Significant improvements in the optical gain properties of oriented liquid crystalline conjugated polymer films

We report significant improvements in the optical gain properties of highly oriented films of the fluorene-based conjugated polymer poly(9,9-dioctylfluorene-co-benzothiadiazole) (F8BT). We demonstrate amplified spontaneous emission (ASE) in optically pumped aligned F8BT planar asymmetric waveguides. The influence of the polymer chain orientation and the excitation polarisation on the optical properties of the materials has been investigated. By selecting specific optimised excitation configurations, high net gain coefficients can be obtained, namely <47 cm−1 compared with <25 cm−1 for standard spin-coated films. The loss coefficients also reduce from α = 7.6 cm−1 (spin-coated film) to 0.9 cm−1 (aligned film). Ellipsometry allows determination of the optical constants and reveals that the refractive indices of the materials are also greatly affected by the orientation of the polymer chains.