacrylate
As an alternative for liquid state gain medium of dye lasers the dye doped the matrices of polymers could be
used. One of the popular materials for this purpose is polymethyl Metacrylate (PMMA). The lasing from dye doped
PMMA solid state laser was first achieved in 1967
25
.
PMMA is a synthetic polymer of metacrylate. PMMA easily changes its form above 100
0
C. PMMA could be
solved in chloroform or dichlorethane. Usually the polymerization of PMMA is taking place by means of radical
polymerization, under UV- light irradiation with use of photo initiators. The presence of free radicals could lead to
chemical reactions with dye molecules, which is the main reason of dye molecule destruction.
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Although the process of preparation of dye doped PMMA gain medium for solid state dye lasers already
contains a reason of dye destruction, easy processing, optical and physical properties make PMMA very convenient
material for experimental studies and creating dye doped solid state active elements for dye lasers.
2. EXPERIMENTAL SETUP AND MATERIALS
The Fig. 2 shows the LC cell containing CLC-R6Gdoped PMAA film-CLC system. The cell consists of two
CLC layers with the thickness of 10µm, and Rhodamine 6G doped PMMA thin film, with the 30 µm thickness.
The system is confined between two glass substrates. For obtaining CLC uniform aligning the glass substrates
were coated with polyvinyl alcohol (PVA), and rubbed in anti-parallel directions. The thicknesses of CLC layers were
given by spacers, see Fig 2. The four sides of the cell were sealed.
Fig. 2 The sketch of the CLC-R6G-CLC multilayer system, g-glass substrate, s-spacers, which kept the thickness of CLC
layers (c) 10 µm, f-R6G doped PMMA film with the 30 µm thickness
The CLC used in experiment was a mixture of 18 wt. % cholesteryl oleate (5-Cholesten-3b-ol 3-Oleate), 32 wt.
% of cholesteryl pelargonate (C
36
H
62
O
2
) and 50 wt. % of commercial Nematic Liquid Crystal E7. The CLC mixture was
heated into the isotropic phase and capillary filled into the cell, containing R6G doped PMMA film on spacers. Then it
was left for two days in the room temperature. The right handed helical CLC structure formed.
The R6G is characterized with high solubility in most known solvents. The R6G doped PMMA film was
obtained by adding R6G into the PMMA dichlorethane solution and spin coated on polyethylene film. After
volatilization of dichlorethane the PMMA film formed and then was separated from polyethylene film. In our experiment
the concentration of R6G in PMMA was about 10
-3
mol/l.
The temperature of the cell was adjusted by a temperature controller which is tuning the temperature of Peltier
thermoelectric module.
For luminescence spectrum study, the Rhodamine 6G doped film was pumped with the 12 mW cw laser, with
the wavelength of the second harmonic of Nd: YAG laser emission (532nm). For sample pumping the oblique angle
geometry of pumping beam incidence was used. The angle between pumping beam and the cell normal was 45⁰, for
obtaining pumping bigger cross section the laser beam was defocused by means of lens, the photoluminescence was
focused on monochromator.
Transmission spectral measurements were carried out by means of UV-VIS spectrophotometer SF-26.
Luminescence spectrum was registered by homemade apparatus, consisting of abbey system monochromator; the
spectrum was registered using CCD camera. The measurements were carried out with image processing special program
working in Lab View environment, which gives the image histogram, along line with given starting and ending
coordinates.
The circularly polarized light was obtained with use of optical wedge, which was acting as a quarter wave plate,
and owing to its wedge form it allows to obtain circularly polarized light for whole visible spectrum. Because of the light
beam size in monochromator, the light passing through the optical wedge was elliptically polarized, with average
elipticity of 0.8.
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2. RESULTS AND DISCUSSION
Transmittance spectra of CLC-R6G doped PMMA film-CLC multilayer system within 450-610 nm wavelength
region was measured in order to study thermal dependences of the system spectral characteristics.
Fig. 3 the transmission spectrum (solid curve) of R6G doped film with the same thickness and concentration of dye as in the
system and the CLC-R6G-CLC system transmission measured with light of different polarizations, at the temperature 28.3⁰ C. We
observe 10 nm of transmittance minimum shift, which corresponds to selective reflection band.
The transmission spectra of R6G doped PMMA 30µm thin film is represented in Fig. 3. As we can see the film
has low transmittance within 410-540 nm wavelength regions, this is due to strong absorption of Rhodamine 6G. In the
same graph the transmission spectra of CLC-R6G doped film-CLC system at
28.3⁰ C is represented, measured using
right, left handed circularly polarized and unpolarized lights.
As can be seen from the graph, low transmission region is matching the same region of R6G doped thin film.
Here we observe interesting result, that transmission minimum of the CLC-R6G-CLC system, measured using LCP and
unpolarized lights is located on 520 nm. Next we can see that the transmission minimum of the system for RCP light is
shifted to the 530 nm. This can mean that the selective reflection band has shifted to the longer wavelength region, with
the rise of the temperature, and results in decrease of transmission of the system. At the same temperature LCP and
unpolarized lights are passing through the sample without any changes for transmittance.
0
10
20
30
40
50
60
70
80
90
100
460 480 500 520 540 560 580 600
Transmission T (%)
Wavelength λ (nm)
T(⁰C)=28.3⁰C Unpol
T(⁰C)=28.3⁰C RCP
T(⁰C)=28.3⁰C LCP
R6G Unpol
0
10
20
30
40
50
60
70
460 480 500 520 540 560 580 600
Transmission T (%)
Wavelength λ (nm)
T(⁰C)=27.1⁰C RCP
T(⁰C)=28.3⁰C RCP
T(⁰C)=29.1⁰C RCP
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Fig. 4 Transmission of CLC-R6G-CLC system measured using RCP light at various temperatures, we observe transmission
minimum 5nm shift with increasing temperature.
Fig. 4 represents the transmission spectra for our system under RCP light, at various temperatures. As could be
seen from graph, transmission spectra minimum at
29.1⁰C is located on 525 nm further increase in shifts the position of the
minimum toward longer wavelength scale about 5 nm.
Because of strong absorption of R6G within 450-580nm and sensitivity of CLC to the change of temperature, it
is very difficult to estimate or measure the width of selective reflection band. The further rise of temperature leads CLC
to the isotropic phase, so no polarization effects could be investigated.
In order to investigate luminescence spectrum of CLC-R6G doped PMMA film-CLC, the system was pumped
with frequency doubled Nd: YAG cw laser, with 532 nm wavelength. Fig. 5 represents the luminescence pattern
obtained from our multilayered system. The luminescence is observed within 540-618 nm wavelength region. The
measurements were carried out at room temperature.
Fig. 5 Photoluminescence of CLC-R6G doped PMMA film-CLC system. Pumping was realized by means of 12 mW cw
laser
The photoluminescence obtained from CLC-R6G doped PMMA film-CLC system is dominated by a narrow
and strong pumping laser irradiation. As can be seen from the figure 5, there is a gap between photoluminescence spectra
and pumping laser beam with wavelength 532 nm.
The important factor for lasing from dye doped CLC system is spectral position of PBG compared
with dye emission spectrum
11
. For achieving lasing with lowest possible threshold from our system it is
important that one of the selective reflection band edges must overlap emission spectrum. In our system this
could be realized with temperature tuning of selective reflection band when selective reflection band is
located within 540-618 nm. When investigating photoluminescence spectrum the room temperature was well
below the temperature when selective reflection band is in visible wavelength region. As we can see from Fig.
3 this temperature is around 28.3⁰ C.
3. CONCLUSIONS
We have experimentally studied transmission spectrum thermal dependence for CLC-R6G doped PMMA film-
CLC multilayer system and observed photoluminescence of R6G.
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The measurements of transmission spectrum at 28.3⁰ C for RCP, LCP and unpolarized lights, show that
transmission minimum corresponds to the selective reflection band.
During experiments we have measured transmission spectrum for RCP light for 460-610 nm visible wavelength
region, at various temperatures of the system. We have observed the 10 nm tuning of transmission minimum, which was
caused by selective reflection band shift, when temperature was varied from 27.1⁰ C to 29.1⁰ C.
We have also observed photoluminescence of our system by pumping R6G with 12 mW cw laser with 532 nm
wavelength.
4. ACKNOLEGMENTS
This work was supported by State Committee of Science Republic of Armenia, grant 11-1c194.
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