Transition from amplified spontaneous emission to laser action in disordered media of R 6 G dye and TiO 2 nanoparticles doped with PMMA polymer

A random laser (RL) based on organic Rhodamine 6G (R6G) laserdye and TiO2 suspended nanoparticles have been prepared with polymethylmethacrylate (PMMA) as a host. Both liquid and spray-coated homogeneous film samples of 22 μm –30 μm thickness range were use. Optimum concentrations have been determined depending on the normal fluorescence spectra which give evidence that the laser dye provides amplification and TiO2 nanoparticles act as scatter center. At the optimum concentrations, results of the random laser (RL) under second harmonic Nd: YAG laser excitation show that the values of bandwidth at full width half-maximum (FWHM) and the threshold energy are about 8 nm and 3 mJ respectively, which represent the minimum value for the liquid samples in the current research. Correspondly, these values become 12 nm and 3 mJ for film sample. The broadening that can be attributed to the concentration quenching of a laser dye at high a concentration level has been observed. [DOI: http://dx.doi.org/10.2971/jeos.2011.11049]


INTRODUCTION
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 [5], polymeric films with and without intentionally introduced scatterers [6], ZnO scattering films and nanoclusters [7], dye-infiltrated opals [8], porous media infiltrated with liquid crystals with dyes [9] and many others.Random lasers are very attractive for a variety of applications, low coherent random laser sources can be advantageous in holography, laser inertial confinement fusion (driver sources for megajoule lasers), transport of energy in fibers for medical applications, and other applications, detailed reviews of random lasers can be found in [10], [11].Random lasers are strongly scattering media that amplify light.There are striking similarities between these systems and more conventional lasers based on a gain medium enclosed in a cavity with two mirrors to provide optical feedback.An example is the observation of a threshold for lasing action and frequency narrowing in random lasers.Evidently, the optical properties of random lasers are quite different from those of conventional lasers: the propagation of pump and fluorescence light is diffusive in a random laser.In contrast with cavity systems, scattering is actually advantageous.Since feedback is provided by multiple scattering, the random laser threshold is lowered by a stronger scattering, i.e., a shorter transport mean free path, because the feedback is more efficient.It has been shown that the threshold in random lasers is reduced dramatically when the photon transport mean free path approaches the stimulated emission wavelength [12].
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 [13].In a random laser the FWHM of the spectrum of the emitted light below the threshold of the laser is approximately the width of the emission spectrum of the gain medium (typical 40 nm) for R6G.However, far above threshold, this FWHM can be as narrow as 10 nm.A measure for the gain narrowing is the narrowing factor NF, defined as the FWHM of the emitted light below threshold (FWHMbelow) divided by the FWHM of the emission spectrum of a random laser far above threshold (FWHMabove) [14], [15].
In this work, RL based on mixtures of suspended TiO 2 nanoparticles of different concentrations and were mixed with R6G and the polymer PMMA was used as a host [16], and both the TiO2 and R6G concentrations were diluted in its corresponding solvent down to 10 −6 mol/l.From fluorescence measurements of the above concentrations, it was noticed that a TiO 2 concentration of 10 −3 mol/l had the highest intensity and the narrowest bandwidth for both liquid and film samples.At this optimum TiO 2 nanoparticles concentration, the emission intensity spectra at different Nd:YAG pumping energies were investigated to determine the lasing threshold [17].

Experimental setup
Figure 3 illustrates the experimental setup of RL measurement.The liquid samples were placed in cuvettes length of 3cm, and width of 0.5 cm.The pumping source of RL is linear polarized Q-switched Nd:YAG 2nd harmonic generation (λ pump = 532 nm, with a pulse width of ≈ 6 ns, repetition rate of 6 Hz, and focal spot size of ≈ 4 mm).A Polarizer, Analyzer and joule meter were used in this setup in order to detect the rate of laser polarity and to get vertical polarization of laser beam.A monochromator and photomultiplier tube was used, successively, to select and detect the emission signals.The liquid sample was stirred at about 5 minutes before recording the spectrum in order to prevent TiO 2 nanoparticles from excessive precipitation.On the other hand, 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.

Results and Discussion
The fluorescence spectra of the liquid samples at 10 −5 mol/l R6G and different concentrations of TiO 2 are illustrated in figure 4. It is obvious that the maximum wavelength occurs at 562 nm with no significant shift.The R6G dye solution of 10 −5 mol/l concentration with the mentioned concentrations of TiO 2 suspension gives the optimum results.In this case, high intensity and hence narrow bandwidth at FWHM (18 nm) were observed comparing with 40 nm in the case of without TiO 2 in accordance to the inset of figure 4. Likewise, minimum value of 20 nm bandwidth at FWHM for 10 −6 mol/l of R6G and of TiO 2 10 −3 mol/l was registered.The latter value is less than that of 45 nm in the case of without TiO 2 .These results are in agreement with the published data in reference [18].These results suggest preliminary that concentrations of both R6G and TiO2 (in liquid and film samples) are the optimum for both amplification and multiscattering.Figure (5) shows the fluorescence spectra of film samples that were prepared at 10 −3 mol/l R6G and the same mentioned concentration of TiO 2 .It can be seen that 25 nm is the minimum bandwidth at FWHM, for λ max of 568 nm, for the film sample derived at concentration 10 −3 mol/l of both R6G and TiO 2 in accordance to the inset of figure 5.
The bandwidth values at FWHM were calculated by applying the following equation 1: Where ∆λ a f f ected affected bandwidth values, I p is the peak intensity, I λ is the wavelength intensity.We can also find the values of bandwidth at full width half-maximum (FWHM) using an Excel program that calculates the area under the curve and distribute them to the highest value of intensity.These results suggest preliminary that concentrations of both R6G and TiO 2 (in liquid and film samples) are the optimum for both amplification and multiscattering processes in this type of random laser.The minimum bandwidth gives an indication that these processes are performed in parallel without noticeable effect on the dye response.Thus, achieving one of the important conditions of RL system.
Figure 6 shows the emission spectra, obtained from RL setup, at different pumping energies for 10 −5 mol/l R6G: TiO 2 10 −3 mol/l in the case of liquid sample.The transition centered at a wavelength of 562 nm, as can be seen from normal fluorescence in figure 4, and RL threshold is approximately 15 mJ as can be determined from figure 9.
At small pumping intensity, only spontaneous emission can be observed which is characterized by the maximum spectral bandwidth value at λ = 562 nm and at (FWHM) of about 12 nm.With increasing the pumping energy up to 25 mJ, a much narrower peak with the same maximum wavelength is found to be ≈ 8 nm at FWHM.The same behavior is observed for the liquid sample of 10 −6 mol/l R6G: 10 −3 TiO 2 mol/l.In this case, RL threshold is the same value as in the previous concentrations, but the spectrum bandwidth is at λ max = 558 nm and at (FWHM) is ≈ 16 nm.Up to 25 mJ pumping energy, a much narrower peak with the same maximum wavelength is found to be ≈ 10 nm at FWHM.The most important difference between liquid and film samples, as shown in figure 6, 7 and figure 8 respectively, is that for the film sample the band- width at FWHM is about 20 nm at 3 mJ pumping energy with λ max 568 nm and at 25 mJ pumping energy the bandwidth at FWHM becomes 12 nm.
It has been noticed that the present emission spectra of film samples show broadening compared with liquid samples.This may be attributed to the concentration quenching at 10 −3 mol/l R6G which reduces emission intensity bandwidth broadening at about 6 nm.

Conclusions
Two types of RL were synthesized via chemical method and spray coating technique.The statistical spectroscopic studies of the concentrations of both R6G dye and TiO2 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 ≈ 8 nm at 25 mJ for the liquid sample at 10 −5 R6G mol/l and TiO 2 10 −3 mol/l.comparatively, a 12 nm bandwidth at also 25 mJ was observed for homogenously prepared spray-coated film sample at 10 −3 R6G mol/l and TiO 2 10 −3 mol/l, these results are not far away from the ones reported elsewhere [4], [6], [12], [17], [18].

2FIG. 2
FIG.2Image of SEM for measuring thickness sample as film.