In recent years, more efforts have been devoted to incorporate organic laser dyes into various host matrices, with the goal of developing solid-state dye laser materials  -  that could possibly replace liquid dye lasers. Compared with liquid dye lasers, solid- state dye lasers have many advantages such as non-toxic, non-flammable, non-volatile compact and better mechanically and thermally etc.  -  . Solid-state tunable dye laser materials can be developed by incorporating stable laser dye molecules into solid host matrices like polymers, silica gels, xerogels, organically modified silicates (ORM- OSILS) and sol-gel glasses  -  .
Coumarin and stilbene-3 (STB-3) laser dyes are very efficient laser dyes for the spectral response in the blue-green region of the electromagnetic spectrum  -  . Several coumarin laser dyes have been widely studied in various solvents and solid hosts.   -  . However very few reports are available on photophysical properties of STB-3 in solid host matrices   . The photophysical properties of laser dyes provide basic information for their use in different applications which in turn depend on the molecular structure of the dye.
The lasing action of dye depends on its photophysical properties    -  . The high value of extinction coefficient and fluorescence quantum yield of a laser dye is favourable for good lasing performance. Therefore, it is necessary to pay more attention to investigate the photophysical properties of dye/sol-gel elements. With these objectives we are reported Spectroscopic properties of prepaired STB-3 impregnated sol-gel solid samples using three distinct methods.
Molecular weight of Stilbene-3 (STB-3)
Stilbene 3 (STB3); Lambda Physik (Laser grade) Mol. wt. 562.62.
Molecular structure of Stilbene-3 (STB-3).
As we know the preparation of sol-gel is a two step process involving hydrolysis and polycondensation of metal alkoxide in presence of a catalyst. The sol-gel materials are prepared in the present work by carrying out hydrolysis and polycondensation of tetraethylorthosilicate (TEOS) using HCl as catalyst.
The materials were prepared by employing three different methods.
1) Method I: Using HCl as catalyst and glycerol as DCCA.
A sol was prepared by mixing 11.2 ml TEOS, 6 ml MeOH, 9 ml-H2O, 1.0 ml HCl (0.1 N) as catalyst and 10 ml glycerol as DCCA to reduce the cracking of monoliths during drying under magnetic stirring at room temperature. After 17 hours of stirring, 3.5 ml sol was poured in the rectangular polystyrene cuvettes of size (4.5 × 1.0 × 1.0) cm3. The cuvettes were sealed with teflon tape to ensure slow drying of sol, which decreases the probability of cracking in solid. Drying and aging of gel were carried out at room temperature in clean and controlled environment.
After about 2 months from the date of preparation, solid blocks were obtained in the form of parallelopipeds with dimensions (0.8 × 0.8 × 2.0) cm3. These glass samples were immersed in MeOH/H2O (50:50 by volume) for 16 hours and then subsequently in 15 ml methanolic solution of STB-3 dye of known concentration for one hour. After removing the samples from the solution they were dried at room temperature. After 15 days of drying the surface of the samples gets dried so that it is handable and can be subjected to various measurements.
2) Method II: Using HCl as catalyst at 60˚C and drying at room temperature.
A sol was prepared by mixing 78 ml TEOS, 102 ml-H2O, 2.4 ml HCl as catalyst under magnetic stirring at 60˚C temperature for 1 hour. After 1 hour stirring, 3.5 ml sol was poured in the rectangular polystyrene cuvettes and then sealed with teflon tape. Drying and aging of gel were carried out at room temperature in clean and controlled environment.
After about 20 days from the date of preparation, solid blocks were obtained in the form of parallelopipeds with dimensions (0.8 × 0. 8 × 2.8) cm3. These glass samples were given dip treatment by immersing them in MeOH/H2O (50:50 by volume) for different intervals of time such as 1 hour, 4 hours, 8 hours, 12 hours and 16 hours and then subsequently in methanolic solution of STB-3 dye of known concentration for one hour. After removing the samples from the solution they were dried at room temperature. After 5 days of drying the surface of the samples gets dried so that sample is handable and can be subjected to various measurements.
3) Method III: Using HCl as catalyst at 60˚C and heated at 600˚C temperature for 3 hours.
A sol was prepared by mixing 78 ml TEOS, 102 ml-H2O, 2.4 ml HCl as catalyst under magnetic stirring at 60˚C temperature for 1 hour. After stirring, 3.5 ml sol was poured in the rectangular polystyrene cuvette and then sealed with teflon tape. Drying and aging of gel were carried out at 60˚C temperature in heating blocks. After about 4 days from the date of preparation, solid blocks were obtained in the form of parallelopipeds with dimensions (0.6 × 0.6 × 1.7) cm3. These glass samples were given heat treatment by heating in programmable microwave furnace at 600˚C for 3 hours and then subsequently they were cooled and kept at room temperature for one day and then the blocks were immersed in 15 ml methanolic solution of dye of known concentration for one hour. After removing the samples from the solution they were dried at room temperature. After 3 days of drying the surface of the samples gets dried so that it is handable and can be subjected to various measurements.
The number density of dye doped molecules in the solid host was calculated by difference method from the optical density (OD) of absorption of dye solution before and after dipping of the glass sample. The desired number density of dye molecules in solid host can be obtained by dipping of glass samples in varied concentration of methanolic solution of STB-3dye. The dried solids obtained by all the above methods were visually of good surface finish with plane parallel side faces. They were used directly for spectroscopic and laser studies without any polishing of the faces.
2.2. Absorption/Fluorescence Measurement
The absorption spectra of STB-3 dye doped samples were recorded with respect to undoped glass sample by a conventional Spectrophotometer. The fluorescence spectra were recorded using an assembled fluorimeter with front surface excitation emission geometry. Fluorescence spectra of STB-3 impregnated sol-gel samples prepared by various methods and STB-3 in methanol were scanned at the same excitation wavelength (350 nm). All the recorded spectra were corrected for the photomultiplier and monochromator sensitivities. The fluorescence intensity (FI) of the dye embedded solid was normalized with the fluorescence intensity of the methanolic solution of the dye to obtain the relative fluorescence intensity.
After measurement of the photophysical properties including fluorescence lifetime, the dye impregnated sol-gel samples prepared by various methods were subjected to laser study under nitrogen laser pumping at 337.1 nm at the rate of 1.67 Hz in transverse dye laser cavity.
3. Results and Discussion
3.1. Absorption/Fluorescence Properties
Figures 1(a)-1(c) and Figures 2(a)-2(c) show the absorption and fluorescence spectra of STB-3 dye doped sol-gel glass samples prepared by Method I, Method II and Method III respectively with increasing drying time. It represents the typical behavior of these solids which is observed for all the concentrations of the dye studied in the present work. The shape of the spectra are similar to those in alcoholic solution suggesting existence of similar forms of dye molecule in the solution and solids. All STB-3 doped solid glass samples prepared by Method I, II and Method III having number densities of the order of 1015/cm3 to 1017/cm3 show single absorption peak peaking at 345 nm, 344 nm and 348 nm respectively. As the drying time increases there is a decrease in the optical density (OD) value of main absorption peak at 345 nm and 344 nm for Method I and Method II samples respectively. But in case of samples prepared by Method III no such change in OD value is observed as drying time increases.
Also slight blue shift (2 nm) is observed in absorption maximum wavelength after 240 days of time of drying for all three types of STB-3 embedded sol-gel glasses. The fluorescence intensity (FI) also decreases with the lapse of time of preparation for Method I and Method II prepared samples, while no change in fluorescence intensity is observed in samples prepared by Method III as the drying time increases as shown in Figure 2(c). However the observed changes in absorption and fluorescence properties with increasing drying time are very less in samples prepared by Method II as compared to samples prepared by Method I.
(a) (b) (c)
Figure 1. Absorption spectra of STB-3 containing sol-gel glass sample (a) Method I; (b) Method II and (c) Method III (no. density Method I: 1.12 × 1016 per·cm3; Method II: 7.50 × 1015 per·cm3; Method III: 6.62 × 1015 per·cm3).
Figure 2. Fluorescence spectra of STB-3 containing sol-gel glass samples (a) Method I; (b) Method II and (c) Method III (no. density Method I: 8.52 × 1016 per cm3; Method II: 6.35 × 1016 per·cm3; Method III: 1.32 × 1017 per·cm3).
Table 1 lists the absorption properties of STB-3 in sol-gel blocks as well as in methanol with low concentration of the dye (number density of the order of 1015 per·cm3). The absorption wavelength maxima (λa) for sample prepared by all the three methods are little blue shifted as compared to that in methanol. The extinction coefficient (ε) of STB-3 in solid prepared by Method I and Method II are less compared to that in solution, where as in solids prepared by Method III it is more than that in solution. But the order of magnitude of ε value is same for all the samples.
The changes observed in the fluorescence spectra of solid glass blocks with increasing dye concentrations (number densities of dye molecules of the order of 1015 - 1018 per· cm3 of the solid) are depicted in Table 2. The fluorescence wavelength (λf) of the dye doped solids is between 430 - 434 nm which is slightly blue shifted compared to that in alcoholic solution (435 nm). The FWHM increases initially as the concentration of the dye increases but decreases again for the higher number density samples. The fluorescence intensity (FI) also first increases and then decreases as the concentration of the dye in solid increases which is due to concentration quenching  -  .
Table 1. Absorption properties of STB-3/sol-gel glass samples in comparison with STB-3/MeOH.
Uncertainty 1 nm in (λa).
Table 2. Fluorescence properties of STB-3/sol-gel glass samples with different concentrations.
Uncertainty 1 nm in (λf); 0.05 in (FI)
Samples prepared by method I and II show that the FI values are lower than those observed for samples prepared by Method III. This may be due to the presence of some residuals along with acid in Method I and Method II samples whereas most residual solvent/chemicals are coming out from the Method III samples during heating. The absorption and fluorescence spectra suggest the existence of single emitting species.
Thus the spectroscopic properties of samples prepared by Method III are superior to those prepared by other two methods. The single lifetime values in the range of 1.12 - 1.42 ns of STB-3 in these samples are an indication of the existence of a single fluorescent species.
3.2. Laser Study
When the blocks prepared by each method containing STB-3 dye with varied concentrations were subjected to laser studies by pumping them with nitrogen laser (337.1 nm) in the transverse dye laser cavity with pump frequency of 1.67 Hz, the laser emission was observed from samples with higher concentration (5.51 × 1018 cm−3) Method I samples, but the output dies within 4 - 5 pulses of the pump laser. Whereas lasing action could not be obtained for Method I and Method II samples. One of the reasons for not obtaining lasing action can be because of very short life time; which is very different from 4 ns the pulse duration of nitrogen laser. Also the 1018 cm−3 concentrations of dye molecules may not be adequate and pump energy of nitrogen laser may not be sufficient for showing lasing action.
Laser dye STB-3 could be embedded successfully in sol-gel glass samples prepared by all three methods. Laser dye STB-3 incorporated in sol-gel glass samples showed extinction coefficient and quantum yield values comparable to that in methanolic solution. The absorption and fluorescence wavelengths showed slight blue shift in case of dye doped solids as compared to that in the solution. This can be combined effect of higher refractive index of solids and environment surrounding the dye molecule.
Though very poor lasing action was observed in case of samples prepared by Method I, the photophysical performance was found to be best in method III samples.
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