Photo-physical characterization of rhodamine 6G in a 2-hydroxyethyl-methacrylate methyl-methacrylate copolymer
Introduction
At present, research on solid-state dye lasers is a very active field [1], [2], [3], [4], [5], [6], [7], [8]. Organic dyes are doped in organic polymers [1], [2], [3], [9], [10], [11], [12], [13], [14], in inorganic glasses [4], [5], [15], [16], [17], [18], or in composite organic–inorganic materials [6], [7], [19], [20], [21]. The processability, the photo-stability, and thermostability of the solid-state dye laser materials are sought to be improved continuously.
Good lasing performance has been achieved for rhodamine 6G doped [2], [22], [23], [24], [25] and coumarin 503 doped [26] HEMA–MMA copolymers. Here, some spectroscopic parameters of rhodamine 6G in a HEMA–MMA copolymer are compared with rhodamine 6G in methanol. The absorption cross-section spectra and stimulated emission cross-section spectra, the fluorescence quantum distributions, quantum yields, and lifetimes are determined. The photo-degradation is studied under low-power steady-state laser excitation and under intense picosecond laser pulse excitation. The saturable absorption behaviour under picosecond laser excitation is also investigated.
Section snippets
Experimental
The structural formulae of rhodamine 6G and of the statistical block-copolymer poly((2-hydroxyethyl-methacrylate)-co-(methyl-methacrylate)) with an equal volume ratio of 2-hydroxyethyl-methacrylate (HEMA) and methyl-methacrylate (MMA) are depicted in Fig. 1. The name of the copolymer is abbreviated by p(HEMA–MMA).
The dye rhodamine 6G (chloride salt, laser grade) was purchased from Lambda-Physik and was used without further purification. For the liquid solution studies, analytical grade methanol
Transmission studies
The transmission spectrum, T(λ), of a 2 mm thick p(HEMA–MMA) disc is shown in Fig. 2(a) (solid curve). The approximate transmission reduction, Tr, due to reflection at the entrance and exit surfaces is shown by the dotted curve. The relations Tr=(1−R)/(1+R) and R=(n−1)2/(n+1)2 with n=1.5044 are used in the calculation of the dotted curve (R is reflection, n is refractive index, dispersion is neglected). In Fig. 2(b), the absorption coefficient spectrum, α(λ), of p(HEMA–MMA) is shown (solid
Conclusions
Some photo-physical parameters of the dye-doped polymer rhodamine 6G in p(HEMA–MMA) have been determined and compared with rhodamine 6G in methanol. The absorption cross-section and stimulated emission cross-section spectra of rhodamine 6G in the polymer and in methanol are found to be quite similar. A small red-shift of the polymer spectrum is discussed in terms of refractive index changes (London dispersion forces [38]). A reduced fluorescence Stokes shift in the polymer matrix is discussed
Acknowledgements
The authors thank D. Riedl, Electronic Department of the University Regensburg, for measuring the relative dielectric constant of p(HEMA–MMA).
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