Enhanced and Polarization-Dependent Coupling for Photoaligned Liquid Crystalline Conjugated Polymer Microcavities

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.