Origin of the amplified spontaneous emission from thiophene/phenylene co-oligomer single crystals: Towards co-oligomer lasers

Photoluminescence (PL) and optical gain measurements have been performed for single crystals of thiophene/phenylene co-oligomer at room and low temperatures. Broad PL bands are transformed to be an ensemble of several spectrally narrower vibronic peaks with decreasing temperature. Very sharp lines as narrow as $∼3meV$ are observed at $10K$ under weak excitation. Intensities of sharp emission lines superlinearly increased at $10K$ under intense excitation, showing the amplified spontaneous emission (ASE). The ASE bands were clearly identified to the sidebands of $B1$ and $A1+B1$ vibronic modes. The ASE is also observed at room temperature under the intense excitation. Spectroscopic investigation at varied temperatures enabled us to identify the origin of the ASE band to the vibronic sidebands of the electronic transition of the thiophene/phenylene co-oligomer crystals. In addition, highly polarized optical gains $∼50cm−1$ were obtained for the two ASE bands at room temperature.