Since the discovery of organic solid-state lasers, great efforts have been devoted to the development of continuous-wave (cw) lasing in organic materials. However, the operation of organic solid-state lasers under optical cw excitation or pulse excitation at a very high repetition rate (quasi-cw excitation) is extremely challenging. In this work, we have demonstrated quasi-continuous-wave (quasi-cw) surface-emitting lasing in a distributed feedback device which combines a second-order grating with an organic thin film of a host material 4,4’-bis(N-carbazolyl)-1,1’-biphenyl (CBP) blended with an organic laser dye 4,4’-bis[(N-carbazole)styryl]biphenyl (BSBCz). When pumping the device with optical picosecond pulse excitation, the quasi-cw laser operation maintained up to a repetition rate of 8 MHz. The lasing threshold was around 0.25 J cm−2 which was almost independent of the repetition rates. For our laser devices, the maximum repetition rate (8 MHz) is the highest ever reported, and the lasing threshold (0.25 J cm−2) is the lowest ever reported. These superior quasi-cw lasing characteristics in BSBCz are accomplished by the less generation of triplet excitons via intersystem crossing because a photoluminescence quantum yield of the blend film is nearly 100% and there is no significant spectral overlap between laser and triplet absorption.[1,2] Triplet quenchers, generally used for the fabrication of organic thin-film lasers, were not necessary in our devices because of negligible accumulation of triplet excitons and a small spectral overlap between emission and triplet absorption. Therefore, we believe that BSBCz is the most promising candidate for the first realization of electrically pumped organic laser diodes in terms of optical characteristics. However, electrical characteristics such as charge carrier mobility, charge carrier capture cross section, etc., are also extremely important and will need further investigation and enhancement for realization of electrically pumped organic lasers.
1. Aimono, T.; Kawamura, Y.; Goushi, K.; Yamamoto, H.; Sasabe, H.; Adachi, C. Appl. Phys. Lett. 2005, 86, 071110–071112.
2. Nakanotani, H.; Adachi, C.; Watanabe, S.; Katoh, R. Appl. Phys. Lett. 2007, 90, 231109.