Continuous-wave (CW) lasing from organic semiconductors is highly desirable for many practical applications. However, to achieve CW lasing, reducing the lasing threshold is a must as high threshold affects degradation and thermal ablation of organic materials. Therefore, in this study, we focus on reducing the lasing threshold by introducing two-dimensional (2D) distributed feedback (DFB) structures, which have the ability to more confine photons and more enhance the feedback of light two-dimensionally when compared with our previously reported one-dimensional DFB structures1. First, we fabricated a series of 2D DFB structures such as second-order 2D square, second order 2D square lattice, second-order circular, and mixed-order circular grating structures. The organic gain layer was a bis[(N-carbazole)styryl]biphenyl (BSBCz) emitter doped into a 4,4′-bis(N-carbazolyl)-1,1′-biphenyl (CBP) host layer at 6 wt.%. This doped layer is well known to have the spectral separation between singlet emission and excited-state triplet absorption1, which can avoid the quenching of singlets by triplets. Out of these four DFBs, the circular mixed-order grating structure provided the best laser performance. The threshold was 10 W cm~(-2) under 1,000 ms of long-pulse photoexcitation (Fig. 1). This performance is much better than that of our previous BSBCz-based laser devices with a mixed-order one-dimensional grating structure, in which the threshold was 1 kW cm~(-2) and the pulse duration was limited at 30 ms1. Overall, this study provides the evidence that largely decreasing the laser threshold and increasing the excitation pulse duration to a quasi-CW level are possible by optimizing the grating structures.
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