Cooperative Behaviors in Amplified Emission from Single Microcrystals of Thiophene/Phenylene Co-Oligomers toward Organic Polariton Laser

After successful commercialization of organic light-emitting diodes (OLEDs), organic semiconductor lasers have become a practical research target for versatile optoelectronic applications. Realization of electrically pumped lasing with molecular materials faces critical issues such as exciton quenching by injected carriers, nonradiative triplet states, and difficulties in introduction of resonators. One of potential routes to overcome them is to utilize polaritonic emission based on cooperative coupling of photons with excitons and/or phonons in molecular crystals. This report overviews cooperative behaviors observed in amplified emissions from single-crystal thiophene/phenylene co-oligomers (TPCOs) and discusses their expected potential toward organic polariton lasers. First, amplified emission accompanied with unusual time delay, spectral splitting, and fluence dependence of lasing threshold is presented, suggesting superfluorescence from macroscopically correlated molecular excitons, not based on the conventional stimulated emission process. Second, selectively enhanced Raman scattering under off-resonant condition is shown, which is interpreted as a result of contribution of coherent vibrations in the TPCO crystals. Then, angle-resolved spectroscopies demonstrate a stable existence of exciton polaritons at room temperature in microcavity devices with TPCO crystals. It is concluded that all those unique behaviors are ascribed to the TPCO excitons and vibrations having large oscillator strength under the uniaxially oriented molecular alignment.