Abstract
Here we report the fabrication and optical characterization of organic microcavities containing liquid crystalline conjugated polymers (LCCPs)-poly(9,9-dioctylfluorene-co-benzothiadiazole) (F8BT), poly(9,9-dioctylfluorene) (PFO), and poly(9,9-dihexylfluorene-co-bithiophene) (F6T2)-aligned on top of a thin transparent sulfuric dye 1 (SD1) photoalignment layer. We extract the optical constants of the aligned films using variable-angle spectroscopic ellipsometry and fabricate metallic microcavities in which the ultrastrong coupling regime is manifest both for the aligned and nonaligned LCCPs. Transition dipole moment alignment enables a systematic increase in the interaction strength, with unprecedented solid-state Rabi splittings of up to 1.80 eV, the first to reach energies comparable to those in the visible spectrum. With an optical gap of 2.79 eV for F6T2 this gives the highest-to-date organic microcavity coupling ratio, 65%. We also demonstrate that the coupling strength is polarization-dependent with bright polariton photoluminescence for TE polarization parallel to the polymer chains and either no emission or weakly coupled emission from the corresponding TM polarization.