Enhancing the Performance of Random Lasers
Effects of Localised Surface Plasmons and Resonance Energy Transfer
Judith M. Dawes
1,2
, Wan Zakiah Wan Ismail
1,2,3
, Ewa M. Goldys
1,4
and David W. Coutts
1,2
1
MQ Photonics, Department of Physics and Astronomy, Macquarie University, Balaclava Road, Sydney, 2109, Australia
2
ARC Centre of Excellence Centre for Ultrahigh-bandwidth Devices for Optical Systems, Sydney 2109, Australia
3
Faculty of Science and Technology, Islamic Science University of Malaysia, Nilai, 71800, Negeri Sembilan, Malaysia
4
ARC Centre of Excellence Centre for Nanoscale BioPhotonics, Sydney, 2109, Australia
Keywords: Random Lasers, Localised Surface Plasmons, Nanoparticles.
Abstract: We investigate the effect of different gain media and different scattering media in random lasers. We
demonstrate an increase in the emission intensity and efficiency of random lasing by incorporating gold rather
than dielectric alumina nanoparticles. There is a trade-off between enhancing the random laser performance
due to the localised surface plasmon resonance field effects and reduction in performance due to fluorescence
quenching by the gold nanoparticles. We use fluorescence resonant energy transfer between dye molecules to
extend the wavelength range of emission.
1 INTRODUCTION
Random lasers offer interesting behaviour compared
with standard (resonant cavity-based) lasers as they
incorporate highly scattering materials combined
with optical gain (Wiersma, 2008). Thus the laser
output typically has reduced coherence, and broader
linewidth than for a comparable standard cavity-
based laser (Wan Ismail et al., 2014). However, these
lasers exhibit typical threshold behaviour and
spatially distributed emission, with a relatively broad
angular range of emission. The lasers can be used in
a range of applications including as broadband – low
coherence sources for Optical Coherence
Tomography, and for probing the properties of the
scattering materials themselves (such as in
biosensing). Here, we describe methods to enhance
the emission intensity or reduce the threshold of
random lasers based on dye solutions with metal and
dielectric scattering particles. We employ
combinations of dyes to extend the available range of
emission wavelengths from these systems.
2 EXPERIMENTS
The random laser solutions were prepared in
methanol, using specified concentrations of titania
nanoparticles (average diameter 200 nm, Sigma
Aldrich), alumina nanoparticles (average size of 150
nm, Sigma Aldrich) and gold nanoparticles (average
diameter 60 nm, Ted Pella) (Wan Ismail et al., 2015).
For the random lasers based on fluorescence
resonance energy transfer, titania, (200 nm, 1×10
11
cm
-3
) is added to a methanol solution of Rhodamine
6G (5×10
-4
to 2×10
-3
M) and Oxazine 17 (5×10
-4
M)
(Sigma Aldrich). Random lasers based on localized
surface plasmon effects incorporated gold
nanoparticles at varying concentrations (from 3 x 10
9
to 1 x 10
12
cm
-3
) with methanol solutions of
rhodamine dyes (10
-3
M). The random laser
suspensions were ultrasonically mixed to break up
aggregates immediately before the laser experiments.
For the laser experiments, the suspensions were
placed in a 1 cm quartz cuvette containing a piece of
teflon to suppress back-face reflections.
The pump source for the experiments was a Q-
switched frequency-doubled Nd:YAG laser operating
at 10 Hz pulse repetition rate with 4 ns pulse width.
The front face of the random laser cuvette was
irradiated at an incident angle of 30
o
to the normal to
the front face with the pump light focussed using a 10
cm focal length lens to produce an excitation area of
1 mm diameter at the cuvette. The emission light at
an angle of 45
o
to the cuvette’s front face was
collected by a lens and delivered to a fibre-coupled
spectrometer (Ocean Optics USB2000 + UV-VS-ES
160
Dawes, J., Ismail, W., Goldys, E. and Coutts, D.
Enhancing the Performance of Random Lasers - Effects of Localised Surface Plasmons and Resonance Energy Transfer.
DOI: 10.5220/0005744001580161
In Proceedings of the 4th International Conference on Photonics, Optics and Laser Technology (PHOTOPTICS 2016), pages 160-163
ISBN: 978-989-758-174-8
Copyright
c
 2016 by SCITEPRESS – Science and Technology Publications, Lda. All rights reserved

with ~1 nm spectral resolution).
3 RESONANCE ENERGY
TRANSFER
3.1 Extended Operating Wavelength
Range of Random Dye Lasers
Figure 1: The peak emission intensity of Oxazine/
Rh6G/titania random lasers with different dye ratio
combination; (a)1:1 (b)1:2 (c)1:4. The lasing threshold is
observed for the oxazine emission peak when the emission
intensity increases nonlinearly with the pump energy
density.
Using combinations of fluorescent dyes, we can
design effective energy transfer from a donor
molecule to an emitter molecule by ensuring strong
spectral overlap of the emitter absorption spectrum
with the emission spectrum of the donor (Cerdan et
al., 2012). This can extend the operational
wavelength of a laser for example (Alee et al., 2013).
We have investigated various dye combinations and
concentrations to explore the efficiency of the energy
transfer. Here we demonstrate near-IR emission
beyond 700 nm from a 532 nm pump laser via
resonant energy transfer from Rhodamine 6G to
Oxazine dye, as an example.
Random lasing emission beyond 700 nm is
achieved by combining two dyes with scatterers.
Oxazine dye is a less efficient dye for random lasers
excited with 532 nm green light but with the addition
of Rhodamine 6G (Rh 6G), Oxazine can reach laser
threshold. We obtain the most efficient transfer in the
weakly scattering regime (with a scattering length of
order 1 mm).
Figure 1 shows laser operation for a series of
random lasers with Rhodamine 6G (5 x 10
-4
to 2×10
-
3
M) and oxazine (5×10
-4
M) incorporating titania
nanoparticles (1×10
11
cm) as scatterers. The dye
mixtures are in the ratios: 1:1, 1:2 and 1:4 for a
mixture of Oxazine: Rh 6G. Figure 2 shows emission
spectra from the lasers below and above laser
threshold.
Figure 1 shows that the random laser threshold for
Oxazine emission reduces gradually when the dye
ratio increases. Increasing the concentration of Rh 6G
increases the Oxazine lasing threshold due to the
competition between Rh 6G emission and Oxazine
emission. By providing high gain to the Rh 6G, the
laser is driven to emit at the Rh 6G emission
wavelength and this disrupts the energy transfer
process to the Oxazine, leading to an increased lasing
threshold.
4 LOCALISED SURFACE
PLASMON RESONANCE
4.1 Random Lasers based on
Rhodamine / Gold Compared with
Rhodamine / Alumina
When metallic (gold) nanoparticles are added to
random lasers incorporating Rhodamine 640 or
Rhodamine 6G dyes, we observe a combination of
resonance energy transfer with localized field
enhancement which leads to increased emission
a) 1:1OxazinetoRh6G
b
)1:2OxazinetoRh6G
c)1:4OxazinetoRh6G
Oxazineemissionpeak
Rh6Gemissionpeak
Enhancing the Performance of Random Lasers - Effects of Localised Surface Plasmons and Resonance Energy Transfer
161

intensity and decreased laser threshold (Sen et al.,
2007). However if there is strong spectral overlap of
the gold particles with the dye, excitation of the gold
nanoparticles quenches the dye fluorescence,
reducing the emission intensity. This may be
overcome using specific nanoparticles or dye
combinations (Xiaoyu et al., 2013).
Figure 2: The emission spectra of Rh 6G (2×10
-3
M) with
oxazine (5×10
-4
M) and titania (1×10
11
cm) random lasers
below (3 mJ/cm
2
) and above (8 mJ/cm
2
) laser threshold.
Table 1 shows the lasing thresholds for Rh 640 / gold
and Rh 6G / gold random lasers, indicating that the
lowest lasing threshold occurs for 8×10
10
cm
-3
of gold
nanoparticle concentration in both cases. The
threshold increases in the weakly scattering regime
for gold nanoparticle concentrations below this
concentration, and also increases in the diffusive
scattering regime above this concentration. For
similar particle concentrations (3×10
10
cm
-3
), the
lasing threshold of the Rh 640 / gold random lasers is
~18 mJ/cm
2
, considerably lower than that for Rh 640
/ alumina random lasers (~43 mJ/cm
2
), shown in
Table 2. Likewise, the lasing threshold of the Rh 6G
/ gold random lasers (~21 mJ/cm
2
) is also
considerably lower than that for Rh6G / alumina
random lasers (~40 mJ/cm
2
).
The decrease of the lasing threshold in these
Rhodamine random lasers with the addition of gold
nanoparticles is attributed to enhanced fluorescence
from localised surface plasmon effects. Emission
from both the Rhodamine dyes is absorbed by the
gold nanoparticles. This leads to a concentration of
the local electric field induced by excited surface
plasmons at the gold nanoparticle surfaces. The
excitation at the particle surface gives a concentrated
local electric field with increased absorption by the
dye and thus more amplification. The increase in the
lasing threshold for higher gold nanoparticle
concentrations is attributed to fluorescence
quenching of the dye molecules due to non-radiative
surface energy transfer from the excited dye to gold
nanoparticles.
The scattering mean free path, l
s at a pump
wavelength of 530 nm for random dye lasers with
alumina ranges from 0.35 mm, in the diffusive
scattering regime (sample size > l
s > λ) to 11.6 mm,
in the weakly scattering regime (l
s ⩾ sample size).
The lasing threshold decreases when the scattering
mean free path is decreased. In the Rh 640 / alumina
and Rh 6G / alumina random lasers, the lasing
threshold reduces with increasing alumina
concentration and there is no additional loss for high
alumina concentrations.
Table 1: Lasing threshold for Rh 640 and Rh 6G (10
-3
M)
random lasers with various concentrations of gold
nanoparticles.
Gold
nanoparticle
concentration
(cm
-3
)
Rh 640 Laser
threshold
mJ/cm
2
Rh 6G Laser
threshold
mJ/cm
2
3x 10
9
27 28
3x10
10
18 21
8x10
10
13 15
3x10
11
21 28
1x10
12
26 32
Table 2: Lasing threshold for Rh 640 and Rh 6G (10
-3
M)
random lasers with various concentrations of alumina
nanoparticles.
Alumina
nanoparticle
concentration
(cm
-3
)
Rh 640 Laser
threshold
mJ/cm
2
Rh 6G Laser
threshold
mJ/cm
2
3x 10
9
No lasing No lasing
3x10
10
43 40
3x10
11
35 33
1x10
12
31 24
In the weakly scattering and diffusive scattering
regimes, the scattering length l
s for a wavelength of
530 nm for random dye lasers with gold nanoparticles
varies from 0.42 mm to 111.1 mm, respectively. This
is roughly as large as the size of the lasing region.
5 CONCLUSIONS
We demonstrate that random laser performance can
be improved with addition of moderate
concentrations of metallic nanoparticles (substituting
for dielectric nanoparticle scatterers), and by
selecting appropriate dyes for efficient radiative
energy transfer. We examine the effect of localised
surface plasmons and Fluorescence Resonance
500 550 600 650 700 750 800
0.0
0.2
0.4
0.6
0.8
1.0
Normalized emission intensity
Wavelength(nm)
Below threshold
Above threshold
PHOTOPTICS 2016 - 4th International Conference on Photonics, Optics and Laser Technology
162

Energy Transfer (FRET) on random laser
performance incorporating Rhodamine dyes with
alternative dyes and metallic nanoparticles.
ACKNOWLEDGEMENTS
Funding and support by the authors’ university and
the Australian Research Council Centres of
Excellence Program CE140100003 and CE11E0091
is gratefully acknowledged.
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Enhancing the Performance of Random Lasers - Effects of Localised Surface Plasmons and Resonance Energy Transfer
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