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

Abstract

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.

Comment on “Cooperative Behaviors in Amplified Emission from Single Microcrystals of Thiophene/Phenylene Co‐Oligomers toward Organic Polariton Laser”

“…[ 4a ] As TPCO used in ref. [1] were synthesized with the use of such chemical reactions; [ 5 ] therefore, it is expected that these TPCO are molecularly self‐doped.…”

“…b) SRRS process explaining the data on the BP3T crystal in Figure 4c of Ref. [1] (adapted from Figure 4d in Ref. [1]).…”

“…In a recent paper by Yanagi et al [ 1 ] presented their progress and vision of the field of amplified emission from single crystals of thiophene/phenylene co‐oligomers (TPCO). These single crystals are an excellent platform for lasing and in particular for a long‐standing challenge in the field—current‐driven (electrically‐pumped) lasing.…”

“…This assignment seems unrealistic as it needs a very high pump intensity and/or an extremely long lifetime of the coherent phonons. Taking, as an upper estimate, the pump flux density of 10 mJ cm −2 with the photon energy of 2.2 eV, the pump pulse duration of 5 ns, [ 1 ] and the molecule concentration on the BP3T crystal face of 0.21 nm −2 from the X‐ray data, [ 2 ] it follows that each BP3T molecule meets (without optical absorption) pump photons once per ≈0.1 ns. However, the longest coherent phonon lifetime reported in various organic systems is two orders of magnitudes shorter.…”

“…In contrast, these (i and ii) and other results from ref. [1] can be naturally explained by an effect of unintentional impurities or the molecular self‐dopants (MSD) that appear in the course of chemical synthesis of TPCO.…”