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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
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].
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 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
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 effective emission linewidth 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.
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.
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.
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 Government (IT-331-07). S. G.-R. acknowledges financial support from the Spanish MEC under the “Juan de la Cierva” program.
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