Lasing characteristics of optically pumped edge-emitting organic semiconductor laser

摘要

Last year, we succeeded in reproducibly producing optically pumped edge-emitting organic semiconductor lasers using a low-temperature cleaving technique. Since the organic layer was generally soft and weak, its edge was damaged by the conventional cleaving at room temperature. This damage reduces the reflectance at the mirror edge and increases the threshold excitation energy. Stiffening the organic layer in liquid nitrogen enabled us to produce high-quality resonators with sufficient reproducibility. Slab waveguide devices consisting of Alq_3:DCM film (5% DCM) were vacuum-deposited onto a polished GaAs (100) substrate coated with an l-μm-thick layer of RF (radio-frequency) sputtered SiO_2- The cleaved samples were optically pumped by a N_2 gas laser (wavelength: 337 nm) resulting in a pulse width of 600 ps at repetition rate of 20 Hz. The laser oscillation was checked by measuring the full width at half maximum of the output spectrum and its polarization characteristics. The threshold density was typically 3 μJ/cm~2 in a sample with a 5-mm-long resonator. We investigated the relationship between the resonator loss and the threshold density by varying the resonator length. The internal loss α and the gain coefficient β were found to be about 10.5 cm~(-1) and 3.2 μJ~(-1)cm, respectively. The threshold density was calculated as a function of the thickness of the emitting layer and compared with experimental values. We found that the optimum thickness is approximately 150 nm. Moreover, the reflectance at the mirror edge was increased by attaching a metal (aluminum) reflector to one side, resulting in a reduction in the threshold.