We report on transient laser action during the photopolymerization process in organic thin films of acrylate monomers doped with a laser dye. The emission spectrum was monitored over a period of time in the direction orthogonal to the incident laser beam which is kept at a constant intensity during the experiments. The emission spectra display the signature of laser action after a certain amount of polymerization. We have also recorded the intensity of fluorescence as well as of the amplified stimulated emission (ASE) using a photodiode. Our results confirmed that all the emission is guided by an increase of the refractive index resulting from the photopolymerization process. The spatial fluctuations in the density of the material are thought to act as micro-cavities leading to a random laser effect.
©2005 Optical Society of America
Organic polymeric materials have become unavoidable in the development of integrated optics. The reason for the increase of interest for these materials lies in the versatility they offered for both one- or two- photon polymerization techniques. Under one photon polymerization [1–2], this materials can be structured through holography or lithography while, in two photon polymerization, the same result can be obtained taking advantage of the spatial resolution inherent to two-photon absorption (TPA) [3–5]. The fabrication of micro- and sub-microstructures for optical applications is nowadays well controlled in photopolymerizable resins. To make them even more attractive for integrated optics, one needs to control their waveguiding properties in situations where the resulting waveguides have been doped by laser dyes or non linear optical chromophores. Recently, we took advantage of the technological potential of the photopolymerization process in many domains. To list some of our work, we have used it to create (2+1) D waveguides . We have achieved single mode propagation in the resin through the total control of the refractive index during the polymerization process. For instance we have used this guide as a passive device to connect two plastic optical fibers. We have also used this technique to fabricate a corrugated surface to serve as a template for a distributed feedback laser . This laser has been achieved in two steps. First, we have designed the corrugated surface by one-photon polymerization using a Lloyd interferometer to obtain a fixed step modulation in the thin film, and second we have added a laser dye to achieve laser emission. Hence, in this experiment laser emission is obtained after the photopolymerization step. In this paper, we investigate the transition between the broad Amplified Spontaneous Emission (ASE) spectrum and the multi-peaked laser emission in organic thin films which have not previously been spatially structured. We show that the guiding of the ASE, obtained at constant incident beam intensity during the polymerization is only due to the increase of the refractive index in the excited region.
The guest-host system is a doped photopolymerizable resin, which consists of a sensitizer dye (eosin y:2’,4’,5’,7’-tetrabromofluorescein disodium salt at 0.1 wt.%), a cosensitizer (methyldiethanolamine at 5 wt.%), and a multifunctional acrylate monomer (pentaerythritol triacrylate) which acts as a solvent. The photopolymerization process is characterized by two optical properties. The first is an increasing refractive index during the polymerization process . The second property lies in weakening of the absorption of the sample during the polymerization as shown in Fig. 1.
Figure 1(a) presents the evolution of the absorption spectra as a function of the duration of polymerization and it clearly shows the bleaching process. In Fig. 1(b), we have added DCM at 0.5 wt.% to make this photopolymerizable material light emitting. The UV-visible spectra confirm that the eosin bleaches much faster than the DCM. Our samples were sandwiched between two 1 mm thick glass plates under mechanical pressure. The top glass was removed after pre-polymerization. The typical thickness of our polymeric films is about 100 µm.
3. Experimental setup
In Fig. 2 (below), we present a schematic drawing of our experimental arrangement. The incident light is generated by a pulsed mode-locked Nd: YAG laser (TFR Spectra-Physics) at 532 nm, produces 10 ns pulses with a variable repetition rate from 1Hz to 1 kHz. The intensity of each pulse was 13.2 mW/cm2. The repetition rate was fixed at 30 Hz for our experimental runs. The pump beam was focused on the cell with a cylindrical lens (f=50 mm) to obtain a narrow stripe (10×3·10-2 mm2) on the polymer film. With this configuration the emission occurs in both ends of the sample. A photodiode was placed at one end to record the evolution of the intensity, while a multimode optical fiber (1 mm core diameter) was placed at the other end to send the emission to a spectrograph equipped with a 1200 lines/mm grating (Princeton SPEC-10, slit width=50 µm, wavelength resolution=1 nm). A camera recording of the output of the spectrograph gives the evolution of the emission spectrum over time.
4. Experimental results
The evolution of the emission intensity in the perpendicular direction is presented in Fig. 3. At the beginning, there is no emission in the orthogonal direction except for the dye fluorescence in the sample in all directions. During the first 10 s, the intensity increases as the photopolymerization progresses. Then, the intensity drops during 15 s before a slow recovery. After one minute the evolution of the luminescence reflects the normal photobleaching of the dye in the organic sample. This behavior of the intensity can be linked to the evolution of the emission spectrum. In Fig. 4 we present the spectrum at three different instances during the photopolymerization. The blue curve represents the spectrum at the beginning of the experiment and corresponds to the ASE spectrum of the doped resin.
Figure 4(b) represents the evolution of the spectrum between 10 and 25 s. This spectrum shows, first, a red-shift (13 nm) on the emission and, second, the appearance of several peaks of random height which signal laser emission. Notice that the peaks fluctuate in intensity but not in their spectral positions. After a while (typically 60 s) the laser peaks disappear while the ASE background regains intensity as shown in Fig. 4(c).
As we have pointed out in a previous paper , the photopolymerization process can be followed by the measurement of the samples refractive index. Thus, as the photopolymerization progresses, the concentration of monomers M(t) decreases in favor of the concentration of polymers P(t). This kinetics have been studied theoretically and experimentally and the evolution of the refractive index has been linked to the polymerization process by the following formula:
with α m(p) the mean polarizability of one mono(poly)mer. For the polymerizable resin used in this work, the refractive index of the sample increased from 1.48 to 1.52 in the first 10 s of the experiment. This leads to strong guiding properties in the thin film. Indeed, the film is sandwiched between air (nair=1) and a glass plate (nglass=1.5) so that, as the polymerization progresses, the refractive index of the film becomes higher than the one of the substrate. The light is then better confined in the sample (view media file). This strong confinement of the light in the film allows the amplification of the fluorescence. Laser threshold is achieved after about 10 s without any increase in intensity of the incoming beam and random laser action occurs. The laser effect is probably due to the spatial fluctuations in the polymer during the polymerization process. Indeed, this process induces a transfer of matter from the dark to the illuminated zones. The appearance of the multi-peaks spectrum can be explained by the spatial fluctuation in density in the polymerized volume. These regions act like micro-cavity resonators which randomly select the laser emission of the cell. These phenomena are similar to those occurring in the random laser created by Quochi and co-workers , except that our samples are not spatially structured before the polymerization starts. The multi-peaks spectrum consists of longitudinal modes separated by a 1.5 GHz free spectral range (FSR) corresponding to a cavity length of 80 µm (see Fig. 4(b)).
To clarify our observations on the random laser effect, we put together all recorded spectra in Fig. 5(a) to show their evolution over time. In this figure, the three characteristic stages described previously appear clearly: the increase and red-shift of the ASE during the first 10 seconds followed by the appearance of the laser peaks, and finally the aging of the dye causing the gradual loss of the laser emission. Figure 5(b) represents the emission intensity integrated between 590 nm and 630 nm which corresponds to the ASE spectrum window. It is quite similar to the total intensity curve (Fig.3). The correspondence between both curves proves that the spectra recorded in our experiments are due to ASE or laser effect only and that the dye fluorescence is negligible. We have simultaneously recorded three parameters (shown in Fig.6) to demonstrate the laser effect in photopolymerizable materials by the fluctuations in density of the materials during the polymerization.
We have reported on a self lasing effect in a photopolymerizable material during the photopolymerization. The laser emission threshold has been achieved by the guiding properties of thin films induced by the intrinsic increase in refractive index during the photopolymerization. We have shown that the spatial micro-fluctuations in the distribution of matter during the polymerization is probably responsible for the mode selection needed for the laser effect to occur.
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