Materials Sciences and Applicatio ns, 2011, 2, 116-129
doi:10.4236/msa.2011.22016 Published Online February 2011 (http://www.SciRP.org/journal/msa)
Different Approaches of Employing Cholesteric
Liquid Crystals in Dye Lasers
Guram Chilaya1, Andro Chanishvili1, Gia Petriashvili1, Riccardo Barberi2, Maria Penelope de Santo2,
Mario Ariosto Matranga2
1Institute of Cybernetics, Euli str. 5, Tbilisi, Georgia; 2CEMIF.CAL, INFM CNR-Licryl Lab., Physics Department, University of
Calabria, Rende (Cs), Italy.
Received June 1st, 2010; revised December 9th, 2010; accepted January 28th, 2011.
ABSTRACT
Two ways of employing cholesteric liquid crystals in tunable dye lasers are considered: the cholesterics as distributed
feedback medium and the cholesterics as resonator mirrors. In the dye doped distributed feedback cholesteric liquid
crystal lasers the frequency tuning is achieved exploiting light induced effects or using a specially designed cell assem-
bling a chiral dopant concentration gradient in combination with suitable distribution of different dyes. Another ap-
proach represen ts the lasing in a multila yer system consisting of a dye doped isotropic so lvent sand wiched between two
CLC layers.
Keywords: Liquid Crystals, Lasers
1. Introduction
Cholesteric Liquid Crystals (CLCs) possess several unique
properties: a supramolecular periodic helical structure
due to the chirality of the molecules (the period - spiral
pitch P - can be set in a wide range from 100 nm up to
infinity), 100% selective reflection of circularly polarized
light and the ability to control selective reflection wave-
length, changing external or internal factors (electric,
magnetic and acoustic fields, temperature, local order
[1,2]. The maximum wavelength 0 and the spectral
width  of the selective reflection band are equal to 0
= P n and  = P n accordingly, where n is the average
index of refraction and n is the birefringence of a layer
perpendicular to the helix axis. According to the modern
conceptions the CLCs can be considered as a one dimen-
sional (1D) photonic crystal [3,4]. The existence of the
selective reflection band and the ability to change smooth-
ly the selective reflection wavelength over a wide range
under the action of applied external forces make it possi-
ble to use in tunable dye lasers. The organic dye lasers
have the distinction of being the first broadly tunable
lasers and one of the most versatile coherent light sources.
These lasers provide broad tunability over a spectral
range that covers the ultraviolet (UV) to near infrared
(NIR). Organic dye lasers were discovered in 1966 [5,6].
Any conventional laser needs two necessary conditions
to operate: the presence of a positive feedback and the
presence of an active medium. As feedback elements
usually resonator mirrors are used. The use of a CLC
layer as a mirror in the dye lasers was proposed long time
ago. The temperature dependence of the selective reflec-
tion band was exploited to tune a broad band dye laser
[7].
But there is a class of lasers in which the external
feedback elements are absent and the active medium acts
as a distributed feedback, in this case the active medium
must have a spatial periodic structure. It is important for
development of compact lasers that the major functional
elements of a mirror-less laser are combined in one cell:
active medium, cavity, and tunable selector. There are
different methods to create periodic structures, but the
optimum is reached when the medium itself possesses a
ready periodic structure with required parameters. In this
respect CLCs are among the materials able to be used as
active medium in Distributed Feedback (DFB) lasers - if
the CLC consists of luminescent molecules or it contains
a luminescent dopant, it becomes possible to build a
tunable dye doped (DD) DFB laser. This approach allows
solving one of the most actual problems in laser tech-
nique i.e. the creation of a simply and smoothly tunable
laser.

Different Approaches of Employing Cholesteric Liquid Crystals in Dye Lasers117
In this review paper two methods to obtain DD CLC
DFB laser tuning will be presented. In the first approach,
the frequency tuning is achieved by exploiting light in-
duced effects. In the second method the spatial tuning of
the laser emitted wavelength was distinguished with the
use of a specially designed cell combining a chiral dopant
concentration gradient and a suitable distribution of dif-
ferent dyes. Also lasing in a multilayer system consisting
of a dye doped isotropic solvent sandwiched between
two CLC cells will be considered.
2. Light Tunable DDDFB CLC Lasers
One of the most exciting developments in LC science
and technology is the possibility of using light to control
the parameters of a material [8,9]. We considered low
molar mass calamatic thermotropic CLCs. According to
the chemical nature of the constituent molecules CLC
subdivided in the following ways: 1) Steroids, mainly
cholesterol esters and their mixtures with each other;
from which derives the name CLC; 2) Non-steroidal cho-
lesterics (so-called chiral nematics); 3) Induced choles-
teric systems comprising a nematic matrix and a liquid-
cryastalline or non-liquidcryastalline optically active do-
pant (OAD). It is known that the pitch in induced cho-
lesteric systems depends on the structure of both the
nematic host and the chiral dopant [10-13].
Reversible and irreversible changes in the helical pitch
have been observed in CLC mixtures as a result of pho-
tochemical transformation of conformationally active
components. In these mixtures, either a chiral component
or a nematic component, or both components, can be
photosensitive. Special photosensitive (chiral or achiral)
compounds can be added to photoinsensitive chiral ne-
matic mixtures. It should be noted that non-photosen-
sitive mixture components should be transparent in the
visible and near-UV regions. Investigations of the ab-
sorption edge spectra of nematic liquid crystals with dif-
ferent rigid fragments and bridging groups [14] have
revealed that the nematic liquid crystals based on cyclo-
hexylcyclohexanes are transparent in the visible and UV
spectral regions up to 200 nm (Figure 1).
In our experiments we used mostly the nematic mix-
ture ZLI-1695 (Merck) as a nematic host, which is the
mixture of cyclohexylcyclohexanes
R = C2, C3, C4, C7 with nematic range 13-72˚C.
A large number of papers investigating pitch control in
CLC mixtures have been devoted to systems consisting
Figure 1. Absorption edge of liquid crystals: 1: Cyclohexyl-
cyclohexanes, 2: Phenylcyclohexanes, 3: Esters, 4: Biphen-
yls, 5: Shiff bases. Layer thickness = 10 m.
of conformationally active molecules, which are capable
of light-induced trans-cis isomerization. Typically an
elongated rod-like molecule (trans isomer) transforms
under the influence of UV radiation to a bent or fractured
form (cis isomer).
Azoxy-based (ZhK 440, NIOPIK: (2/3 p-n butyl-p-
methoxyazoxybenzene + 1/3 p-n-butyl-p-heptanoylazo-
xybenzene) LCs are transparent in a wider part of a spec-
trum than azo-compounds (ZhK 286, NIOPIK: p-ethoxy-
phenyl-p0-azophenyl hexanoate) as shown in Figure 3
when a small concentration of the compounds was dis-
solved in a transparent nematic.
In recent years, nematic liquid crystals themselves
have been used as a photoizomerizable component in the
study of the light-induced phenomena in chiral nematic
mixtures. Usually the amount of photoactive material in
these systems is small. More recently, CLC mixtures
with a photoisomerizable nematic component typically
comprising 60-80 wt% of the sample, with the rest of the
sample being a photoinactive chiral compound has been
investigated [15-21]. Different nematic liquid crys- tals
based on azo and azoxy compounds, as well as on cin-
namic acids, which can undergo photoisomerization,
were investigated.
The color change of CLC mixtures under UV irradia-
tion can be used for realization of a solar UV-B irradia-
tion sensor. For example, the mixture based on a photo-
sensitive derivative of cinnamate [22-24] has the absorp-
tion has the spectrum very close to the spectrum of hu-
man skin sensitivity to sunburn (UV-B range: 290-300

Different Approaches of Employing Cholesteric Liquid Crystals in Dye Lasers 118
Figure 2. Scheme of of photoisomerization of azo- and azoxy-benzene compounds.
Figure 3. Absorption spectra of the azo- (ZhK 286) and
azoxy- (ZhK 440) compounds. Figure 4. Absorption spectrum for a mixture with 99% of
an UV transparent nematic and 1% of 4-cyanophenyl ester
of 4' heptylcinnamic acid (Zhk-537, NIOPIK).
320 nm) and thus is able to precisely measure UV irra-
diation dose in accordance with the Erythemal Response
Spectrum. The properties of this sensor are shown in
Figures 4-6.
The idea of tunable lasing in CLCs was expressed in
patent of Goldberg and Shnur [25]. The theoretical ana-
lysis performed by Kukhtarev [26] demonstrated that the
observation of lasing with a distributed feedback in a
cholesteric liquid-crystal mixture containing a lumines-
cent dye requires a pump power of ~105 W/cm2. Il’chishin
et al. [27] were the first to design a dye-doped cholesteric
liquid-crystal laser. More recently, the consideration of
chiral LCs as a photonic gap medium promoted explana-
tion of the appearance of laser emission near the edge of
the selective reflection band [28] and also stimulated the
investigation of this emission. This approach allowed
those authors to explain the observed laser emission at
the edge of the selective reflection band (SRB) in the DD
CLC. Within the band gap, the wave exponentially de-
cays when propagating deep into the crystal and, corre-
spondingly, the density of states in the band gap becomes
considerably lower. Since the degree of spontaneous
emission according to the Fermi law is proportional to
the number of photon states, the spontaneous emission is
suppressed within the band gap and, accordingly, in-
Figure 5. The selective reflection wavelength as a function
of UV radiation time for the mixture 76.5% (50%ZLI-1695
+ 50%Zhk-537) + 23.5%ZLI-6248.
creases at the band edges. A comprehensive analysis of
the CLC as a photonic crystal is given in the review by
Kopp et al. [29].
Tunability of the lasing in these systems can be achi-
eved by the change of chiral dopant concentration, by
temperature variation, by mechanical stress or applying
an electric field.

Different Approaches of Employing Cholesteric Liquid Crystals in Dye Lasers119
Figure 6. Change of color under direct sun irradiation.
(76.5% (25%ZLI1695 + 75%ZhK537) + 23.5% MLC- 6248).
Gold – no UV exposure. Green – 4 min direct sun irradia-
tion. Blue – 8 min irradiation.
In [30] was proposed the tuning of the lasing wave-
length by light. In this pioneering work the controlling of
the generated laser wavelength was carried out by means
of phototransformation of a well known chiral dopant
from Merck ZLI-811 [left-handed (S-811) and right-
handed ZLI-3786 (R-811)] whose light sensitivity was
never considered before. The experimentally observed
phenomenon of photo-transformation of the OAD ZLI-
811 under UV irradiation is explained by the photo-Fries
rearrangement of its chiral molecules, which is usually
observed in complex aromatic esters. The Fries rear-
rangement (Figure 7), which is irreversible, can be in-
duced photo-chemically or chemically [31,32].
The absorption spectrum of ZLI-811 solved in the
transparent nematic ZLI-1695 is shown in Figure 8.
The dependence of the selective transmission on the
irradiation time of ZLI-811 solved in the transparent
nematic mixture ZLI 1184 + ZLI 1185 (cyclohexanes,
Merck) is shown in Figure 9. The samples were irradi-
ated by light from a Mercury lamp with a 240- to 400-
nm filter.
In order to test the phototuning of the laser emission, a
40-mm-thick cell was prepared with a pitch that provides
laser emission at 382 nm, in correspondence of the long
wavelength edge. The cholesteric mixture was prepared
mixing a nematic ZLI-1695 66 wt% and left-hand chiral
dopant ZLI-811 34 wt%. 0.5 wt% of a photoluminescent
dye 4-cyano-4''-decyloxy-p-terphenyl
Figure 7. Photo-fries rearrangemen.
Figure 8. Absorption spectrum of 1% solution of S-811 in
ZLI 1695.
Figure 9. Transmission spectra of the 36% ZLI-1184 + 36%
ZLI-1185 + 28% ZLI-811 mixture irradiated for different
times. 1: Before irradiation, 2: 3 min or irradiation, 3: 7
min, 4: 11 min, 5: 16 min, 6: 21 min, 7: 26 min, 8: 30 min, 9:
34 min, 10: 37 min.

Different Approaches of Employing Cholesteric Liquid Crystals in Dye Lasers 120
was then added to the mixture. This is a liquid crystalline
dye with the following phase sequence: Cr -99˚C - SA -
227˚C = N - 230˚C - I. The material exhibits a violet emis-
sion with a maximum at about 390-400 nm wavlength.
The scheme of experiment is shown in Figure 10.
The pumping with third-harmonic radiation of a Q-
switched Nd:YAG laser (Continuum, Surelite II, 355 nm,
6 ns, 5-10 Hz) led to laser emission, which could be
tuned under exposure to light from a 100 W mercury
lamp with a filter transparent in the range 240-400 nm.
After step-by-step light irradiation from the UV lamp,
shifts of the lasing wavelength were observed. In Figure
11 laser lines for different exposure times are shown. The
total shift encompasses more than 30 nm wavelength
range, from the near UV to the visible. Illuminating the
sample over an hour, the laser emission at the long
wavelength edge of the band gap vanishes. At this stage,
an unstable peak appears on the short edge of the gap,
jumping to the long wavelength edge and back.
Since the publication of this work, there have appeared
a number of papers concerned with phototunable CLC
[33-37]. In [36,37], the reversible tuning was performed
using a trans-cis photoisomerization of the chiral azo
Figure 10. Scheme of the photo tunable CLC laser.
Figure 11. Laser lines from the CLC exposed for several
time durations. Numbers indicate the irradiation time in
minutes.
component of a cholesteric liquid-crystal mixture. Lin et al.
[37] achieved a change in the helical pitch up to 110 nm
through exposure to light from an ultraviolet lamp for 20
min. However, the reverse relaxation transition to the
initial trans state was observed only within 20 hours.
Note that the high light sensitivity of azo and azoxy
compounds made it possible to use Light-Emitting Di-
odes (LEDs) instead of a mercury lamp in order to con-
trol the helical pitch of CLCs. The reversible tuning of
the lasing wavelength in CLCs containing nematicazo- or
azoxy- photo-sensitive components was accomplished by
varying the time of irradiation of samples with the use of
two LEDs: at 405 nm wavelength for blue shift and at
466 nm wavelength for red shift [38]. The second har-
monic signal from a Nd:YAG laser was used as light
source for pumping the cholesteric resonators. The pulse
wavelength, width, and repetition rate were 532 nm, 4 ns,
and 1 Hz, respectively.
The investigations indicate that in spite of the fact that
the azo compounds were more sensitive to light irradia-
tion, and that the isomerization processes were faster for
these compounds than for the azoxy compounds, the
azoxy materials were more suitable for use in lasing de-
vices. Furthermore, azoxybenzenebased liquid crystals
are transparent over a broader region of the electromag-
netic spectrum than azobenzene compounds, which may
enable lasing at short wavelengths: for these measure-
ments, a small amount (not more than 2%) of the com-
pound was dissolved in a transparent nematic matrix.
Finally, the azoxy-compounds were less sensitive than
the azo-compounds, and also exhibited better optical
properties. The shift in the lasing wavelength for differ-
ent exposure times is shown in Figure 12 for the mixture
99.5% (71%ZhK-440 + 29%MLC-6247) + 0.5%DCM;
where ZhK-440 is an azoxy-component and the dye is
DCM-p-dimetylaminostyryl 4H-pyran, by Exciton.
These experiments demonstrate LED-controlled re-
versible tuning in both directions, from longer to shorter
wavelengths and vice versa, of a DD CLC laser. The use
of LEDs to induce photo-transformations and laser tun-
ing represents a very simple but relevant innovation over
similar light-controlled tunable CLC laser systems that
use UV lamps, primarily because of the possibility of
incorporating such small devices inside novel compact
laser systems.
3. Spatially Tunable DD CLC DFB Lasers
An alternative approach to the generation of laser radia-
tion tunable over a wide range of wavelengths in DD
CLCs based on preparation a cell with gradient of the
helical pitch was proposed in [39]. The pitch gradient
was produced by filling of the cell with a cholesteric liq-
uid-crystal mixture having a concentration gradient of

Different Approaches of Employing Cholesteric Liquid Crystals in Dye Lasers121
(a)
(b)
Figure 12. Lasing spectra of a 99.5% (71%ZhK-440 + 29%
MLC-6247) + 0.5 %DCM mixture upon exposure to a 405-
nm light emitting diode (a) and after subsequent exposure
to radiation of a 466-nm light-emitting diode (b) Numbers
indicate the irradiation time in minutes.
chiral dopants. In this way one achieves a continuous
tuning of laser emission only moving the CLC cell with
respect to the position of the pump laser beam. These
experiments have been carried out in CLC mixtures with
the nematic ZLI-1695, the chiral dopant ZLI-811, and a
terphenyl derivative as dye. Two mixtures with different
concentrations of the chiral dopant were prepared: the
first one with SRBin red – near-infrared range and the
second one with SRB in near UV-violet range. The start-
ing empty cell was half filled by capillarity with the first
mixture and the filling was then completed using the
second mixture. The pitch gradient was stable for a few
months. In order to test the laser emission tuning, the cell
was placed on a translational stage that let the sample
move orthogonally to the exciting laser beam. The laser
action was then investigated at several sample positions.
Figure 13 shows the lasing spectra of the DD CLC ob-
tained for tiny displacement of the cell, at 2 µJ excitation
energy source from the third harmonic of a Nd:YAG
laser at pulse wavelength 355 nm. The total shift encom-
passes more than 30 nm, from about 393 nm to 427 nm.
The tuning range of laser radiation was limited by the
width of the dye emission band. The tuning range can be
broadened with the use of the second dye whose absorp-
tion band coincides with the luminescence band of the
first dye. In this case, there occurs an energy transfer,
which is known as the Förster transfer [40]. In [41] the
range of laser wavelengths was extended with the use of
a specially designed cell by help of developed method of
preparing a cell containing several dyes. The CLC mix-
ture contained the following six dyes:
D1 4-cyano-4’-decyloxy-p-terphenyl (ICMA Univer-
sidad de Zaragoza, Spain);
D2 2,5-bis (5-tert-butyl-1, 3-benzoxazol-2-yl) tiophene
[41];
D3 N- (3-metoxypropyl) -4 (3-metoxypropylamino) -1,
8-naphtalimide [42];
D4 2,3,5,6-1H, 4H-tetrahydro-8-methylquinolizino- (9,
9a, 1gh) (Coumarin 102 from Exciton);
D5 9- (4-decylxoycarbonylphenylethynyl) -10-(4-ethy-
loxyphenylethynyld antracene (ICMA Universidad de
Zaragoza, Spain);
D6 (4-Dicyanomethylene-2-methyl-6-(p-dimethylami-
nostyryl) -4H-pyran (DCM from Exciton).
The dyes are selected in order to cover a wide wave-
length range from UV to visible, and employ the same
pump source. D1 has been used for UV luminescence;
D2 for violet luminescence, and also in Förster energy
transfer combined with D3 for blue-green luminescence.
D4 and D5 have been used in Förster transfer for green
Figure 13. Wavelength tuning of DD CLC laser obtained by
shifting the cell with concentration gradient.

Different Approaches of Employing Cholesteric Liquid Crystals in Dye Lasers 122
demonstrate a possibility of designing compact DD CLC
lasers quasicontinuously tunable over a wide range of
wavelengths.
luminescence. Finally, D6 is used for yellow, orange, and
red luminescence. The total luminescence band of these
dyes covered the range from 373 to 684 nm. As seen
from Figure 14 the luminescence spectrum of the blue-2
dye overlaps well the absorption spectrum of the green
dye.
Recently, lasing in DFB CLC cells with spatial helical
pitch gradient obtained in different ways was shown. A
spatially tunable laser emission from DD CLC cell with
one-dimensional temperature gradient was proposed in
[43]. The lasing wavelength was widely tunable from
577 to 670 nm by shifting the position of the dye-doped
As a CLC the mixture of the nematic liquid crystal
ZLI-1646 and the optically active dopant MLC-6247
both from Merck was employed. The Nitrogen laser (337
nm, 4 ns, 1-10 Hz) was used as a pump source. Figure
15 shows the lasing spectra of the dye doped CLC at
several pitches corresponding to different cell positions,
obtained by translation. The total shift covers more than
300 nm wavelength range, from 370 to 680 nm.
In conclusion, tuning of a DD CLC mirror-less laser
has been obtained in one single sandwich cell with sev-
eral mixtures dye-CLC combined in order to cover al-
most completely the wavelength range from UV to red,
also making a suitable cholesteric pitch gradient inside
the cell. The relevant aspect of the method is that a sim-
ple translation of the cell with respect to a single exciting
beam enables fine tuning of the laser wavelength in a
very large spectrum range. Thus, the obtained results
Figure 14. The absorption and emission spectra of blue sen-
sitizer (D2) with solid lines and green emitter (D3) with
dashed lines.
350 400 450 500 550 600 650 700
0.25
0.50
0.75
1.00
Pump
/nm
I/a.u.
Figure 15. Some laser lines from the DD CLC obtained shifting the cell with respect to the pumping beam.

Different Approaches of Employing Cholesteric Liquid Crystals in Dye Lasers123
CLC cell with respect to the pumping beam. In the other
case by introducing short-pitch CLC and Coumarin dye
from the left side of an empty cell and long-pitch CLC
and two dyes (Coumarin and DCM) from the right side
simultaneously a structure with helical pitch and dye
concentration gradients was created [44]. Using the För-
ster energy transfer between two dyes the lasing from the
structure continuously covered full visible range from
470 nm to 670 nm. The tuning was performed by dis-
placement of the cell with respect to the pumping beam.
Also a spatial pitch gradient in CLC with two dyes (För-
ster couple) with a concentration gradient was created by
means of a photo-controlled effect [45]. In this case posi-
tion-sensitive laser emission covering the visible region
from 480 nm to 670 nm was obtained.
4. Dye Lasers with CLC Mirrors
One of the main drawbacks of CLC lasers, limiting their
technological application is their low stability. This is
connected with two phenomena occurring under the in-
fluence of a powerful pumping: gradual deformation of
the CLC layer planar orientation and degradation of the
luminescent dye molecules. The problem of dye mole-
cules degradation is common to conventional dye lasers
as well and, to solve it, the dye solvent is circulating con-
tinuously through the laser chamber, avoiding the satura-
tion effects from pumping. Since in DD CLC lasers it is
impossible to make the dye circulate separately from the
CLC structure, a rotating cell was suggested to avoid
deformation of the CLC texture and bleaching of the dye
due to the high energy of the pumping beam [46]. This
rotating mechanism dramatically increased the stability
of the emitted laser beam from several minutes up to
several hours.
Several attempts were made to optimize the lasing
conditions and performance characteristics of DD CLC.
Enhanced laser emission is demonstrated by incorporate-
ing a single external CLC reflector in a DD CLC laser
[47].
Another way to enhance the lasing property of DD
CLC systems is the introduction of defects in the choles-
teric structure. In this case lasing is observed inside the
band gap. The introduction of a defect into the CLC can
be achieved in two ways: replacing a part of the host me-
dium with a material that has a different dielectric con-
stant e.g. two layers of CLC sandwich a thin layer of an
isotropic medium [48]; introducing a phase jump inside
the CLC cell [49]. The introduction of a defect and the
tuning of photonic defect modes in the CLC, by means of
the local deformation of the helix in the middle of the
CLC layer, were considered theoretically in [50]. Ex-
perimentally the defect mode emission was investigated
mostly in polymeric CLCs. Defect mode lasing was ob-
served in a DD cholesteric polymeric network [51],
where the defect was produced by a phase jump of the
cholesteric helix at the interface of two staked layers of
polymer film.
The lasing in three layers cell where dye solution is
sandwiched between two CLCs can be considered as
defect mode type. Diffrerent dye solutions were investi-
gated: in three layered structures where between two
CLC layers with the same pitch and handedness a dye
doped nematic LC [52] or a dye doped CLC [53] was
sandwiched. Also the defect mode type lasing was stud-
ied when between dielectric multilayers a dye doped
CLC [54] or a dye doped nematic LC [55] was sand-
wiched. It should be noted that the lasing threshold ob-
served with the three-layered helical CLC is lower than
for conventional DD CLC lasers [54,56].
In this paragraph the investigations of lasing in a mul-
tilayer system consisting of a dye doped isotropic solvent
sandwiched between two CLC cells will be considered
[57-60]. The separation of the CLC and the active me-
dium allows: 1) To avoid the degradation of the CLC
structure caused by the absorption of the pumping energy,
2) To use dyes not soluble in LCs and 3) To use the opti-
mal thickness both for the CLC layer and for the dye
solution layer (thicker dye layer and thinner CLC layer).
CLC mixture was prepared by mixing a nematic MLC-
6816 (Merck, cyclohexylcyclohexanes) with right handed
chiral MLC-6248 (Merck). Not soluble in LCs Rhoda-
mine-6G was used as a dye and glycerol was used as an
isotropic solvent. In Figure 16 the scheme of three layer
structure is shown.
The second harmonic of a Q-switched Nd: YAG laser
(Continuum, Surelite II) was used as a pumping light
source. The pulse wavelength, width, and repetition rate
were 532 nm, 4 ns, and 1 Hz, respectively.
At the beginning of investigations we employed three
layered structures with CLC layers possessing equal
pitches [57]. In this case typical defect mode lasing was
expected. Indeed, as shown in Figure 17 multimode
lasing inside the stop band with several emission peaks
was observed.
To achieve single-mode lasing the CLC cells were filled
with two distinct cholesteric mixtures, whose pitches
were shifted in such a way that only the edges of the
band gaps overlapped. In Figure 18 the transmission
spectra of each CLC layer, the spectra of dye solution
emission and lasing in this cell are shown. A single mode
lasing occurs in the overlapping part of CLC band gaps.
In contrast with the conventional dye lasers with usual
dielectric mirrors, in this kind of CLC lasers the wave-
length of lasing is strongly connected with the CLC
pitches. By choosing other CLC pitches, one obtains
lasing at another wavelength.

Different Approaches of Employing Cholesteric Liquid Crystals in Dye Lasers 124
Figure 16. Scheme of the sandwich cell.
Figure 17. Transmission spectrum of the cholesteric (1),
fluorescence of the dye (2) and lasing spectrum from the
sandwich cell (3).
Figure 18. Transmission spectra of two cholesterics with
different pitches (1 and 2), fluorescence of the dye (3) and
lasing spectrum from the sandwich cell (4).
Besides, Rhodamine-6G, another dye, Stilbene-420,
whose absorption and emission peaks are located in UV
and violet ranges, was exploited as well. In this case, the
CLC pitches were set to get lasing in violet spectrum
range and a Nitrogen laser (337 nm) was used for pump-
ing. Multimode lasing in the structures with equal CLC
pitches and single mode one in the shifted pitch configu-
ration were observed, in violet spectrum range and a ni-
trogen laser (337 nm) was used for pumping. Multimode
lasing in the structures with equal CLC pitches and single
mode one in the shifted pitches configuration were ob-
served confirming the general behavior.
The next step of our investigations was obtaining the
wide range tunable laser emission in this multilayer sys-
tem [58-60]. Two approaches for this purpose were ex-
ploited: spatially tunable and temperature tunable systems.
In the first case the assembled sandwich cell was
composed by a first cholesteric layer containing a mate-
rial with a well defined pitch and a second cholesteric
layer containing chiral materials with different pitches.
The CLC layer with invariable pitch contains a wide
band gap material while the second layer consists of nar-
row band gap material with spatially changing pitch. The
parallel combination of these two layers forms a spatially
tunable optical resonator. A different laser wavelength is
emitted from different regions of the cell under the
pumping beam irradiation. To obtain the wide photonic
band gap layer, BL-006 or alternatively BL-090, with ∆n
approximately 0.3, as nematic compounds and MLC-
6248 as optically active dopant have been used. To pre-
pare the second cholesteric layer, MLC-6816 as a ne-
matic compound, ZLI-3786 as a photosensitive chiral
compound and RM-257 as photo- polymer (all the above
materials supplied by Merck), the Irgacure 2100 (Ciba)
as photoiniziator, as an isotropic solvent glycerol and as
luminescent dyes ADS680HO (American Dye Source),
not soluble in liquid crystals have been used.
The pitch gradient across the cell with narrow band
gap was achieved using two different methods for dif-
ferent CLC mixtures. In first case to the nematic MLC-
6816 was doped with a chiral compound ZLI-3786 whose
chemical structure is identical to the one of ZLI-811.
Like ZLI-811 the mixture NLC + ZLI-3786 undergoes a
photo-Fries transformation [30,39] if irradiated at wave-
lengths shorter than 300 nm. To create a pitch gradient
within the CLC layer containing ZLI-3786, distinct re-
gions of the layer have been exposed to UV for different
times using a customized mask. The addition to the mix-
ture of a photopolymer and a photoiniziator is needed to
stabilize the cholesteric structure. The used illumination
times have been from 1 up to 5 minutes, to have the
helical pitch varying continuously within the second
cholesteric layer. Figure 19 shows the transmission spec-

Different Approaches of Employing Cholesteric Liquid Crystals in Dye Lasers125
trum for both wide and narrow band gap cells.
In order to test the laser tuning, the cell has been
placed on a translation stage and the sample has been
moved with respect to the pumping laser beam in the
plane of the CLC layer. Using the dye ADS680HO a laser
emission in the near infrared at 790 nm has been ob-
served for the first time. The laser emission wave-length
shift obtained shifting the cell is shown in Figure 20.
The second strategy to obtain the modulation of the
photonic band gap position of the second chiral layer
relies on the use of a series of CLC mixtures in which the
chiral compound concentration changes. Three mixtures
with different concentrations of the optically active
Figure 19. Transmission spectra of the wide band gap cho-
lesteric (1) 75%BL-006 + 25%MLC-6248 and of the narrow
band gap cholesteric mixture 99% (74%MLC-6816 + 26%
ZLI-3786) + 1% (99%RM-257 + 1%Irgacure 2100) after 0
min (2), 1 minute (3), 2 min (4), 3 min (5), 4 min (6) and 5
min (7) of exposure to a UV lamp.
Figure 20. Wavelength tuning of the three-layer laser ob-
tained by shifting the sample.
dopant have been prepared. The cell has been partially
filled, by capillarity, with one of the three mixtures; fill-
ing has been then completed using in sequence the other
two mixtures. After assembly, different wavelengths of
the visible range are selectively reflected by the whole
system. The tuning of the laser emission, from UV to
near IR, was observed in different cells using different
luminescent dyes. Figure 21 shows the wavelength tun-
ing of laser emission obtained from three different cells.
Luminophore OF(mb) (ICMA Universidad de Zaragoza,
Spain) used in the first cell is an oligomer with chiral
2-(S)-methylbutyl pendant chains [61].
Also we studied the temperature controllable tuning of
lasing in these three layered structures. Owing small bi-
refringence (n) and high temperature sensitivity and
variety of temperature dependence of the pitch the mix-
tures of cholesteryl ethers are suitable to obtain tempera-
ture tunable single mode lasing. Several years ago J.
Adams, W. Haas, and J. Wysocki invesigated tempera-
ture and concentration characteristics of the pitch of two-
component cholesteryl chloride and a different carbox-
ylic ester [62]. Varying concentration of components in
these mixtures, the spectral range, the sign of dp/dt and
slope of dp/dt can be controlled. To obtain tunable lasing,
a two component mixture: cholesteryl chloride (24%) +
cholesteryl pelargonate (76%) with CLC temperature
range 30˚-75˚ and positive temperature dependence of the
pitch (dp/dt > 0) was selected. In this mixture the selec-
tive reflection appears in the region from 560 nm to 600
nm. A three layer cell, where the central part filled with a
solution of glycerol and Rhodamine 6G, was sandwiched
between two external layers of CLC, was assembled as
shown in Figure 22.
Figure 21. Wavelength tuning of laser emission from three
different cells filled with solution of glycerol and the lu-
minophore OF(mb) (1), Rhodamine 6G (2), and LDS698 (3).

Different Approaches of Employing Cholesteric Liquid Crystals in Dye Lasers 126
Figure 23 shows the transmission of the cholesteric
mixture and luminescence spectrum of 0.4% Rhodamine
6G solution in glycerol.
To obtain the laser effect and the tuning of the emitted
Figure 22. Sketch of the three layer cell in the set-up for
temperature tuning of the laser.
Figure 23. Transmission of the cholesteric mixture [24%
cholesteryl chloride + 76% cholesteryl pelargonate] (dashed
line) and luminescence spectrum of 0.4% Rhodamine-6G
solution in glycerol (solid line).
Figure 24. Tuning of the laser peaks obtained by changing
the temperature of the three layer structure.
wavelength, the three layer cell was fitted in a thermal
stage (Figure 22) and placed in front of the pumping
beam. The pumping light source was a Nitrogen laser
(VSL-337ND-S). The pulse wavelength, width, and repe-
tition rate were 337 nm, 4 ns, and 1-10 Hz, respectively.
The tuning of the emitted laser wavelength was achieved
by varying the temperature of the entire cell. Increasing
the temperature from 40˚C to 73˚C, a shift of the laser
peak of about 40 nm towards longer wavelengths was
observed (Figure 24).
To summarize, we have demonstrated recent advances
in obtain widely tunable lasing from ultraviolet wave-
lengths, through the visible range, to infrared wave-
lengths by employing cholesteric liquid crystals in dye
lasers [63].
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