Miniaturization of the tunable laser equipment is an important factor for further development in various optoelectronic technologies. To realize the compact tunable laser devices, an optical gain medium having a broadband optical gain characteristic is required. In this study, we propose a promising strategy for preparing an optical gain film that exhibits a ∼250 nm gain bandwidth in the visible wavelength region. This film consists of a polymer matrix co-doped with organic luminescent molecules that form a complex of the excited state, i.e., exciplex. The exciplex state can co-exist with the monomolecular exciton state, leading to broadband (400–650 nm) optical gain with a large Stokes shift. Optically pumped lasing action is also possible when the film is combined with an optical resonator. This strategy would be useful for developing a compact tunable laser device without active medium replacement.
We investigated optical gain and lasing characteristics of a polymer thin film co-doped with Rhodamine 6G and 4-(Dicyanomethylene)-2-methyl-6-(4-dimethylaminostyryl)-4H-pyran dye molecules. By the co-doping of these dyes, the optical gain coefficient and its spectral range were enhanced and widened, respectively. These results cannot be accounted for by the linear combination of the optical gain properties of single-doped films. Owing to this high-gain and wide-band optical amplification property, a distributed feedback lasing with a low threshold of 70 μJ/cm as well as a widely tunable wavelength range of 587-613 nm was achieved. We attribute this amplification property of the co-doped film, which cannot be achieved with the single-doped films, to the appearance of a coupled excited state between the two molecules.
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