Real time random laser properties of Rhodamine-doped di-ureasil hybrids
Edison Pecoraro,1 Sara García-Revilla,2 Rute A. S. Ferreira,3
Rolindes Balda,2,4 Luís D. Carlos3, and Joaquín Fernández2,4* 1Instituto de Telecomunicações, Universidade de Aveiro, 3810 −193 Aveiro, Portugal
2Departamento de Física Aplicada I, Escuela Técnica Superior de Ingeniería, Alda. Urquijo s/n 48013 Bilbao, Spain 3Departamento de Física, CICECO, Universidade de Aveiro, 3810-193 Aveiro, Portugal
4Centro de Física de Materiales CSIC-UPV/EHU and Donostia International Physics Center, Apartado 1072, 20080
San Sebastián, Spain
Abstract: This investigation explores, for the first time, the random laser
behavior of ground powder obtained from organic-inorganic hybrid
materials based on Rhodamine 6G incorporated into a di-ureasil matrix. The
experimental results, both in the spectral and temporal domains, obtained by
pumping with picosecond laser pulses, show the existence of efficient
random laser emission in this system. Finally, the random laser performance
is compared with the one of other Rhodamine-doped solid state silica
compounds.
©2010 Optical Society of America
OCIS codes: (140.3380) Laser materials; (300.6500) Spectroscopy, time-resolved; (290.4210)
Multiple scattering; (320.7090) Ultrafast lasers.
References and links
1. R. Reisfeld, “Prospects of sol-gel technology towards luminescent materials,” Opt. Mater. 16(1-2), 1–7 (2001).
2. C. Sanchez, B. Lebeau, F. Chaput, and J. P. Boilot, “Optical Properties of Functional Hybrid Organic-Inorganic
Nanocomposites,” Adv. Mater. 15(23), 1969–1994 (2003).
3. S. Y. Lam, and M. J. Damzen, “Characterisation of solid-state dyes and their use as tunable laser amplifiers,”
Appl. Phys. B 77(6-7), 577–584 (2003).
4. O. García, L. Garrido, R. Sastre, A. Costela, and I. García-Moreno, “Synthetic strategies for hybrid materials to
improve properties for optoelectronic applications,” Adv. Funct. Mater. 18(14), 2017–2025 (2008).
5. D. Avnir, D. Levy, and R. Reisfeld, “The nature of the silica cage as reflected by spectral changes and enhanced
photostability of trapped Rhodamine-6G,” J. Phys. Chem. 88(24), 5956–5959 (1984).
6. R. Reisfeld, D. Brusilovsky, M. Eyal, E. Miron, Z. Burstein, and J. Irvi, “A new solid-state tunable laser in the
visible,” Chem. Phys. Lett. 160(1), 43–44 (1989).
7. E. T. Knobbe, B. Dunn, P. D. Fuqua, and F. Nishida, “Laser behavior and photostability characteristics of
organic-dye doped silicate gel materials,” Appl. Opt. 29(18), 2729–2733 (1990).
8. J. C. Altman, R. E. Stone, B. Dunn, and F. Nishida, “Solid-state laser using a Rhodamine-doped silica-gel
compound,” IEEE Photon. Technol. Lett. 3(3), 189–190 (1991).
9. T. H. Nhung, M. Canva, F. Chaput, H. Goudket, G. Roger, A. Brun, D. D. Manh, N. D. Hung, and J. P. Boilot,
“Dye energy transfer in xerogel matrices and application to solid-state dye lasers,” Opt. Commun. 232(1-6), 343–
351 (2004).
10. S. Grandi, C. Tomasi, P. Mustarelli, F. Clemente, and C. M. Carbonaro, “Characterisation of a new sol-gel
precursor for a SiO2-Rhodamine 6G hybrid class II material,” J. Sol-Gel Sci. Techn. 41, 57–63 (2007).
11. G. Valverde-Aguilar, “Photostability of laser dyes incorporated in formamide SiO(2)ORMOSILs,” Opt. Mater.
28(10), 1209–1215 (2006).
12. V. de Zea Bermudez, L. D. Carlos, and L. Alcácer, “Sol-gel derived urea cross-linked organically modified
silicates. 1. Room temperature mid-infrared spectra,” Chem. Mater. 11(3), 569–580 (1999).
13. D. C. Oliveira, A. G. Macedo, N. J. O. Silva, C. Molina, R. A. S. Ferreira, P. S. André, K. Dahmouche, V. de
Zea Bermudez, Y. Messaddeq, S. J. L. Ribeiro, and L. D. Carlos, “Photopatternable di-ureasil-zirconium
oxocluster organic-inorganic hybrids as cost effective integrated optical substrates,” Chem. Mater. 20(11), 3696–
3705 (2008).
14. C. M. S. Vicente, E. Pecoraro, R. A. S. Ferreira, P. S. André, R. Nogueira, Y. Messaddeq, S. J. L. Ribeiro, and L.
D. Carlos, “Waveguides and gratings fabrication in zirconium-based organic/inorganic hybrids,” J. Sol-Gel Sci.
Technol. 48(1-2), 80–85 (2008).
#122573 - $15.00 USD Received 11 Jan 2010; revised 9 Mar 2010; accepted 14 Mar 2010; published 25 Mar 2010(C) 2010 OSA 29 March 2010 / Vol. 18, No. 7 / OPTICS EXPRESS 7470
15. C. Molina, P. J. Moreira, R. R. Gonçalves, R. A. S. Ferreira, Y. Messaddeq, S. J. L. Ribeiro, O. Soppera, A. P.
Leite, P. V. S. Marques, V. de Zea Bermudez, and L. D. Carlos, “Planar and UV written channel optical
waveguides prepared with siloxane poly(oxyethylene)-zirconia organic-inorganic hybrids. Structure and optical
properties,” J. Mater. Chem. 15(35-36), 3937–3945 (2005).
16. L. D. Carlos, R. A. S. Ferreira, V. de Zea Bermudez, and S. J. L. Ribeiro, “Lanthanide-Containing Light-
Emitting Organic-Inorganic Hybrids: A Bet on the Future,” Adv. Mater. 21(5), 509–534 (2009).
17. E. Stathatos, P. Lianos, U. L. Stangar, and B. Orel, “Study of laser action of Coumarine-153 incorporated in sol-
gel made silica/poly(propylene oxide) nanocomposite gels,” Chem. Phys. Lett. 345(5-6), 381–385 (2001).
18. D. C. Oliveira, Y. Messaddeq, K. Dahmouche, S. J. L. Ribeiro, R. R. Gonçalves, A. Vesperini, D. Gindre, and J.-
M. Nunzi, “Distributed feedback multipeak laser emission in Rhodamine 6G doped organic-inorganic hybrids,”
J. Sol-Gel Sci,” Techn. 40, 359–363 (2006).
19. V. S. Letokhov, “Stimulated emission of an ensemble of scattering particles with negative absorption,” JETP
Lett. 5, 212–215 (1967).
20. D. S. Wiersma, “The physics and applications of random lasers,” Nat. Phys. 4(5), 359–367 (2008).
21. M. A. Noginov, Solid-State Random Lasers, (Springer, Berlin, 2005).
22. H. Cao, “Lasing in random media,” Waves Random Media 13(3), R1–R39 (2003).
23. S. Mujumdar, V. Turck, R. Torre, and D. S. Wiersma, “Chaotic behavior of a random laser with static disorder,”
Phys. Rev. A 76(3), 033807 (2007).
24. S. John, and G. Pang, “Theory of lasing in a multiple-scattering medium,” Phys. Rev. A 54(4), 3642–3652
(1996).
25. D. S. Wiersma, and A. Lagendijk, “Light diffusion with gain and random lasers,” Phys. Rev. E Stat. Phys.
Plasmas Fluids Relat. Interdiscip. Topics 54(4), 4256–4265 (1996).
26. X. Jiang, and C. M. Soukoulis, “Time dependent theory for random lasers,” Phys. Rev. Lett. 85(1), 70–73
(2000).
27. A. L. Burin, M. A. Ratner, H. Cao, and R. P. H. Chang, “Model for a random laser,” Phys. Rev. Lett. 87(21),
215503 (2001).
28. C. W. Lee, K. S. Wong, J. D. Huang, S. V. Frolov, and Z. V. Vardeny, “Femtosecond time-resolved laser action
in poly(p-phenylene vinylene) films: stimulated emission in an inhomogeneously broadened exciton
distribution,” Chem. Phys. Lett. 314(5-6), 564–569 (1999).
29. G. Zacharakis, G. Heliotis, G. Filippidis, D. Anglos, and T. G. Papazoglou, “Investigation of the laserlike
behavior of polymeric scattering gain media under subpicosecond laser excitation,” Appl. Opt. 38(28), 6087–
6092 (1999).
30. D. Anglos, A. Stassinopoulos, R. N. Das, G. Zacharakis, M. Psyllaki, R. Jakubiak, R. A. Vaia, E. P. Giannelis,
and S. H. Anastasiadis, “Random laser action in organic-inorganic nanocomposites,” J. Opt. Soc. Am. B 21(1),
208–213 (2004).
31. S. García-Revilla, J. Fernández, M. A. Illarramendi, B. García-Ramiro, R. Balda, H. Cui, M. Zayat, and D. Levy,
“Ultrafast random laser emission in a dye-doped silica gel powder,” Opt. Express 16(16), 12251–12263 (2008).
32. S. García-Revilla, J. Fernández, R. Balda, M. Zayat, and D. Levy, “Real-time spectroscopy of novel solid-state
random lasers,” Proc. SPIE 7212, K1–K11 (2009).
33. S. García-Revilla, M. Zayac, R. Balda, M. Al-Saleh, D. Levy, and J. Fernández, “1Low threshold random lasing
in dye-doped silica nano powders,” Opt. Express 17(15), 13202–13215 (2009).
34. C. Kim, D. V. Martyshkin, V. V. Fedorov, and S. B. Mirov, “Middle-infrared random lasing of Cr2+ doped ZnSe,
ZnS, CdSe powders, powders imbedded in polymer liquid solutions, and polymer films,” Opt. Commun.
282(10), 2049–2052 (2009).
1. Introduction
Solid-state dye laser’s research has attracted much attention during the last two decades,
because of technical and economical advantages in comparison with classical liquid dye
lasers, e.g. more compact, non-toxic, non-volatile, non-flammable and mechanically stable.
The main research has been devoted to the development of material hosts which disperse well
the dye molecules protecting them efficiently from photobleaching effects, and for which
laser efficiency and operational photostability are high [1–4]. Therefore, the interest in dyes
containing organic-inorganic hybrids has grown considerably during the last fifteen years
since first reports demonstrated the possibility of applying of these materials to solid-state
dye-lasers [5–8]. The potential of these materials relies on the possibility of fully exploiting
the synergy between the laser action of organic dyes and the intrinsic characteristics of sol–
gel derived hosts, namely the excellent optical quality, the low processing temperature (< 200
°C) which allows the incorporation of functional dye molecules into the hybrid matrix and the
f of large amounts of emitting dyes isolated from each other and protected by the hybrid host
[9–11].
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Among the various organic-inorganic hosts that have been developed in the last years, the
so-called di-ureasils – in which the hybrid framework is composed of poly(ethylene oxide)
chains of variable molecular weight grafted at both ends to a siloxane backbone through urea
functionalities [12] – present acceptable transparency, mechanical flexibility and thermal
stability to be processed both as thin films and as transparent and shape controlled monoliths
with potential applications in integrated optics (IO) for optical telecommunications
(essentially passive long haul components). In particular, some of us have recently
demonstrated the use of di-ureasils as cost effective IO substrates, namely in the production of
patternable gratings, channels and monomode planar waveguides with low propagation losses
(< 0.3 dB/cm) and Fabry-Perot cavities [13–15]. However, until now active IO components
based on organic dyes containing di-ureasils had not been strictly reported, despite the ability
of the hybrid host to easily encapsulate large amounts of light emitting centers (organic dyes
or trivalent lanthanide ions [16]). In fact, Coumarine-153 was incorporated in an analogous
hybrid host, the organic counter-part of which is formed by low-molecular weight poly
(propylene oxide) chains [17] and Rhodamine 6G (Rh6G) was embedded in di-ureasils
containing metacrylate modified zirconium propoxide [18]. Whereas laser action was reported
in the former material, distributed feedback laser emission was demonstrated in the latter
hybrids.
Since first proposed by Letokhov in 1967 [19], lasing in random media has become a
subject of intense theoretical and experimental studies due to the important scientific and
technological implications of this new research field [20]. Random lasing has been observed
in a wide range of scattering systems such as solutions of microparticles dispersed in a laser
dye, neodymium doped crystal powders, ceramic and polymeric systems, semiconductor
nanoparticles, organic tissues, liquid crystals, etc (see Refs [20–22]. and references therein).
The nature and morphology of each amplifying disordered medium determine their specific
feedback mechanism and random laser behavior [21–23], making it difficult to study and
compare all the previously mentioned systems by means of a unique theoretical treatment
[24–27]. Only recently, the real-time behavior of solid state random laser systems both in the
spectral and temporal domains has been investigated [28–34]. Here we explore the random
laser behavior of the above mentioned organic-inorganic hybrid compounds based on Rh6G
incorporated into the di-ureasil host. The experimental results obtained both in the spectral
and temporal domains are compared with those already obtained in the ground powder of a
silica gel containing dye-doped silica nanoparticles. Account taken of the important physical
properties found in this hybrid compounds, we believe that the experimental random laser
results presented here pave the way for future applications of this kind of materials.
2. Experimental
2.1 Sample preparation
The reagents O,O′-Bis(2-aminopropyl) polypropylene glycol-block-polyethylene glycol-
block-polypropylene glycol (Fluka), commercially known as Jeffamine-ED600®, average
molecular weight 600 g.mol−1
, 3-isocyanatepropyltriethoxysilane (ICPTES) (Aldrich 95%),
ethyl alcohol absolute P.A. (Carlo Erba), tetrahydrofuran P.A. (stabilized - Riedel-de Haën),
HCl (ACS Reagent 37% - Sigma-Aldrich) and Rhodamine 6G hydrochloride 95% (Sigma)
were used as received. The di-ureasil host, termed as d-U(600), contains 8.5 (OCH2CH2)
polymer chains with both ends grafted to a siliceous network by means of urea linkages. The
cross-links between the organic and the inorganic components were formed by reacting the
NH2 groups of Jeffamine-ED600® with the –N=C=O group of ICPTES, in THF, under
magnetic stirring and reflux at 80 °C for 18 h. The non-hydrolyzed d-U(600) precursor was
isolated after complete THF evaporating at 45 °C in rotary bench evaporator. A solution of
Rh6G chloride in 1 mL of ethanol was incorporated into the di-ureasil host, under magnetic
stirring. Two Rh6G molar concentrations, 3.75 mM and 7.50 mM, were used. The Rh6G
#122573 - $15.00 USD Received 11 Jan 2010; revised 9 Mar 2010; accepted 14 Mar 2010; published 25 Mar 2010(C) 2010 OSA 29 March 2010 / Vol. 18, No. 7 / OPTICS EXPRESS 7472
solutions were added to 3 g of d-UPTES after 15 min. The suspensions were kept under
magnetic stirring for 15 min at room temperature. Just after, 300 µL of HCl 2M were added to
shift the pH from 9 to 2, the pH region that speeds up the time for sol-gel transition. The
suspensions were then cast into a polystyrene mould (1×1×3 cm) and left to gel, which
happened within 3 min. After gelation, the mould was covered by Parafilm® and kept at room
temperature for 24 h. Then the cover was removed and a three-step heat treatment at 40 °C
(72 h), 50 °C (24 h) and 60 °C (24 h) was performed to eliminate residual solvents (including
ethanol and water produced by polycondensation). The final volumes were not significantly
affected by shrinkage process (less than 5%). The two samples prepared were denoted by d-
U(600)@R6G/3.75, and d-U(600)@R6G/7.50.
2.2. Experimental techniques
The random laser behavior found in the d-U(600)@R6G/3.75 and d-U(600)@R6G/7.50
samples was studied by using the frequency doubled output (532 nm) of a 10Hz, Q-switched
Nd: YAG laser as the excitation source. Details about the experimental technique can be
found elsewhere [31]. Note that both samples were ground by using a mixer mill. The
resulting ground powder was compacted in a quartz cell with no front window for handling
ease and optical characterization. The use of ground powder increases the multiple scattering
efficiency favoring the laser-like emission generation.
Room-temperature emission decay curves were recorded on a Fluorolog TCSPC
spectrofluorometer coupled to a TBX-04 photomultiplier tube module (950 V), 200 ns time-
to-amplitude converter and 70 ns delay. The exciting source was a Horiba-Jobin-Yvon pulsed
diode (NanoLED-390, peak at 388 nm, 1.2 ns pulse duration, 1 MHz repetition rate, and 150
ns synchronization delay).
The absolute emission quantum yields of d-U(600)@R6G/3.75 and d-U(600)@R6G/7.50
samples were measured at room temperature using a quantum yield measurement system
C9920-02 from Hamamatsu with a 150 W Xenon lamp coupled to a monochromator for
wavelength discrimination, an integrating sphere as sample chamber and a multi-channel
analyzer for signal detection. The reported value is the average of three measurements
performed for each sample. The method is accurate within 10%.
3. Results and discussion
The relevant random lasing properties of d-U(600)@R6G/3.75, and d-U(600)@R6G/7.50
samples such as pump power dependence of the emission spectra, emission kinetics, and
laser-like emission threshold were studied in the ground powder after picosecond optical
pumping (pump pulse duration of 40 ps) in single shot measurements. On the other hand, the
lifetimes of the excited state and emission quantum yields of both samples obtained under
Xenon lamp excitation are shown in Table 1.
Table 1. Rh6G excited state lifetime and emission quantum yield of the studied samples
Sample τ (ns) 388 nm Quantum Yield (%)
(380 nm) d-U(600)@R6G/3.75 8.15±0.03 70 d-U(600)@R6G/7.50 7.51±0.04 53
Figure 1(a) shows the normalized emission spectra of the ground powder of d-
U(600)@R6G/3.75 obtained with excitation pulse energies of 10.3, 14.7, 20.7, 24.5, and 103
µJ/pulse. At the lowest excitation energy the emission spectrum shows the broad fluorescence
band of Rh6G centered at 582 nm. However, when the pump energy increases the emission
linewidth is significantly reduced which reveals the appearance of laser-like emission. In
particular, at 103 µJ/pulse the broad tails of the photoluminescence are completely suppressed
and only the gain-narrowed peak survives (orange line in Fig. 1(a)). Figure 1(b) shows the
#122573 - $15.00 USD Received 11 Jan 2010; revised 9 Mar 2010; accepted 14 Mar 2010; published 25 Mar 2010(C) 2010 OSA 29 March 2010 / Vol. 18, No. 7 / OPTICS EXPRESS 7473
effective emission linewidth max
( )=
eff
I d
I
λ λλ
∆ ∫
collapse from 54 nm to 13 nm obtained in
this sample upon increasing the pump pulse energy. From these experimental data, a laser
threshold (defined as the energy value above which a suddenly drop of the spectral linewidth
is observed) of around 17 µJ/pulse was found in the ground powder of d-U(600)@R6G/3.75.
In addition, Fig. 1(a) shows that the spectral position of the laser-like emission peak of this
sample is redshifted as the excitation energy is increased. This emission feature was also
observed in previously studied Rh6G doped silica powders [33].
Fig. 1. (a) Normalized emission spectra of the ground powder of d-U(600)@R6G/3.75
obtained at 10.3 µJ/pulse (red), 14.7 µJ/pulse (blue), 20.7 µJ/pulse (green), 24.5 µJ/pulse
(black), and 103 µJ/pulse (orange). (b) Pump energy dependence of the emission linewidths.
The temporal characteristics of the pulse emitted from the ground powder of d-
U(600)@R6G/3.75 were also investigated. Figure 2(a) shows the normalized emission decays
obtained at different excitation energies. Note that the wavy pattern of these curves is due to
the typical response of the fast photodiode used to perform this set of experiments. At 1.8
µJ/pulse, the emission is spontaneous and the output pulse duration is limited by the Rh6G
lifetime in d-U(600)@R6G/3.75 (7.2 ns). Nevertheless, as can be observed when comparing
the emission decays obtained at 1.8, 4.5, 13.3, 16.9 and 29.5 µJ/pulse, a marked shortening of
the pulse profile is found when increasing the pump pulse energy. In particular, the full width
at half maximum (FWHM) of the time profile was reduced down to 400 ps approximately
above the onset of laser-like action. This minimum time-width corresponds to the actual time
resolution of the detection system which was used in the experimental set-up. Therefore, our
real temporal width might be much narrower.
#122573 - $15.00 USD Received 11 Jan 2010; revised 9 Mar 2010; accepted 14 Mar 2010; published 25 Mar 2010(C) 2010 OSA 29 March 2010 / Vol. 18, No. 7 / OPTICS EXPRESS 7474
Fig. 2. (a) Normalized temporal profiles of the ground powder of d-U(600)@R6G/3.75
obtained at 1.8 µJ/pulse (point line), 4.5 µJ/pulse (dashed line), 13.3 µJ/pulse (dash-dot line),
16.9 µJ/pulse (thin full line), and 29.5 µJ/pulse (thick full line). (b) FWHM of the temporal
profiles as a function of the pump pulse energy.
The spectral and temporal emission characteristics of the ground powder of d-
U(600)@R6G/7.50 were also studied as a function of the pump pulse energy in order to
investigate the effect of the Rh6G dye concentration on the previously described random laser
phenomenon. Figure 3(a) compares the normalized emission spectra of this sample measured
at 11, 18, 20, 24, and 100 µJ/pulse. As it can be clearly seen, a very similar behavior to the
one depicted in Fig. 1(a) for d-U(600)@R6G/3.75 was found. In particular, the laser
thresholds estimated from the excitation energy dependence of their emission linewidths
(Figs. 3(b) and 1(b)) are almost the same. Therefore, the increase of dye concentration has no
significant effect on the onset of laser-like emission. In addition, Fig. 3(a) shows the redshift
of the laser-like emission peak found in d-U(600)@R6G/7.50 as a function of the pump pulse
energy. Note that if compared with the emission spectra of d-U(600)@R6G/3.75 (Fig. 1(a)),
the ones measured in d-U(600)@R6G/7.50 (Fig. 3(a)) appear 2 nm shifted towards smaller
wavelengths.
Fig. 3. (a) Normalized emission spectra of the ground powder of d-U(600)@R6G/7.50
obtained at 11 µJ/pulse (red), 18 µJ/pulse (blue), 20 µJ/pulse (green), 24 µJ/pulse (black), and
100 µJ/pulse (orange). (b) Pump energy dependence of the emission linewidths.
#122573 - $15.00 USD Received 11 Jan 2010; revised 9 Mar 2010; accepted 14 Mar 2010; published 25 Mar 2010(C) 2010 OSA 29 March 2010 / Vol. 18, No. 7 / OPTICS EXPRESS 7475
Concerning the temporal emission dynamics of the ground powder of d-
U(600)@R6G/7.50, Fig. 4(a) shows the normalized emission decays obtained at 1.6, 4.8, 7.5,
11.2 and 25.3 µJ/pulse and the resulting reduction of the pulse duration. It is worthy to notice
the smaller Rh6G lifetime value found in this sample by pumping at 1.63 µJ/pulse (point line),
i.e., well below the laser threshold, if compared to that of d-U(600)@R6G/3.75 sample (6 and
7.2 ns, respectively, which in turn, are a little lower than the ones shown in Table 1 for the
same samples obtained by pumping with a pulsed diode at 388 nm). This lifetime quenching
evidences the increasing contribution of non-radiative de-activation channels when the
amount of Rh6G dye is enhanced. On the other hand, the reduction of the output pulse
duration found as a function of the pump energy is depicted in Fig. 4(b). Also in this case, a
minimum time-width around 400 ps was obtained.
Fig. 4. (a) Normalized temporal profiles of the ground powder of d-U(600)@R6G/7.50
obtained at 1.6 µJ/pulse (point line), 4.8 µJ/pulse (dashed line), 7.5 µJ/pulse (dash-dot line),
11.2 µJ/pulse (thin full line), and 25.3 µJ/pulse (thick full line). (b) FWHM of the temporal
profiles as a function of the pump pulse energy.
We have also explored how the random laser performance (in terms of laser threshold and
slope efficiency) of this new di-ureasil hybrids compares with the ground powder of silica
gels containing Rh6G-SiO2 nanoparticles previously reported by some of us [33]. As an
example, Fig. 5(a) compares the spectral narrowing found in the ground powder of d-
U(600)@R6G/3.75 and in the ground powder of a silica gel containing 4 wt% Rh6G-SiO2
nanoparticles by using the same experimental conditions. This plot evidences the larger onset
of laser-like emission of d-U(600)@R6G/3.75 (around 17 and 7 µJ/pulse, respectively). On
the other hand, Fig. 5(b) shows the integrated emission intensity of their corresponding
emission spectra as a function of the pump pulse energy. As can be observed, the slope
efficiency of the ground powder of the silica gel containing 4 wt% Rh6G-SiO2 nanoparticles
is slightly larger. Finally, from the linear fits of these experimental data, laser thresholds of 17
and 11 µJ/pulse are found in the ground powder of d-U(600)@R6G/3.75 and in the ground
powder of the silica gel containing 4 wt% Rh6G-SiO2 nanoparticles, respectively. These
values are in good agreement with the laser threshold estimation given above.
#122573 - $15.00 USD Received 11 Jan 2010; revised 9 Mar 2010; accepted 14 Mar 2010; published 25 Mar 2010(C) 2010 OSA 29 March 2010 / Vol. 18, No. 7 / OPTICS EXPRESS 7476
Fig. 5. (a) Spectral narrowing of the ground powders of d-U(600)@R6G/3.75 (red dots) and
bulk silica gel containing 4 wt% Rh6G-SiO2 nanoparticles (blue triangles). (b) Integrated
intensity of the emission spectra of these samples as a function of the pump pulse energy. The
red and blue lines represent the linear fits of the corresponding experimental data.
4. Summary and conclusions
Random laser-like effects such as spectral narrowing, emission intensity increase and pulse
shortening have been observed and characterized in the ground powder of d-
U(600)@R6G/3.75 and d-U(600)@R6G/7.50. Both spectral and temporal domains show
parallel behaviors as a function of the pump pulse energy. Moreover, a very similar random
laser behavior is obtained regardless the Rh6G-concentration. The laser threshold value of
both samples is around 17 µJ/pulse.
The lifetime value of the Rh6G dye in d-U(600)@R6G/7.50 is smaller than in d-
U(600)@R6G/3.75. This lifetime quenching suggests the presence of additional non-radiative
de-activation channels in the former case. This hypothesis is confirmed by the quantum
efficiency measurements shown in Table 1.
The emission spectra found in the ground powder of d-U(600)@R6G/3.75 are slightly red-
shifted with respect to the ones of d-U(600)@R6G/7.50. On the other hand, the laser-like
emission peaks of the ground powder samples are redshifted when increasing the excitation
energy.
A first comparison between the emission features of the ground powder of the d-
U(600)@R6G/3.75 di-ureasil and those of a silica gel containing 4 wt% of Rh6G-SiO2
nanoparticles reveals that a slightly larger slope efficiency and lower threshold for laser-like
emission occur in the later case. However, it is worthy noticing that these random laser
performances (threshold and efficiency) in d-U(600)@R6G have been obtained with a dye
concentration (1015
molecules/g of sample) four orders of magnitude lower than the one used
in the Rhodamine doped silica gel (1019
molecules/g of sample) which makes the di-ureasil
hybrids far more attractive for applications.
Account taken of the potentialities of these di-ureasil organic/inorganic hybrids for
flexible IO devices, we think that these novel experimental results on random laser emission
pave the way for future applications as access/indoor components in the new generation of
optical telecommunications.
Acknowledgments
Funding for this research is provided by the Fundação para a Ciência e a Tecnologia, FEDER
(PTDC/CTM/72093/2006), by the Spanish Government MEC under Projects MAT2008-
05921/MAT and Consolider SAUUL CSD2007-00013, and by the Basque Country
#122573 - $15.00 USD Received 11 Jan 2010; revised 9 Mar 2010; accepted 14 Mar 2010; published 25 Mar 2010(C) 2010 OSA 29 March 2010 / Vol. 18, No. 7 / OPTICS EXPRESS 7477