Advances in Materials Physics and Chemistry, 2011, 1, 20-25
doi:10.4236/ampc.2011.12004 Published Online September 2011 (http://www.SciRP.org/journal/ampc)
Random Laser of R6G Dye and TiO2 Nanoparticles
Doped in PMMA Polymer
Baha T. Chiad1, Kamil H. Latif 2, Firas J. Kadhim1, Mohammad. A. Hammed1
1Department of Physics, College of Science, University of Baghdad, Baghdad, Iraq
2Ministry of Environment, Baghdad, Iraq
Received July 29, 2011; revised August 3, 2011; accepted September 5, 2011
Abstract
The random laser (RL) based on organic Rhodamine 6G (R6G) laser- dye and Titanium dioxide (TiO2) sus-
pended nanoparticles have been prepared with polymethylmethacrylate (PMMA) as a host. Both liquid and
spray-coated homogeneous film samples of 22.4 µm - 30.1 µm thickness range were use. Optimum concen-
trations have been determined depending on the normal fluorescence spectra which give evidence that the
laser dye provides amplification and TiO2 nanoparticles as scatter center. At the optimum concentrations,
results of the random laser (RL) under second harmonic Nd: YAG laser excitation show that the values of
bandwidth at full width half-maximum (FWHM) and the threshold energy are about 9 nm and 15 mJ respec-
tively, which represent the minimum value for the liquid samples in the current research. Correspondly, these
values become 14 nm and 15 mJ for film sample. The broadening that can be attributed to the concentration
quenching of a laser dye at high concentration levels has been observed.
Keywords: Random Laser, Random Gain Media, Laser Resonator
1. Introduction
Random lasers (RL) are unique sources of stimulated
emission in which the feedback is provided by scattering
in a gain medium [1,2]. Random laser effects have been
observed in a variety of organic and inorganic gain me-
dia including powders of solid-state luminescent and
laser crystals [3,4], liquid laser dyes with scatterers [5],
polymeric films with and without intentionally intro-
duced scatterers [6], ZnO scattering films and nanoclu-
sters [7], dye-infiltrated opals [8], porous media infil-
trated with liquid crystals with dyes [9] and many others.
Random lasers are very attractive for a variety of appli-
cations, low coherent random laser sources can be ad-
vantageous in holography, laser inertial confinement
fusion (driver sources for megajoule lasers), transport of
energy in fibers for medical applications, and other app-
lications, detailed reviews of random lasers can be found
in [10,11]. Random lasers are strongly scattering media
that amplify light. There are striking similarities between
these systems and more conventional lasers based on a
gain medium enclosed in a cavity with two mirrors to
provide optical feedback. An example is the observation
of a threshold for lasing action and frequency narrowing
in random lasers. Evidently, the optical properties of
random lasers are quite different from those of conven-
tional lasers: the propagation of pump and fluorescence
light is diffusive in a random laser. In contrast with cav-
ity systems, scattering is actually advantageous. Since
feedback is provided by multiple scattering, the random
laser threshold is lowered by a stronger scattering, i.e., a
shorter transport mean free path, because the feedback is
more efficient. It has been shown that the threshold in
random lasers is reduced dramatically when the photon
transport mean free path approaches the stimulated emis-
sion wavelength [12].
On other hand, gain narrowing denotes a decrease of
the width of the spectrum of the emitted light triggered
by an increase in the pump fluence. The width will be
characterized by the at full width half maximum (FW
HM). Gain narrowing is observed in all laser systems
[13]. In a random laser the FWHM of the spectrum of the
emitted light below the threshold of the laser is approxi-
mately the width of the emission spectrum of the gain
medium (typical 40 nm) for R6G. However, far above
threshold, this FWHM can be as narrow as 10 nm. A
measure for the gain narrowing is the narrowing factor
NF, defined as the FWHM of the emitted light below

21 B. T. CHIAD ET AL.
threshold (FWHM below) divided by the FWHM of the
emission spectrum of a random laser far above threshold
(FWHM above) [14,15].
In this work, RL based on mixtures of suspended TiO2
nanoparticles of different concentrations and were mixed
with R6G and the polymer PMMA was used as a host
[16], and both the TiO2 and R6G concentrations were
diluted in its corresponding solvent down to 10–6 mol/l.
From fluorescence measurements of the above concen-
trations, it was noticed that a TiO2 concentration of 10–3
mol/l had the highest intensity and the narrowest band-
width for both liquid and film samples. At this optimum
TiO2 nanoparticles concentration, the emission intensity
spectra at different Nd:YAG pumping energies were in-
vestigated to determine the lasing threshold [17].
2. Experimental
2.1. Chemicals and Preparation
Rhodamine 6G laser dye (C28H31N2O3Cl) with molecular
weight 479.02 g/mol supplied by Lambda Physics LC
(5900): Ethanol alcohol (C2H5OH) with spectroscopic
grade purity supplied by Gainland Chemical Company
and chloroform (CH3CCl3) with spectroscopic grade pu-
rity supplied by Philip Harris chemical company UK.
Poly-Methyl methyleacrylate (PMMA) with chemical
forms (CH2CH3COOCH3), supplied by fulka (Switzer-
land), and used as a host for laser dye and nanoparticles.
Titanium dioxide (TiO2, nanoparticles 50 nm) of Anatase
crystal structure was acquired from Dupont Inc. The
mean particle size of TiO2 suspension is 48.7 nm, which
is prepared as film, determined by electron microscope.
The solutions of laser dye prepared by dissolving the
required amount of the dye in ethanol. The concentra-
tions of dye solutions were: 10–3, 10–4, 10–5 and 10–6
mol/l. The powder of TiO2 was suspended in chloroform
and its concentration was estimated based on the known
weight fraction of TiO2 in the suspension. The mixing
volume ratio of R6G 10–5, 10–6 mol/l: TiO2 10–3, 10–4,
10–5, 10–6 mol/l: PMMA 10–2 mol/l is 3:4:3 ml for the
samples in liquid phase. Spray pyrolysis technique used
to prepare the film samples. Figures 1 (a)-(b) shows the
photographs of some prepared samples.
Figure 2 illustrates a scanning electron microscope
(SEM) topography image for the film sample. The thick-
ness of coating in three different regions of the film as
approximately the same giving indication of the homo-
geneity of the prepared film.
2.2. Experimental Setup
Figure 3 illustrates the experimental setup of RL meas-
urement. The liquid samples were placed in cuvettes
length of 1 cm, and width of 0.5 cm. The pumping source
Gold coating
(a) (b)
Figure 1. Photos image of some prepared samples; (a) liq-
uid phase, (b) films.
Figure 2. Image of SEM for measuring thickness sample as
film.
of RL is random polarized Q-switched Nd: YAG 2nd
harmonic generation (λpump = 532 nm, with a pulse width
of ~6 ns, repetition rate of 6 Hz, and focal spot size of ~4
mm). A monochromator and photomultiplier tube was
used, successively, to select and detect the emission sig-
nals. The liquid sample was stirred at about 5 minutes
before recording the spectrum in order to prevent TiO2
nanoparticles from excessive precipitation. On the other
hand, cylindrical lens was used to extend the laser spot
along the film sample and thus, enabling a precise meas-
urement of the emission signals by monochromator and
photomultiplier tube. The emission spectra ware record-
ed at different gradually-increasing pumping energies.

B. T. CHIAD ET AL.
22
Analyzing
Monochrometer
Sample
PMT
X-Y recorder
H.V power
supply
SHG
Nd: YAG
Figure 3. Schematic diagram of RL experimental setup.
3. Result and Dissection
The fluorescence spectra of the liquid samples at 10–5
mol/l R6G and different concentrations of TiO2 are
illustrated in Figure 4. It is obvious that the maximum
wavelength is occurred at 562 nm with not significant
shift. The R6G dye solution of 10–5 mol/l concentration
with the mentioned concentrations of TiO2 suspension
gives the optimum results. In this case, high intensity and
hence narrow bandwidth at FWHM (18 nm) were
observed comparing with 40 nm in the case of without
TiO2 in accordance to the inset of Figure 4. Likewise,
minimum value of 20 nm bandwidth at FWHM for 10–6
mol/l of R6G and of TiO2 10–3 mol/l of was registered.
The latter value is less than that of 45 nm in the case of
without TiO2. These results are agreement with the
published data in reference [18].
Figure 5 shows the fluorescence spectra of film
samples that prepared at 10–3 mol/l R6G and the same
mentioned concentration of TiO2. It can be seen that 25
nm is the minimum bandwidth at FWHM, for λmax of 568
nm, for the film sample derived at concentration 10–3
mol/l of both R6G and TiO2, according to the inset of
Figure 5.
The bandwidth values at FWHM were calculated by
applying the following equation (1):
effectedI dI
p


   (1)
where Δλeffected bandwidth values Ip peak intensity Iλ
wavelength intensity.
We can also find the values of bandwidth at full width
0
20
40
60
80
100
120
140
160
180
200
220
240
260
280
530540550560570580 590600 610
Wavelength (nm)
Fluores ce nce Intensity (a r b.. uint)
TiO2: 10^-3 M
TiO2: 10^-4 M
TiO2: 10^-5 M
TiO2: 10^-6 M
0
20
40
60
80
100
120
140
160
180
200
220
240
260
280
520 540 560 580 600620
Wavelength (nm)
Fluorescence Intensity (arb. uint)
R6G 10^-5 with T iO 2 10^-3
R6G 10^-5 with ou t TiO2
Figure 4. The fluorescence spectra of liquid sample with
10–5 mol/l R6G and 10–3, 10–4, 10–5, 10–6 mol/l hosted by
polymer PMMA.
0
20
40
60
80
100
120
140
520570 620
Wavelength (n m)
Fluorescence Intensity (arb. uint)
TiO2:10^-3 M
TiO2:10^-4 M
TiO2:10^-5 M
TiO2:10^-6 M
0
20
40
60
80
100
120
140
520 540560 580 600620640
W av e length (n m)
Fluorescence Intensity (arb. uint)
R6G 10^-3 with TiO 2 10^-
3
R6G 10^-3 with out TiO 2
Figure 5. The fluorescence spectra of film samples with 10–3
mol/l R6G and 10–3, 10–4, 10–5, 10–6 mol/l hosted by polymer
PMMA.

23 B. T. CHIAD ET AL.
half-maximum (FWHM) using Excel program that
calculates the area under the approach and distribute
them to the highest value of intensity.
These results suggest preliminary that concentrations
of both R6G and TiO2 (in liquid and film samples) are
the optimum for both amplification and multiscattering
processes in this type of random laser. The minimum
bandwidth gives an indication that these processes are
performed in parallel without noticeable effect on the
dye response, Thus, achieving one of the important
conditions of RL system.
Figure 6 shows the emission spectra, obtained from
RL setup, at different pumping energies for 10–5 mol/l
R6G: TiO2 10–3 mol/l in the case of liquid sample. The
transition centered at wavelength approximately of 562
nm, as can be seen from normal fluorescence in Figure 4,
and RL threshold is approximately 15 mJ as can be de-
termined from Figure 7.
Figure 6. Emission spectra of liquid sample: 10–5 mol/l
R6G:10–3 mol/l at different pumping energies.
0
2
4
6
8
10
12
14
16
18
20
0510 15 2025 30 35 40 45
Pump en ergy (mJ)
FWHM(nm)
-2
2
6
10
14
18
22
26
30
Intens it y(ar b. uint
)
Figure 7. Random laser threshold determined from the re-
lation of maximum intensity and FWHM at different of
pumping energies for a liquid sample : 10–5 mol/l R6G:
TiO2 10–3 mol/l.
Figure 8. Emission spectra of film sample: 10–3 mol/l
R6G:10–3 mol/l at different pumping energies.

B. T. CHIAD ET AL.
24
At small pumping intensity, only spontaneous emi-
ssion can be observed which is characterized by the
maximum spectral bandwidth value at λ = 562 nm and at
(FWHM) of about 15 nm. With increasing the pumping
energy up to 40 mJ, a much narrower peak with the same
maximum wavelength is found to be about ~9 nm at
FWHM. The same behavior is observed for the liquid
sample of 10–6 mol/l R6G:10–3 mol/l TiO2. In this case,
RL threshold is the same value as in the previous con-
centrations, but the spectrum bandwidth is at λ max =
558 nm and at (FWHM) is about ~18 nm. Up to 40 mJ
pumping energy, a much narrower peak with the same
maximum wavelength is found to be about ~10 nm at
FWHM.
The most important difference between the liquid
samples, as shown in Figures 6-8 respectively, is that for
the film sample the bandwidth at FWHM is about 22 nm
at 15 mJ pumping energy with λmax 568 nm and at 40 mJ
pumping energy the bandwidth at FWHM becomes 14
nm.
It has been noticed that the present emission spectra of
film samples show broadening compared with liquid
samples. This may be attributed to the concentration
quenching at 10–3 mol/l R6G which reduces emission
intensity bandwidth broadening at about 6 nm.
4. Conclusions
Two types of RL were synthesized via chemical method
and spray coating technique. The statistical spectroscopic
studies of the concentrations of both R6G dye and TiO2
scatter centers were achieved giving an indication about
the optimum required concentrations. The results of RL
measurements show that the minimum bandwidth at
FWHM is about ~9 nm at 40 mJ for the liquid sample at
10–5 R6G mol/l and TiO2 10–3 mol/l. Comparatively, a 14
nm bandwidth at also 40 mJ was observed for homoge-
nously prepared spray-coated film sample at 10–3 R6G
mol/l and TiO2 10–3 mol/l, these results are not far away
from the ones reported elsewhere [4,6,12,17,18].
5. Acknowledgements
We would like to appreciate the valuable technical assis-
tance of Dr. Hani J. Kbashe, Dr. Qahtan G. AL-Zaidi and
Dr. Raied K. Jamil at the optics research center of the
physics department. Also, the financial support of the
collage of science, Baghdad University is acknowledged.
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