Time-resolved photoluminescence of thiophene/phenylene co-oligomer (TPCO) single crystals has been measured at 7 K. The amplified pulse emissions with time delay are demonstrated in the 0–2 vibronic transition band of the TPCO single crystal under high-intensity excitation. We identify the delayed pulse emission to the 2A
1 vibration line. The pulse peak of the 2A
1 line exhibits a maximum delay time of 70 ps at 7 K. We consider that the amplified pulse emissions with time delay in single crystals occur at a vibronic transition band with the largest spontaneous emission intensity in the PL spectrum under weak excitation.
Photoluminescence spectral narrowing of 2,5-bis(4-biphenylyl)thiophene (BP1T) single crystals has been studied for various excitation lengths at 12 K to investigate the origin of its light amplification. In the range of excitation length L < 1:5 mm, the excitation energy threshold for the spectral narrowing of the 0 -1 vibronic transition band decreases with increasing excitation length. This behavior is not consistent with that of stimulated emissions. The present results give a plausible interpretation that a cooperative emission process contributes to the light amplification of the 0 -1 vibronic transition of the BP1T single crystals.
Amplified Raman scattering was observed from single crystals of thiophene/phenylene co-oligomers (TPCOs). Under ns-pulsed excitation, the TPCO crystals exhibited amplified spontaneous emission (ASE) at resonant absorption wavelengths. With increasing excitation wavelength to the 0-0 absorption edge, the stimulated resonant Raman peaks appeared both in the 0-1 and 0-2 ASE band regions. When the excitation wavelength coincided with the 0-1 ASE band energy, the Raman peaks selectively appeared in the 0-2 ASE band. Such unusual enhancement of the 0-2 Raman scattering was ascribed to resonant stimulation via vibronic coupling with electronic transitions in the uniaxially oriented TPCO molecules.
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