The synthesis, structural characterization, and amplified spontaneous emission spectroscopy of dye-scattering particles in inorganic medium based on Rhodamine 610-TiO2 nanoparticles confined in silica xerogel matrix have been reported. Optimum concentrations have been determined depending on the normal fluorescence spectra for laser dye, in order to provide amplification, and TiO2 nanoparticals as scatter center. Random Laser has been studied under second harmonic Nd: YAG laser excitation. At the optimum concentrations, the results show that the values of bandwidth at full width half-maximum (FWHM) and the threshold energy are about 11 nm and 3 mJ respectively. The scattered and amplified probe light has been collected on a PC-interfaced CCD camera system.
Random Lasers; Sol-Gel; Nanoscale Materials and Structures1. Introduction
Laser action in random structures where light is multiply scattered and amplified becomes a subject of intense theoretical and experimental studies because of its important potential applications [1]. Following the pioneering theoretical work by Letokhov in the late 1960s [2], experiments by Markushe et al. on powdered Na5La1−xNdx (MoO4)4 [3] and Lawandy et al on solutions containing an organic dye, Rhodamine 640, and TiO2 particles [4] demonstrated scattering-assisted optical gain upon laser pumping.
Random lasing has been demonstrated with various material structures, including semiconductor [5], and rare earth based materials [6] in powder form, nanocomposite hybrids [7], organic dyes [8,9], liquid crystals [10] and biological tissues [11]. Many research works have been done on the preparation and characterization of disordered gain media based on organic dye and scattering particles doped into a solid-state organic host [12-15]. This type of disordered media has been to be bulks or films and based on external scatters.
Sol-gel method has been used to produce several types of inorganic host matrices doped with laser dyes. In this way, the ground powder of a silica gel containing 2 wt% R6G-SiO2 nanoparticles prepared via the sol-gel method has been experimentally demonstrated one and twophoton pumped random lasing [16,17]. The advantages of this dye random laser system have been specified in its solid-state nature, quenched disorder, high laser-like emission efficiency and the possibility of being functionalized for various applications such as biomedical sensors [18]. Also, a novel sol-gel method has been used to synthesize zinc oxide buffer layer for use in random laser media [19].
In the present study, sol-gel method has been used to synthesis silica xerogel as inorganic transparent solid host for both R610 dye as active dopant and TiO2 nanoparticles as scatters. Suitable sol-gel parameters have been used to produce a suitable silica matrix that contains the suspended titanium nanoparticles. Consequently, the statistical spectroscopic studies of the concentrations of R610 dye and TiO2 scatter center have been achieved to select the optimum required concentrations. At these optimum concentrations, the emission spectra at different energies of Nd:YAG, 2nd harmonic generation, has been investigated to determine the lasing threshold.
2. Experimental Details2.1. Samples Preparation
The silica xerogel samples were prepared using sol-gel method. Two recipes, illustrated schematically in Figure 1, were prepared and mixed together. Tetraethylorthosilicate TEOS (Si(OC2H5)4, Purity > 98%; supplied by Schuchardt, was used as SiO2 precursor. For recipe 1,
TEOS and pure ethanol were mixed at molar ratio of 1:4 to prepare silica solution. The catalyst solution was contained deionized water of pH1 (using hydrochloric acid HCl, 0.15 M) mixed with pure ethanol at molar ratio of 2:4. The catalyst solution was slowly added to silica solution with R-molar ratio (H2O/TEOS) of 24:0.06. In recipe 2, Titanium dioxide (TiO2) acquired from DuPont Inc., with the crystal structure of anatase was used as scatter center. TiO2 particles were suspended in chloroform with different concentrations estimated from the known weight fraction of TiO2 in suspension. R610 laser dye was dissolved in ethanol at concentration of 10−3 mol/l and then diluted to get the other concentrations: 2 × 10−4, 2 × 10−5 and 10−5 mol/l. After mixing the two recipes, N, N dimethylformamide (C3H7NO) was added as drying control chemical additive. The gelation of monoliths was achieved by pouring the final sol into covered glass tube and kept it at 60˚C.
2.2. Samples Characterization
The samples were prepared as bulk nature in rod shape. The diameter and length of rod samples were measured using digital micrometer (0 - 150 mm), Hardened. The diameter and length ranges were 3 - 4.5 mm and 13 - 18 mm respectively.
Titanium particle size determination was carried out with scanning electron microscope (SEM) after preparing TiO2 suspension as film by spin coating process. The device of spin coating (achieved in our laboratory) was constituted of a metal body which holds the motor and the sample holder as well as the glass windows, the metal covers and the upper cover (sailing) has a hole for liquids delivery, variable legs for horizontal adjustment of the equipment using spirit level, variable power supply of 220 V input and variable output from 0 V to 220 V and A.C. motor 100 V, 220 W, maximum speed at 100 V is 8000 rpm. Figure 2 shows topography image of TiO2 film where the mean particle size was determined to be 52.49 nm.
Mid-IR spectra were obtained for the prepared samples using FT-IR spectrometer, Shimadzu, on KBr pellets of the samples. The characteristic vibrational bands of silica were determined in the spectra of silica xerogel samples doped with R610 dye and TiO2 nanoparticles as shown in figure 3.
The band centered at 462.88 cm−1 is due to the bending vibrations of (Si-O-Si) groups, and the absorption band peaking at 805 cm−1 is attributed to the symmetric stretching (Si-O-Si) groups, while the wide band at around 1070 cm−1 is the characteristic Si-O-Si asymmetric stretching vibrations [20]. The band at 962 cm−1 is
ascribed to stretching vibration of silanol (Si−OH) groups [21], indicates the existence of these groups in the silica network. Because of the drying process at 150˚C cannot completely trap the water molecules from the pores structure of silica xerogel, the absorption bands at 1660.8 cm−1 is observed and due to bending vibrations of (O-H) bond in H2O molecules [22].
The important result, that could be seen from the FTIR spectra, is the existence of two absorption bands centered at around 447 cm−1, that overlapped with bending vibration band of siloxan (462.88 cm−1), and 667 cm−1, assigned for Ti-O-Ti bonds [23,24]. This result indicates that there is randomly distribution of TiO2 nanoparticles, interstitially as defects within the structure of silica matrix near the pores surfaces and thus act as scatter centers.
2.3. Experimental Setup
Figure 4 shows the experimental setup for measuring the spectra of amplified spontaneous emission (ASE). A quartz cell with 0.5 cm width and 3 cm length has been used as sample cell. The excitation source is Q-switched Nd:YAG 2nd harmonic generation (λ = 532 nm, with a pulse width of 6 ns, repetition rate of 6 Hz and focal spot size of ≈4 mm). Monochromator and photomultiplier tube were used, respectively, to select and detect the emission signals.
To explore the laser output characteristics (intensity distribution); a PC-interfaced CCD camera system (working in the spectral range of 400 - 1100 nm) was used with
a 532 nm interference filter to rid the excitation wavelength as shown in the inset of figure 4.
3. Result and Dissection
Firstly, the dropping in the fluorescence intensity was observed for the rod samples containing 10−3 and 2 × 10−4 mol/l of R610 laser dye and 4.5 × 1013 cm−3 TiO2 particle density as shown in figure 5. This behavior can be seen with other concentrations of TiO2 and due to the aggregation of dye molecules inside the pores structure of silica xerogel, and thus the concentration quenching can be occurred. The concentrations: 10−4 mol/l of R610 dye and 4.5 × 1013 cm−3 TiO2 particle density give the best results, in which high fluorescence intensity and hence narrow band width at FWHM (20 nm) can be observed comparing with 39 nm in the case of without doping of TiO2 as shown in the inset of figure 5. The values of bandwidth at full width half-maximum (FWHM) were determined using an Excel program that calculates the area under the curve and distribute them to the highest value of intensity.
These optimum concentrations of dye molecules and nanoparticles doped silica xerogel give a primarily indication that the amplification and multiscattering processes are performed parallel without noticeable effect on the dye response. Thus, TiO2 nanoparticles scatter photons, increase the path length photons walking in the matrix of silica xerogel and consequently can cause amplified spontaneous emission (incoherent laser).
The emission spectra of rod sample, containing these optimum concentrations were recorded with different
pumping energies (1 - 25 mJ). Thus, the threshold energy and the pumping energy that gives a minimum band width were determined. Figure 6 shows the emission spectra, obtained from the experimental setup, of silica xerogel rod containing: 10−4 mol/l of R610 dye and 4.5 ×
1013 cm−3 of TiO2.
The R610 dye fluorescence was centered at approximately 582 nm as can be seen in Figures 5 and 6. The relation of both band width at FWHM and intensity at peak emission as a function of pumping energy can be illustrated in Figure 7. From this figure, it was found that
the random laser threshold was 3 mJ and the minimum value of band width at FWHM (with maximum intensity) was 11 nm at 25 mJ pumping energy.
Figures 8(a) and (b) shows photo of a charge-coupled device (CCD) camera, as clarified in the inset of Figure 4, for the selected sample at optimum concentrations. This is the laser beam profile test which involves 2D and 3D images where the apparent spikes verify the results described in the ASE intensity spectra. By this test, it can be explored the random gain media spectral intensity distribution, spot size (it represents the quantum yield of the random laser emitted towards the camera) and the spectral purity characteristics for the tested sample.
4. Conclusion
A new type of random gain media was synthesized via sol-gel method. The positions of TiO2 nanoparticles within silica xerogel matrix were primarily determined by FTIR spectra, in which these particles cannot locate within the pores structure of such matrix because of the large size of these particles compared with the pores size (approximately 10 nm according to pH value of the solution). The influence of TiO2 scatter on light emission was explored and the optimum concentrations of particle and dye were obtained. The results of random laser measurements show that the minimum band width at FWHM was 11 nm at 25 mJ and laser threshold at 3 mJ for a homogeneous silica xerogel sample doped with 10−4 mol/l of R610 dye and 4.5 × 1013 cm−3 of TiO2. This threshold was considered to be lower than that of organic hosts such as polymers [25]. It was also found that titanium particles contribute for small photodegradation of the dye molecules.
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