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<sub>3</sub>:DCM film (5% DCM) were vacuum-deposited onto a polished GaAs (100) substrate coated with an 1-μm-thick layer of RF (radio-frequency) sputtered SiO<sub>2</sub>. The cleaved samples were optically pumped by a N<sub>2</sub> 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<sup>2</sup> 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<sup>-1</sup> and 3.2 μJ<sup>-1</sup>.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.
While preparing parallel mirrors for edge-emitting organic semiconductor lasers (OSLs) by cleaving, the edge of an
organic layer is severely damaged typically by the force of cleaving the substrate. By using an organic layer hardened
at a low temperature, we were able to cleave an organic layer along the facet of the substrate and reproducibly obtain
smooth and parallel mirror surfaces. Slab waveguide OSL structures consisting of 200 nm-thick Al<sub>q3</sub>:DCM films (5%
DCM) were vacuum-deposited onto polished GaAs (100) substrates coated with an 800 nm-thick layer of RF-sputtered
SiO<sub>2</sub>. The edge facets were prepared by cleaving the OSL structures in liquid nitrogen, and the facets of the organic
layers were evaluated by scanning electron microscope. The samples were optically pumped using a nitrogen laser
(λ=337 nm) with 600 ps pulse width at a 20 Hz repetition rate. The typical threshold power density was 68 μJ/cm<sup>2</sup> in
the sample with about a 10 mm cavity length. The lasing peak wavelength was 644.5 nm. The full width at half
maximum of the photoluminescence spectrum, dependence of light output power on input power, directional
characteristics and polarization characteristics were measured. Our method is very useful to realize electrically pumped
edge emitting OSLs.