Prethreshold Lasing with Time-Delayed Pulse Emission from a Single Crystal of Thiophene/Phenylene Co-Oligomer

Unusual time-delayed pulse emissions with spectrally narrowed band splitting were observed from a single-crystal thiophene/phenylene co-oligomer at room temperature. Under pulsed optical excitation, the Fabry–Perot lasing oscillation caused by a pair of the crystal facets emerges with peculiar spectral splitting in the prethreshold fluence range. Time-resolved measurements of those "prethreshold lasing" peaks reveal a pulse-shaped profile with a time delay up to 300 ps. This observation is not explained in terms of usual cooperative interaction reported as superfluorescence for inorganic emitters at low temperature, but might be more relevant to unknown phase correlation overwhelming thermal perturbation among perfectly aligned molecules.