Laser oscillation of whispering gallery modes was observed in microring structures of semiconducting thiophene/phenylene co-oligomer (TPCO) crystals at room temperature. Microring structures were formed by dry etching from thin film crystals of TPCO. The thresholds for the laser oscillation of a microring and a thin film crystal are 200 and for picosecond excitation, respectively. Therefore, the threshold for the microring was reduced to of that for the thin film crystal. The dramatic reduction of threshold clearly demonstrates the importance of microcavity in making efficient organic semiconductor lasers.
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We confirmed the molecular arrangement by angle-resolved transmission spectra. With increasing the incident angle, the absorption band at grows in polarization but not in polarization. This shows that the absorption band is ascribed to the H band and that the transition dipole moment parallel to the long molecular axis is perpendicular to the surface.
