Efficient anisotropic polariton lasing using molecular conformation and orientation in organic microcavities

Abstract

Organic exciton-photon polariton lasers have recently been shown to be remarkable candidates for the realization of efficient sources of coherent light operating at room temperature. While their thresholds are now comparable with conventional organic photon lasers, tuning of molecular conformation and orientation as a means to further enhance their performance remains largely unexplored. Here, we first report a two-fold reduction in the threshold of a polariton laser based on a high-Q microcavity filled with an active layer of poly(9,9-dioctylfluorene) (PFO) when 15% {\beta}-phase conformation is introduced. We then take advantage of the liquid crystalline properties of PFO and use a thin transparent sulfuric dye 1 (SD1) photoalignment layer to induce homogeneous nematic alignment of the polymer chains. The resulting transition dipole moment orientation increases the Rabi energy, bringing the system into the ultra-strong coupling regime where we observe anisotropic polariton lasing with an eight-fold reduction in absorbed threshold, down to 1.14 pJ / 0.36 {\mu}Jcm-2 for the direction parallel to the orientation, with no emission along the orthogonal direction. To our knowledge, this threshold is lower than demonstrated with state-of-the art optically pumped organic vertical cavity surface-emitting photon and polariton lasers. This demonstration opens new opportunities for more efficient polaritonic devices and the observation of fundamental effects at low polariton numbers.