Laser oscillation of whispering gallery modes was observed in microring structures of semiconducting thiophene/phenylene co-oligomer ͑TPCO͒ 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 1400 J/cm 2 for picosecond excitation, respectively. Therefore, the threshold for the microring was reduced to 1 / 7 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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Emission dynamics of the organic semiconductor material thiophene/phenylene co-oligomer (TPCO) single crystals has been studied under high-density optical excitation at 5 K. The optical amplification phenomenon is observed both on 0 -1 and 0 -2 vibration transitions, where these intensities increase superlinearly with the increase in excitation density. In particular, when optical amplification has started, the 0 -1 emission line shows symmetric pulse-shaped emission and its pulse peak has delay time of 50 ps. Delay time and pulse width decrease with the increase in excitation density. The excitation density dependence of delay time is useful for optical devices with the variable delay time with amplification.
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