Random gain medium based on organic Rhodamine 590 (R590) laser-dye and Titanium dioxide (TiO2) suspended nanoparticles have been prepared with polymethylmethacrylate (PMMA) as a host. Spray-coated homogeneous film samples of 22.4 μm thickness. Optimum concentrations have been determined depending on the normal fluorescence spectra which give evidence that the laser dye provides amplification and TiO 2 nanoparticles act as scatter center. At the optimum concentrations, results of the random gain medium under second harmonic Nd: YAG laser excitation shows that the values of bandwidth at full width half-maximum (FWHM) and the threshold energy are about 11 nm and 10 mJ respectively.
Random lasers (RL) are unique sources of stimulated emission in which the feedback is provided by scattering in a gain medium [1,2]. Random laser effects have been observed in a variety of organic and inorganic gain media including powders of solid-state luminescent and laser crystals [3,4], liquid laser dyes with scatterers [
On other hand, gain narrowing denotes a decrease of the width of the spectrum of the emitted light triggered by an increase in the pump fluence. The width will be characterized by the at full width half maximum (FWHM). Gain narrowing is observed in all laser systems [
In this work, random gain medium based on mixtures of suspended TiO2 nanoparticles of different concentrations and were mixed with R590 and the polymer PMMA was used as a host [
Rhodamine 590 laser dye (C28H31N2O3Cl) with molecular weight 479.02 g/mol supplied by Lambda Physics LC (5900): Ethanol alcohol (C2H5OH) with spectroscopic grade purity supplied by Gainland Chemical Company and chloroform (CH3CCl3) with spectroscopic grade purity supplied by Philip Harris chemical company U.K. Polymethylmethyleacrylate (PMMA) with chemical forms (CH2CH3COOCH3), supplied by fluka (Switzerland), and used as a host for laser dye and nanoparticles. Titanium dioxide (TiO2, nanoparticles 50 nm) of Anatase crystal structure was acquired from Dupont Inc. The mean particle size TiO2 nanoparticles which is prepared as film, determined by a scanning electron microscope (SEM).
The solutions of laser dye are prepared by dissolving the required amount of the dye in ethanol. The concentrations of dye solutions were: 10–3, 10–4, 10–5 and 10–6 mol/l. The powder of TiO2 was suspended in chloroform and its concentration was estimated based on the known weight fraction of TiO2 in the suspension. The PMMA solution is prepared with chloroform solvent at 10–2 mole/ liter. While final solutions, which are used in the decomposition process, prepared at maxing volume ratio: 4:3:3 of R590:TiO2:PMMA respectively. Spray pyrolysis technique was used to prepare the film samples. Figures 1 and 2 show the photographs of some prepared film samples.
The SEM electron beam produces charges on the surface
of the sample under test. These charges prevent getting a clear image and thus, the particles size can never be measured. This problem is eliminated by covering part of the sample surface with a gold coating to facilitate the discharging of the surface charge as shown in
spot size of ~4 mm). A monochromator and photomultiplier tube was used, successively, to select and detect the emission signals. A cylindrical lens was used to extend the laser spot along the film sample and thus, enabling a precise measurement of the emission signals by monochromator and photomultiplier tube. The emission spectra ware recorded at different gradually-increasing pumping energies.
The liquid samples derived from mixing R590 dye and TiO2 nanoparticles in the suspension, which are prepared as random gain media. In these samples, the TiO2 nanoparticles have not been dissolved and are mixed within the solutions (but have not changed the color of the solutions even at high concentrations of TiO2 nanoparticles).
The film samples, which are prepared by the spray pyrolysed technique. These samples are shown to be highly homogeneous. The reason for the homogeneity of the prepared films is due to several factors such as selecting the special appropriate parameters of the chemical spray technique like the number of sprays and the substrate temperature (maintained fixed at 80˚C). Furthermore, the spraying is done in an isolated ambient (i.e., clean atmosphere). On the other hand, the incorporation of the PMMA polymer as a host for both the R590 dye and the TiO2 nanoparticles has an advantage of obtaining transparent and homogeneous films in view of the many benefits it possesses such as the good compatibility in the laser dyes and the high optical homogeneity.
The photo stability obviously increases when the dye chromoform bind with the polymer chains. It’s for these reasons the polymer could be chosen as a host. These media are characterized by having appropriate thermal optical coefficients as well as they work in a wide temperature range and in small volumes.
The absorption and fluorescence spectra are recorded of all prepared samples as a function of concentrations of both R590 dye (as active molecules) and TiO2 nanoparticles (as scatter center). These measurements give us an indication about the optimum concentrations for the samples which are prepared as random gain media.
Film samples as shown in
that the addition of the polymer PMMA as a host to R590 dye and TiO2 nanoparticles causes a red shift in the absorption spectrum as compared to solutions without polymer. This is due to the increase in the perturbation field among molecules in addition to dimer formation as a result of dye molecules aggregation giving rise to its being absorbed at a higher energy than that of the monomers and being emitted at a lower energy.
Upon mixing the different solution concentrations of the R590 dye with PMMA polymer as a host medium and measuring the fluorescence spectra, the obtained results revealed that the 10–4 mole/liter dye concentration within the polymer is the premium as illustrated in
Shows
The combining of R590 dye within polymer PMMA host as an active laser medium decreases degradation of the dye. These active media have eliminated the use of large volumes of organic solvents and in addition, it involves other benefits like good compatible with the dyes and the optical homogeneity, these media enabled the use of high-concentration dyes without dimer formation and dye quenching. It can be noticed that the increase of polymer solution solidity prevents the disintegration of excited dye molecule resulting from the photo chemical reactions. Thus, the dye stability increases and the fluorescence intensity increases as well. This is a result of dimer disintegration that ends the absorption process at the re-absorption fluorescence.
The polymer PMMA film samples doped with R590 dye and TiO2 nanoparticles have been prepared in which the TiO2 nanoparticles are randomly distributed within the PMMA host. These particles scatter photons and increased the path length photon walking in the film and cause amplified spontaneous emission (incoherent laser).
The AES emission intensity variation with the wavelength for the different excitation energies of the film selected sample, it can be noticed from these spectra that the emission intensity increases and the FWHM decreases within the 5 - 40 mJ range increase of the excitation energy. For the film sample under study, the obtained narrowest bandwidth at FWHM is 11 nm. This dependence is more clarified in the
The evaluation of the film threshold energy (Eth = 10 mJ) at which the gain exceeds the loss, the dependence of the emission intensity and the FWHM on the excitation energy are shown in
Random gain medium is synthesized via spray coating technique. The statistical spectroscopic studies of the concentrations of both R590 dye and TiO2 nanoparticles scatter centers were achieved giving an indication about the optimum required concentrations. The results of RL measurements show that the minimum bandwidth at FWHM is ~11 nm at 40 mJ for the film sample at 10–4 R590 mol/l and TiO2. Was observed for homogenously prepared spray-coated film sample these results are not far away from the ones reported elsewhere [4,6,12,17].