Patterned Block-Copolymer-Silica Mesostructures as Host Media for the Laser Dye Rhodamine 6G

Rhodamine 6G-doped mesostructured silica is prepared by an acidic sol−gel route using poly-b-poly(propylene oxide)-b-poly(ethylene oxide) (EO x −PO y −EO x ) block copolymer surfactants. Using low-refractive-index (n ∼ 1.2) mesoporous SiO2 as a support, the synthesis is combined with soft lithography to produce high-quality waveguides. This enables efficient waveguiding in the line-patterned rhodamine 6G-doped mesostructured domains, which have a higher refractive index than both the mesoporous support and cladding. For the structure-directing block copolymer surfactants used, (EO)20(PO)70(EO)20 (P123) and (EO)106(PO)70(EO)106 (F127), X-ray diffraction patterns and transmission electron microscopy reveal hexagonal mesophases, whose longitudinal cylinder axes are aligned predominantly parallel to the substrate plane. For samples made by micromolding-in-capillaries (MIMIC), the longitudinal axes are also aligned along the longitudinal waveguide axes. Samples made by micromolding also possess a high mesostructural order, though in the absence of an aligning flow field, their long-range order (ca. several hundred nanometers) is lower than for samples processed using the MIMIC technique. When optically pumped, the rhodamine 6G-doped waveguides exhibit amplified spontaneous emission with thresholds as low as ∼6 kW cm-2, substantially lower than rhodamine 6G-doped sol−gel glasses. This is attributed to the ability of the polymeric surfactant to co-assemble with the dye molecules, thereby leading to high dye dispersions and reduced dye dimerization. Additionally, rhodamine 6G shows good photostablility in the mesostructured waveguides, similar to that of rhodamine 6G in organically modified silicates.