Random laser action is demonstrated in two kinds of powder samples containing rhodamine 6G (Rh6G) doped SiO2 nanoparticles which are either directly dispersed within pure silica particles or embedded in a silica gel matrix which is subsequently ground. Both organic-inorganic hybrid materials present different laser thresholds and emission features which are systematically studied and compared. The dependence of the emission kinetics, emission spectrum, random laser threshold and slope efficiency on the dye doped nanoparticles concentration is investigated in both cases. We also explore if the incorporation of additional TiO2 scatterers could enhance the random laser operation of the studied systems.
© 2009 Optical Society of America
Since first proposed by Letokhov in 1967 , 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. Nevertheless, it is only recently that researchers have started to fully understand the physical mechanisms behind general signatures of random lasing such as an overall spectral narrowing (with the eventual presence of narrow spikes in the emission spectrum) a threshold behavior, and an output pulse shortening. Random lasing has been observed in a wide range of scattering systems such as solution of microparticles dispersed in a laser dye, neodymium doped crystal powders, ceramic and polymer systems, semiconductor nanoparticles, organic tissues, liquid crystals, etc (see Refs. [2–4] and references therein). The nature and morphology of each amplifying disordered medium determine their specific feedback mechanism and random laser behavior [3–5] making it difficult to study all the previously mentioned systems with a unique theoretical treatment [6–9]. A detailed discussion of the latest results and theories concerning random lasers can be found in Ref. .
Within the class of dye random lasers, laser dyes are used to provide the optical gain amplification whereas the scattering typically comes from a suspension of particles in a dye solution [10–12]. In the present study a different approach is followed to achieve random lasing. The rhodamine 6G (Rh6G) dye is embedded in solid porous silica nanoparticles which can be either incorporated into a bulk silica gel matrix or directly dispersed within undoped silica nanoparticles. The advantages of such a material are its solid-state nature, quenched disorder, high laser-like emission efficiency, and the possibility to be functionalized for various applications in the field of biosensors or biotracers. In a recent work, the authors have demonstrated random laser action in a grown powder made of a silica gel containing 2 wt% Rh6G-SiO2 nanoparticles and in a dispersed powder sample containing the same amount of fluorescent nanoparticles [13, 14]. Moreover, we have demonstrated that their random laser emission dynamics can be accurately described by using a light diffusive propagation model with the feedback provided by the powder. The aim of the present work is thus to deeply investigate the spectroscopic properties of these new proposed materials as a function of the dye-doped nanoparticles concentration in order to minimize the laser threshold energy and to optimize the random laser output under short pump pulse excitation. In analogy to other liquid random laser studies [10, 11, 15–18], in our work, we also explore if the incorporation of additional TiO2 scatterers to our dye-doped silica powders could enhance the random laser operation.
The comparison of the laser-like threshold both in frequency and time domains of the studied laser powders shows that the smallest onset of laser-like emission and the highest slope efficiency occur in a ground silica gel sample doped with a 4 wt% of Rh6G-SiO2 nanoparticles. On the other hand, the incorporation of small (~15 nm) TiO2 scatterers has no influence on the laser threshold whereas the dispersion of larger (~405 nm) TiO2 nanoparticles within the dye-doped silica nanopowders slightly improves the laser threshold but decreases the slope efficiency.
2.1 Synthesis and characterization of the laser samples
Two kinds of powder laser samples were prepared via the sol-gel method containing different concentrations of Rh6G doped silica nanoparticles and TiO2 scatterers. In the first stage the Rh6G-SiO2 nanoparticles (~10 nm) were prepared following the synthesis procedure described in Ref. . The concentration of Rh6G in the particles was 0.75 wt%. Then, the fluorescent nanoparticles and the TiO2 scatterers were either embedded in a silica matrix to obtain doped silica gels which were subsequently ground (ground powder samples, GP samples), or dispersed within pure silica nanoparticles (dispersed powder samples, DP samples).
For the preparation of the doped silica gels, the corresponding amount of Rh6G-SiO2 nanoparticles and TiO2 nanoparticles were dispersed in ormosil sols to form the fluorescent particles doped hybrid gels with and without TiO2 scatterers (~15 nm). Note that homogeneity of the silica gel samples cannot be preserved by doping with larger TiO2 nanoparticles. The doped silica gels were finally dried at 50 °C for two weeks. In order to obtain GP samples, a mixer mill was employed. The polydispersity of the measured ground powders was evaluated from SEM (scanning electron microscope) photographs. The average particle size was estimated to be around 4 µm.
In the DP samples, different concentrations of Rh6G doped silica nanoparticles were mixed with undoped SiO2. SEM photographs of the dispersed powders reveal an average particle size of 92 nm. In some of these samples different concentrations of TiO2 particles (~405 nm) were also added.
All the powder laser samples were compacted in a quartz cell for handling ease and optical characterization.
2.2 Experimental techniques
The spectral and temporal measurements were performed at room temperature by using the frequency doubled output (532 nm) of a 10 Hz, Q-switched Nd: YAG laser as the excitation source. The pump pulse duration was 40 ps. The laser excitation energy was controlled by using a pair of polarizers and measured with an energy meter. The powder samples were compacted in a 6 mm high cylindrical cuvette with no front cell window. This sample holder has a diameter size larger than the incident laser spot (6 mm and 2.8 mm, respectively). The emission from the front face of the sample was collected with an optical fiber by use of two lenses. This geometry, also used to perform temporal measurements, is particularly useful to avoid reflection effects of the pump and emitted radiation in the cuvette walls. A long-pass filter was used to remove light at the pump frequency.
In the spectral measurements, the emitted light was dispersed by a 0.5 m imaging spectrograph and recorded by a gated intensified CCD camera. This camera allows time-resolved detection of luminescence with exposure times down to 200 ps and variable delays after excitation. Time reference was provided by a fast photodiode which monitored a small fraction of the incident laser beam. The excited state decays were recorded by coupling the optical fiber with a fast phototodiode connected to a 13 GHz bandwidth digital oscilloscope. In this case, the temporal response was limited by the 100 ps detector resolution. It is worth mentioning that both time-resolved spectral and temporal studies were performed in single shot measurements.
3. Results and discussion
The laser-like effects of the GP and DP random laser samples containing Rh6G doped SiO2 nanoparticles were investigated as a function of the dye doped nanoparticles concentration. TiO2 particles were also incorporated in some of these samples in order to explore the effect of additional passive scatterers in the random laser phenomenon of both solid-state organic-inorganic hybrid materials.
3.1 Ground powder samples
Figure 1(a) shows the normalized emission spectra of the GP of a silica gel containing 4 wt% Rh6G-SiO2 nanoparticles at different pump energies. At 8 µJ/pulse the emission spectrum shows the typical broad fluorescence band of Rh6G centered at 576.8 nm. However as the pump energy is increased the effective emission linewidth () is significantly reduced, which reveals the appearance of laser-like emission. In particular, at 100 µJ/pulse the broad tails of the photoluminescence are completely suppressed and only the gain-narrowed peak survives (orange line in Fig. 1(a)). This confirms that although the corresponding bulk dye doped silica gel is transparent at daylight, the ground powder acts as efficient enough light scatterer in the spectral range of the Rh6G optical gain. The linewidth collapse upon increasing the pump pulse energy is depicted in Fig. 1(b). These experimental data indicate that the laser threshold of this sample (defined as the energy value above which a suddenly drop of the spectral linewidth is observed) is around 8.5 µJ/pulse.
In addition, Fig. 1(a) shows the redshift of the laser-like emission peak observed in the same sample when increasing the pump pulse energy. Note that at 11 µJ/pulse the emission peak is centered at 573.7 nm whereas at 900 µJ/pulse it is centered at 583.7 nm. Figure 1(c) presents the spectral position of this emission peak as a function of the excitation energy. Similar spectral shifts depending on the pumping power were previously reported in colloidal dye solutions containing scattering particles . This emission feature was explained with a combination of saturation absorption in the excitation process and re-absorption in the luminescence process.
The temporal profile of the GP samples was also explored. Figure 2(a) compares the normalized emission decays of the 4 wt% Rh6G-SiO2 sample obtained at 3, 8, 11, and 900 µJ/pulse. Note that the wavy structure of these curves is due to the typical response of the fast photodiode used in the experimental set-up. At the lowest excitation energies, the output pulse duration (see point line in Fig. 2(a)) is limited by the lifetime of the Rh6G dye in this sample (5 ns). However, as pump energy increases, the pulse profile becomes strongly shortened to a pulse in the subnanosecond range. Figure 2(b) shows the resulting sharp drop of its full width at half maximum (FWHM). A minimum time-width of around 100 ps is obtained at high pump pulse energies. This is the actual time resolution of the detector we used in these measurements, so, our real pulse duration might be much narrower. As a matter of fact, studies on liquid dye and polymer sheet random lasers have given emission pulses as short as 50 ps or even less [11, 19, 20].
In order to study the effect of the dye-doped nanoparticles concentration on the previously described random laser behavior we have also performed spectral and temporal measurements in the GP of different silica gels containing 0.1, 1, 2, 3, 4, and 5 wt% Rh6G-SiO2 nanoparticles. Figure 3(a) compares the spectral narrowing of these samples as a function of the pump pulse energy. As it can be clearly seen, a similar behavior is obtained regardless the dye-doped nanoparticles concentration, only a slightly larger laser threshold is found in the ground powder containing 0.1 wt% Rh6G-SiO2 nanoparticles. This is the less efficient sample as evidenced in Fig. 3(b) where the integrated intensity of the corresponding emission spectra is plotted as a function of the pump pulse energy. In contrast, the GP of the silica gel containing 4 wt% Rh6G-SiO2 nanoparticles has the largest slope efficiency.
Figure 3(c) shows the temporal shortening of the output pulse obtained as a function of the excitation energy in the same set of GP of silica gel samples. In agreement with the spectral results, a slightly larger laser threshold is found in the sample containing 0.1 wt% Rh6G-SiO2 nanoparticles. This plot also exhibits the different output pulse FWHM at low excitation energies. It is worthy mentioning the enhancement of the lifetime value when increasing the fluorescent nanoparticles concentration from 0.1 wt% (τ=4.6 ns) to 2 wt% (τ=6.2 ns) and the subsequent lifetime quenching obtained when the amount of Rh6G-SiO2 nanoparticle is further increased up to 5 wt% (τ=5 ns). This behavior could be qualitatively explained by an enhanced re-absorption and the appearance of new non-radiative de-activation channels due to the formation of dye aggregates. In the first case, an increase of the lifetime is expected owing to the radiation trapping effect. The increasing contribution of the reabsorption phenomenon when the Rh6G-SiO2 nanoparticles concentration is enhanced could account for the initially observed lifetime rise. On the contrary, the presence of dye aggregates could be the source for the lifetime quenching found when using 5 wt% Rh6G-SiO2 nanoparticles. Note that the formation of dye aggregates is expected to be a gradual process as the concentration of the fluorescent nanoparticle increases. Their effect could thus become the dominant lifetime contribution at high concentrations.
Our explanation above is supported by the emission spectral shift observed as the fluorescent nanoparticles concentration increases. Figure 4(a) shows the normalized emission spectra of the GP of silica gel samples containing 0.1, 1, 2, 3, 4, and 5 wt% Rh6G-SiO2 nanoparticles at 900 µJ/pulse. As it is depicted in Fig. 4(b), the corresponding laser-like peak maximum is redshifted (18.7 nm) when the amount of Rh6G-SiO2 nanoparticles increases. Exciting below the threshold, the Rh6G fluorescence of these GP samples is also redshifted. However, the spectral shift is smaller in this case (12 nm at 5 µJ/pulse). Redshifts of the Rh6G fluorescence and of the laser-like emission have been previously reported as a function of the dye concentration in gel silica matrices  and in colloidal dye solutions [15, 17]. This spectral shift was interpreted either by the presence of re-absorption [15, 17, 22] or by the formation of aggregates . The lifetime behavior obtained as a function of the dye-doped nanoparticles concentration in our GP samples suggests that in these samples both competing phenomena are present, and that the dye aggregates contribution becomes more important for the highest concentrations of fluorescent nanoparticles.
In order to investigate the possible influence of passive scatterers in the random laser threshold of our GP samples based on silica gel matrices we have studied the spectral and temporal emission properties of the GP of different silica gels containing 4 wt% Rh6G-SiO2 nanoparticles and 0, 0.1, and 0.8 wt% TiO2 nanoparticles. These nanoparticles were to be small enough (~15 nm) to preserve the homogeneity of the gel samples. Nevertheless, the introduction of such small TiO2 scatterers does not lead to a significant change in the emission spectra of the resulting GP samples. In particular, regardless the TiO2 concentration, the linewidth collapse occurs around the same energy value (data not shown). This evidences that the scattering effect of these passive dispersors is not enough to reduce the random laser threshold. This might be due to the rather small size of the employed TiO2 scatterers.
A parallel behavior is found in the time domain. Figure 5 shows the FWHM of the temporal profiles obtained in the three previously mentioned samples as a function of the pump pulse energy. The pulse shortening takes place around the same energy value (~8.5 µJ/pulse) in all of them. Nevertheless, a significant reduction of their temporal FWHM is found at low excitation energies when the TiO2 concentration is increased. In fact, the Rh6G lifetime of the GP sample without TiO2 dispersors is 5 ns whereas the lifetime value of those samples containing 0.1 wt% and 0.8 wt% TiO2 scatterers are 4 and 3.2 ns, respectively. These values suggest the presence of additional non-radiative de-activation channels in those samples where TiO2 nanoparticles are embedded in the silica matrix.
3.2 Dispersed powder samples
Basic properties of random lasing such as emission spectrum, emission kinetics and stimulated emission threshold were also investigated in the DP samples. As an example, Fig. 6(a) presents the normalized emission spectra of 8 wt% Rh6G-SiO2 nanoparticles dispersed in silica powder measured at 23, 37, 45, 75, and 900 µJ/pulse. This figure illustrates the spectral narrowing of this sample as the pump pulse energy is increased demonstrating that no embedding medium for the Rh6G-SiO2 nanoparticles is required to achieve random lasing. Nevertheless, it is clear from the excitation energy dependence of its emission linewidth (see Fig. 6(b)) that this DP sample has a significant larger laser threshold than the GP where the fluorescent nanoparticles are embedded in the silica matrix (~40 µJ/pulse and ~10 µJ/pulse, respectively). In addition, Fig. 6(a) shows the redshift of its laser-like emission. The spectral position of the emission peak is plotted as a function of the pump energy in Fig. 6(c). A redshift of 3.8 nm was found in the studied pump energy range.
On the other hand, the normalized emission decays obtained at 3, 46, 55, and 900 µJ/pulse (see Fig. 7(a)) show the reduction of the output pulse duration of 8 wt% Rh6G-SiO2 nanoparticles dispersed within silica nanoparticles as the excitation energy increases. The resulting drop of the FWHM output pulse is depicted in Fig. 7(b) as a function of the pump energy. A minimum time-width around 100 ps was also found. Nevertheless, in this case a larger pump pulse energy value is required to achieve this pulse duration if compared to GP samples.
Different Rh6G-SiO2 nanoparticles concentrations were also tried to study their effect in the optical properties of the DP samples. Figure 8(a) shows the pump energy dependence of the effective emission linewidth of 2, 4, 6, and 8 wt% Rh6G-SiO2 nanoparticles dispersed in silica powder. From this experimental data a laser threshold around 40 µJ/pulse was inferred for the four dispersed powder samples. Therefore, the increase of the fluorescent nanoparticles concentration does not remarkably improve the laser threshold. However, the increase of the Rh6G-SiO2 nanoparticles concentration dispersed within undoped silica nanoparticles yields an enhancement of the slope efficiency. Figure 8(b) shows their spectrally integrated emission intensity as a function of the pump pulse energy.
Concerning the temporal dynamics, Fig. 8(c) compares the pulse shortening found increasing the pump pulse energy in the DP samples containing 2 and 8 wt% fluorescent nanoparticles. As can be seen, the powder sample containing 2 wt% Rh6G-SiO2 nanoparticles seems to have a slightly smaller laser threshold. Moreover, below the onset of the laser-like action, a remarkable reduction of the FWHM of the temporal profiles occurs in the sample containing 8 wt% Rh6G-SiO2 nanoparticles (Rh6G lifetime 2.9 ns) if compared to the one containing 2 wt% (Rh6G lifetime 4 ns). On the other hand, when the pump energy is beyond the threshold level, the pulse duration is reduced up to the 100 ps temporal resolution in both cases.
Figure 9(a) shows the normalized emission spectra of the DP samples containing 2, 4, 6, and 8 wt% Rh6G-SiO2 nanoparticles above threshold at 900 µJ/pulse. As it is shown in Fig. 9(b), a redshift of 5.8 nm is observed. This redshift was attributed to the presence of dye aggregates account taken of the lifetime quenching found with the fluorescent nanoparticles concentration (see Fig. 8(c)).
Comparing Figs. 4(a) and 9(a), significant changes in the spectral position and spectrum bandshape of Rh6G are seen in the two explored kinds of powder laser samples. The emission spectra of the GP samples are redshifted compared to the ones found in the DP. As an example, the emission band of the DP sample containing 2 wt% Rh6G-SiO2 nanoparticles is centered at 556.4 nm at 900 µJ/pulse whereas in the GP sample containing the same amount of fluorescent nanoparticles the laser-like emission peak appears centered at 579.9 nm. There is thus a redshift of 23.5 nm between both. It is also noteworthy the broader emission band found in the GP above the threshold. In particular, the effective emission linewidths of the DP sample and the GP sample both containing 2 wt% Rh6G-SiO2 nanoparticles are 11.5 nm and 13.6 nm at 900 µJ/pulse, respectively. Moreover, the corresponding lifetimes below threshold are 4 ns and 6.2 ns. All these experimental results seem to indicate a larger contribution of the re-absorption phenomena in the GP. It is important to bear in mind that the actual Rh6G concentration might be higher in a GP sample than in a DP sample although both contain the same concentration of dye-doped nanoparticles. Note that a dye leakage is expected in the final preparation step of the DP due to the method employed for the dispersion in solution of the pure SiO2 particles and the fluorescent nanoparticles.
To explore a possible enhancement of random laser emission in DP samples we have also tested DP samples in which passive scatterers were added. In this case, larger TiO2 particles (~405 nm) were employed in order to increase the light scattering effect. We thus performed a detailed spectroscopic investigation by varying the dye-doped nanoparticles concentration and the amount of TiO2 scatterers dispersed within the silica particles. First, the fluorescent nanoparticles concentration was varied from 2 to 8 wt% with the TiO2 concentration kept constant (0.5 wt%). In these samples no significant changes in the emission spectra, integrated intensities or laser thresholds were found if compared with the ones without dispersors. We have just observed small differences at low excitation energies in the FWHM of their temporal profiles.
Second, the dye-doped nanoparticles concentration was kept constant at 4 wt% whereas the scattering particle concentration (~405 nm) was varied from 0.7 to 7.1 wt%. Figure 10(a) shows the effective emission linewidth of 4 wt% Rh6G-SiO2 nanoparticles dispersed in silica powder with 0.7, 1.4, 2.8, and 7.1 wt% TiO2 dispersors as a function of the pump energy. This plot reveals the slightly smaller laser threshold of the powder sample containing 7.1 wt% TiO2 scatterers. Figure 10(b) shows the pump energy dependence of the spectrally integrated intensity for the different scattering particle concentrations. As we can see, the slope efficiency increases when increasing the TiO2 concentration up to 2.8 wt%. However, it is significantly reduced in the dispersed powder sample containing 7.1 wt% TiO2.
Figure 10(c) shows the FWHM of the time profiles obtained in the DP samples containing 0.7 and 7.1 wt% TiO2 scatterers as a function of the pump pulse energy. As can be observed a reduction of the laser threshold energy, from 40 to 27 µJ/pulse, was found when increasing the TiO2 concentration. On the other hand, it is worthy to remark that the laser threshold of the DP sample containing 7.1 wt% TiO2 is still around three times larger than in GP samples. This result demonstrates that without the need of passive dispersors GP samples behave as more efficient systems to achieve random lasing.
Finally, as the GP samples have the lowest laser threshold and therefore seem to be the most attractive ones for potential future applications, we have explored their susceptibility to optical damage under VIS laser pulse irradiation. As an example, Fig. 11 shows that there is almost no change in the integrated emission intensity of the GP sample containing 2 wt% Rh6G-SiO2 nanoparticles when pumping at 35 µJ/pulse (i.e. above the onset for laser-like emission) after an irradiation time of eight hours. This is a clear evidence of the good photostability of our powder random laser samples.
Random laser-like effects such as spectral narrowing and pulse shortening have been observed and characterized in two different kinds of silica based powder samples. Both were synthesized by the sol-gel technique and contain Rh6G doped SiO2 nanoparticles which were either dispersed within pure silica particles (DP samples) or embedded in a silica gel matrix which was subsequently ground (GP samples). Spectral and temporal ultrafast spectroscopies were performed as a function of dye-doped nanoparticles and TiO2 scatterers concentrations to find the most efficient system as random laser emitter. The comparison between the emission features of both kinds of powder laser samples reveals that lower thresholds for laser-like emission are achieved in the GP samples, even in the absence of passive scatterers. As we have mentioned in Sec. 3.2, the different preparation procedures between GP and DP samples could allow for an actual larger Rh6G concentration in GP samples which could favor an enhancement of the optical gain required for random lasing. Moreover, the different average particle size between GP and DP samples (4 µm and 92 nm, respectively) could influence the multiple scattering efficiency and therefore the onset of laser-like emission.
The experimental results show that the ground powder sample containing 4 wt% of Rh6G-SiO2 nanoparticles has the lowest laser threshold (~8.5 µJ/pulse). In addition, the increase in the fluorescent nanoparticles concentration leads to a laser-like emission redshift together with an enhancement of the slope efficiency in both kinds of powder samples. On the other hand, the incorporation of small TiO2 scatterers (~15 nm) in the silica gel matrix of the GP samples has a negligible effect on the onset of their laser-like emission. In contrast, we have observed a reduction of the random laser threshold of the DP samples by using larger TiO2 scatterers (~405 nm) but their slope efficiency decreases as well. This fact together with the Rh6G lifetime quenching found in the DP samples indicate the presence of some additional non-radiative de-activation channels when using TiO2 scatterers.
We also note that the emission spectra of the GP samples are redshifted if compared with the ones measured in the DP samples containing the same amount of fluorescent nanoparticles. This spectral shift is attributed to an enhanced re-absorption effect in the GP samples. In addition, the good photostability of the GP samples has been proven under VIS laser pulse irradiation by exciting above the threshold.
These results confirm that GP based samples are the most efficient for practical purposes in light emitting devices. Moreover, the low laser-like threshold found in these systems opens a challenging field of new applications such as biosensors or biotracers based on these dye-doped nanoparticles.
This work has been supported by the Spanish Government MEC under Projects No. MAT2008-05921/MAT, MAT2008-00010/NAN, Consolider SAUUL CSD2007-00013, and 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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