Development of Wavelength-Tunable Solid-State Lasers with Low Thresholds Comprising Nano-Wires of Thiophene/Phenylene Co-Oligmers

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(i) Development of Organic Laser MaterialsWe have newly developed various thiophene/phenylene co-oligomers whose molecular terminals of either side are modified with chemical groups such as alkyls, methoxy, cyano, and trifluoromethyl. Those co-oligomers comprise thiophenes and phenylenes (benzenes) and are potentially useful as organic laser materials. Their electronic states and polarity of carriers (holes and electrons) can appropriately be controlled by suitably choosing the chemical groups as substituents.(ii) Observation of Laser OscillationWe have made single crystals and nano-wires of high purity using the above-mentioned co-oligomer materials. The laser oscillation has definitively been observed from them. In particular, we chose as model materials 2,5-bis(4-biphenylyl)thiophene (BP1T) and 5,5'''''-diphenyl-2,2':5',2'':5'',2''':5''',2'''':5'''',2'''''-sexithiophene (P6T) and closely inspected the emission characteristics of the nano-wires that resulted from the laser excitation of the materials under various excitation conditions. As the result, we found BP1T and P6T to cause the laser oscillation around 466 and 688 nm, respectively. In either case either edge plane of the crystals forms the Fabri-Perot resonator. Thus we have observed for the first time the laser oscillation from the organic crystals. The results are important as opening up a route to wavelength-tunable lasers of wide spectra.(iii) Evaluation of Laser Oscillation CharacteristicsWe have investigated the size dependent features of the laser oscillation from the nano-wires of BP1T. We confirmed on the basis of the relevant measurements that the thresholds of the laser oscillation decrease with increasing the size of the nano-wires. The stripe excitation with a focused beam of 20 μm in width is particularly effective in reproducibly causing the laser oscillation. The stripe excitation is thought advantageous to the optical confinement.